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Minerals and rocks, Notas de estudo de Engenharia de Produção

Minerals and rocks

Tipologia: Notas de estudo

2010

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Baixe Minerals and rocks e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! EJor => BO0N WWW.BOOKBOON.COM ). RICHARD WILSON MINERALS AND ROCKS ALT DMD GE CT BOOKBOON.COM NO REGISTRATION NEEDED J. Richard Wilson Minerals and Rocks Download free books at BookBooN.com Please click the advert Minerals and Rocks Contents 31 3.2 3.3 34 41 411 4.1.2 4.1.3 4.14 4.1.5 4.1.6 4.2 4.21 4.2.2 4.2.3 4.24 4.2.5 m w Crystallography Symmetry Crystal systems Crystal clas; Indices of crystal faces Systematic Mineralogy Silicate minerals Nesosilicates Sorosilicates (epidote) Cyclosilicates Inosilicates Phyllosilicates Tectosilicates Non-silicate minerals Native elements Sulphides Oxides Chlorides and fluorides Carbonates Sulphates Phosphates Igneous rocks Classification of igneous rocks Today's job mark AgrE ir got Swedish Institute skills and a competi 24 22 26 31 31 41 41 42 so 51 58 values ambitious, innovative, perceptive team players. Swedish universities foster these qualities through a forward-thinking culture where you're close to the latest ideas and global trends. reer goals may be, studying in Sweden will give you valuable ive advantage for your future. wiww.studyinsweden.se Download free books at BookBooN.com Please click the advert Minerals and Rocks Contents 5.11 Plutonic rocks 78 5.1.2 Volcanic rocks 85 5.1.3 Mineral assemblages 91 5.2 Magma 92 521 Where does magma come from? 93 5.2.2 The composition of magma 96 5.23 Temperature 99 5.24 Viscosity 99 5.2.5 Density 100 53 Eruption of magma 100 531 Non-explosive eruptions 100 53.2 Explosive eruptions 103 54 Volcanoes 105 541 The shapes of volcanoes 105 54.2 Calderas 105 5.5 Plutonic rocks 106 5.51 Minor intrusions (dykes and sills) 106 5.5.2 Major intrusions (plutons) 108 5.5.3 Chilled margins 11 5.6 The origin of magma 11 5.6.1 Distribution of volcanoes 11 5.6.2 Origin of basaltic magma 112 5.6.3 Origin of andesitic magma n4 5.64 Origin of rhyolitic magma n4 5.6.5 Crystallization of magmas n4 ramaahairendde oticon PEOPLE FIRST Download free books at BookBooN.com Please click the advert Minerals and Rocks Contents 5.7 Igneous mineral deposits 122 6. Sedimentary rocks 123 61 Introduction 123 6.2 Weathering 123 63. Classification of sedimentary rocks 128 631 Clastic sedimentary rocks 129 63.2 Biochemical sedimentary rocks 132 63.3 Organic sedimentary rocks 132 634 Chemical sedimentary rocks 133 64 Sedimentary structures 134 641 Layering (bedding) 134 64.2 Surface markings 135 643 Graded bedding 136 6.5 Where do sediments form? 136 6.5.1 Terrestrial environments 137 6.5.2 Marine environments 139 7. Metamorphic rocks 142 71 Introduction 142 7.2 Metamorphism — causes and effects 143 7.21 Heat 143 7.2.2 Pressure 143 7.23 Water 144 7.24 Differential stress 144 Today's job market values ambii universities foster these quali close to the latest ideas and glob; hatever your career goals Swedish Institute skil 1 a competitive advan! a Download free books at BookBooN.com Minerals and Rocks Introduction 1. Introduction The solid part of the Earth is made up of rocks. Rocks are made up of minerals. A mineral is a naturally occurring inorganic solid. It has a specific chemical composition and a characteristic crystal structure. Quartz is a very common mineral. Most beach sand is composed of quartz. It has the composition SiO, and forms elongate 6-sided crystals that terminate at a point (Picture 1.1). Picture 1.1: Quartz crystals. Pyrite is also a naturally occurring mineral that forms cubic crystals (Picture 1.2). It is also known as iron pyrites and has the composition FeS,. Diatuma 19 Demito nao Download free books at BookBooN.com 10 Minerals and Rocks Introduction Rocks are naturally occurring, coherent solids consisting of an aggregate of minerals. Glass may be present in some volcanic rocks. There are three main groups of rocks, classified on the basis of how they formed: * | IGNEOUS * SEDIMENTARY e METAMORPHIC 1.1 Igneous rocks Igneous rocks form by the solidification of melts. Molten rock is called magma. The most common magma (basaltic magma) forms as a result of partial melting of the Earth's mantle. Magma formed in the mantle 1ises towards the surface because it has a lower density than the surrounding mantle rocks. If the magma 1eaches the surface, a volcano is formed. Magma at the surface of the earth is called lava. Volcanic eruptions can produce mainly lava, but some eruptions are explosive and produce large volumes of ash and other fragmentary volcanic rocks, such as pumice. Igneous rocks that form at the surface of the earth are volcanic rocks. Because volcanic rocks cool rapidly they are fine-grained. They may contain some large crystals but the matrix is always fine-grained. Lava may cool so fast that crystals do not have time to nucleate and grow and glass forms. Magma below the surface of the earth contains dissolved fluids — mostly water. As magma 1ises to the surface the pressure decreases and some of the fluids escape as gas. Evidence of escaping gas is common in volcanic rocks in the form of bubbles that are called vesicles. Magma formed in the mantle may not reach the surface but accumulate in a magma chamber in the crust. Magma here cools slowly and the resulting rock is entirely crystalline and (relatively) coarse-grained. Igneous rocks that form below the surface are called plutonic rocks. 1.2 Sedimentary rocks All rocks exposed at the surface of the earth are subject to weathering. Rocks break into fragments that are transported by wind, ice and water and can be deposited to form a sediment. Small fragments are transported further than large fragments. During weathering many of the original minerals break down to produce clay minerals. An important mineral that does not break down is quartz. As they become buried, loose sediments (sand, silt, clay) become consolidate d and form compact rocks — sedimentary rocks. An example is consolidated clay that is called shale. Other sedimentary rocks form as a result of the precipitation of minerals from water; rock salt is an example. A wide variety of life forms exist in sedimentary environments and sedimentary rocks often contain evidence of life in the form of fossils. Fossils and fragments of fossils can accumulate to form limestone. One of the most obvious features of sedimentary rocks is layering. Download free books at BookBooN.com 11 Minerals and Rocks Introduction 1.3 Metamorphic rocks All rock types can be subjected to elevated temperature and/or pressure conditions. For example, sedimentary rocks near a magma chamber will be heated. The magma may have a temperature in the vicinity of 1200ºC. Sedimentary rocks close to the chamber will be heated more than those finther away. If shale is heated the clay minerals break down to form new minerals that are stable at higher temperatures. The original sedimentary rock changes its mineralogy and structure because it has been heated as a result of its proximity to an igneous rock - it has become thermally or contact metamorphosed. Rocks below the surface of the earth may be subjected to deformation at the same time as they are heated in what is known as regional metamorphism. Regionally metamorphosed rocks develop a foliation — a new layered structure — which is evident in, for example, schists and gneisses. 1.4 The Rock Cycle Sedimentary and volcanic processes take place at (or very near) the surface of the earth. Plutonic and metamorphic processes take place below the surface. The three groups of rocks: igneous, sedimentary and metamorphic, are related to each other by the Rock Cycle (Fig. 1.1). Igneous rocks at the surface of the earth are subjected to weathering and erosion. Material derived from this is deposited to form sediments. The loose sediment consolidates to form a sedimentary rock. This sedimentary rock becomes buried and subjected to heating and/or deformation — it becomes metamorphosed. Metamorphism can be so intense that the rock begins to melt and form an igneous rock. This can be exposed at the surface and the process continues. Download free books at BookBooN.com 12 Minerals and Rocks Minerals — an Introduction 2.1.1 Polymorphs Each mineral has a unique crystal structure. The same chemical composition can, however, in some cases develop more than one crystal structure. For example carbon (C) occurs in nature in two forms: GRAPHITE - hexagonal crystals, very soft (marks paper), density 2.3 g/cm” DIAMOND - cubic crystals, hardest known mineral, density 3.51 g/cm” Diamonds occur in nature in rocks that were formed at extremely high pressure. To conveit graphite to diamond in the laboratory requires a pressure of ca. 25 kilobars (25.000 times atmospheric pressure) at low temperature, increasing to 100 kb at about 2500ºC (Fig. 2.1). To form diamonds in nature therefore requires a pressure of >30 kb which is equivalent to a depth of about 100 km below the surface of the Earth. 3000 graphite Má v 3 E 2000 v a 5 diamond tamon É 1000 100 200 300 Pressure (kilobars) Fig. 2.1: Pressure — temperature phase diagram for carbon The stable phase of carbon at pressures below -30 kb is graphite, but (luckily for the diamond industry) the conversion from diamond to graphite at low temperatures is extremely slow. 2.2 Properties of minerals These are determined by the composition and crystal structure of the mineral. Relevant mineral physical properties include external shape, cleavage, lustre, colour, hardness and density. Some minerals have a characteristic taste; others are magnetic. The ability of a mineral to react with dilute HCI is a commonly used chemical property. Download free books at BookBooN.com 15 Minerals and Rocks Minerals — an Introduction 2.2.1 Crystal form, growth habit and twinning As you will leam in crystallography, there are seven crystal systems. These are: Crystal system cubic tetragonal hexagonal trigonal orthorhombic monoclinic triclinic mineral examples garnet, pyrite, halite (rock salt), magnetite zircon beryl calcite olivine, topaz gypsum, orthoclase (K-feldspar) plagioclase (Ca-, Na-feldspar) The 3-dimensional appearance of a mineral is referred to as its "habit". This is, of course, largely influenced by the crystal structure. For example, a hexagonal mineral can form long 6-sided needles or short “stubby” crystals. Both are hexagonal but they have different “habits”. Common descriptive terms include: flakes needle-like tabular prismatic fibrous thin plates (typical of mica minerals) thin, long crystals flattened but not "flaky” elongate - but not needle-like typical of e.g. asbestos minerals Crystals of a mineral often occur together in an aggregate. These crystal aggregates can develop in a wide variety of forms: radiating skeletal reniform (kidney-shaped) rosette e.g. pyrite e.g. native copper e.g. hematite e.g. corundum A feature shown by many crystals is a symmetrical intergrowth referred to as hvinning. Twins can be of a variety of types including; a) contact wins where it looks as if part of a crystal has been rotated (e.g. swallow tail twins in gypsum). The two individuals are separated by a definite surface (Fig. 2.2a). b) penetration iwins consist of two interpenetrating individuals, such as is common in staurolite (where a cross-like structure is formed) (Fig. 2.2b) and in K-feldspar (where it is called Carlsbad twinning) c) repeated (also called multiple or polysynthetic) fwins are made up of three or more parts twinned in the same fashion. This is very common in plagioclase feldspar. Download free books at BookBooN.com 16 Please click the advert Minerals and Rocks Minerals — an Introduction Fig. 2.2: Some common examples of twinning a) A single (monoclinic) crystal of gypsum is shown on the left. The two “halves” of a gypsum twin are joined along a plane in a contact twin. b) Staurolite commonly develops penetration twins where it appears that two crystals have “grown into” each other. do GCHQ it's an interesting world Get under the skin of it. Graduate opportunities Cheltenham | £824,945 + benefits One of the UK's intelligence services, GCHQ's role is two-fold: to gather and analyse intelligence which helps shape Britain's response to global events, and, to provide technical advice for the protection of Government communication and information sy In doing so, our specialists - in IT, internet, engineering, languages, information assuranoe, mathematics and intelligence — get well beneath the surface of global affairs. 1f you thought the world was an interesting place, you really ought to explore our world of work. tems. www.careersinbritishintelligence.co.uk Deer » 4 INVESTORS LUG) E 4 PINPEOPLE ants tom Download free books at BookBooN.com 17 Minerals and Rocks Minerals — an Introduction 2.2.7 Tenacity This is the resistance that a mineral offers to breaking, crushing, bending or tearing i.e. its cohesiveness. Commonly used terms are: brittle breaks or powders easily (e.g. quartz) malleable can be hammered out into thin sheets ductile can be drawn into a wire flexible bends but does not resume its original shape on pressure release elastic resumes its original shape after bending (e.g. mica) The terms malleable, ductile and flexible are mostly relevant for metallic minerals. 2.2.8 Density Density, mass per unit volume, is measured in gm/cm”. We use the term specific gravity (G) which is the ratio of the weight of a substance to the same volume of water. Since this is a ratio there are no units for G. This is often a very useful criterion for mineral identification. Some useful reference values are: Gold (Au) 19.3 Galena (PbS) 7.6 Pyrite (FeS,) 5.2 Halite (NaCl) 2.2 Calcite (CaCO;) 27 K-feldspar (K[AlSis0s]) 2.6 Olivine (Mg, Fe)[SiO,] 32-44 Quartz (SiO,) 26 2.2.9 Other properties Other useful properties can be relevant in special cases. For example, halite, also called rock salt (NaCl), has a characteristic taste and magnetite (Fe;O,) is strongly magnetic. Calcite (CaCOs) reacts readily with cold hydrochloric acid (HCI) to release bubbles of carbon dioxide: Caco, + 2HCI= CaCl, +H;0 + CO, Many minerals are birefringent. This is usually only observable with the aid of a microscope, but in calcite double refraction is visible in hand specimen — you can see “double” through transparent crystals. Download free books at BookBooN.com 20 Please click the advert Minerals and Rocks Crystallography 3. Crystallography Ciystallography is the science related to crystals. The word is derived from the Greek “crystallus” which means ice. The term was used in ancient Greece for colowrless quartz which was believed to be “fossilised” ice. The most obvious feature of a quartz crystal (Picture 1.1) is that it is defined by a series of flat surfaces formed during growth and must, therefore, reflect the intemal structure of the material. This feature is shared by many materials and the term used for them is crystals. A vital feature of crystals of the same type is that the angle between the crystal faces is identical, no matter how large or small the crystal is. This law of the “constancy of interfacial angles” was demonstrated by Nicolaus Steno in 1669. Minerals are crystalline. When a mineral grows from a melt (magma) it forms crystals with perfect crystal faces. As more crystals grow and the amount of melt decreases the crystals begin to collide with each other. When all the melt has crystallised the solid rock consists of the minerals that formed but their crystal faces will seldom be preserved. Even though the crystalline nature of minerals may not be obvious in a solid rock, it can usually be confirmed with the aid of a microscope and always using X- ray diffraction. out of thin air. As SR ERES A es of Knowledge Engineering = x CAR CRU Te A Co = ae LR e a pf Download free books at BookBooN.com 21 Minerals and Rocks Crystallography 3.1 Symmetry The shapes illustrated in Fig. 3.1 have symmetry planes. The two sides of the face in Fig. 3.1a, are identical. They are the mirror image of each other. The face has one symmetry plane. The diamond in Fig. 3.1b has two symmetry planes, one horizontal and one vertical. The shape in Fig. 3.1c has four symmetry planes. à bl sm Ie SP1 sp3 .,. SP2 SP2 SP4 Fig. 3.1: Planes of symmetry in two-dimensional objects a) contains one symmetry plane; b) contains two; c) has four (SP1 to SP4). In addition to two symmetry planes, the diamond shape in Fig. 3.1b also has a symmetry axis. When it is 1otated 180º about an axis through the centre (perpendicular to the plane of the paper) the same image is achieved as before rotation (Fig. 3.2). The axis is called “two-fold” because rotation through 360º repeats the same shape twice. The “distorted diamond” in Fig. 3.3 has a two-fold axis of symmetry but no symmetry planes. sm sm sm Fig. 3.2: Axis of symmetry Two-fold symmetry axis in the diamond shape from Fig. 3.1b. Download free books at BookBooN.com 22 Minerals and Rocks Crystallography Symmetry elements are therefore: SYMMETRY PLANES SP SYMMETRY AXES AZ A3 AS A6 CENTRE OF SYMMETRY c Several symmetry elements are often combined in crystals. The shape in Fig. 3.6 has a four-fold symmetry axis (A4) and four two-fold symmetry axes (442). There is also a total of 5 symmetry planes and a centre of symmetry. a d A4 a] , SP1 tT q . I ] ! (| “rs o L b [e] TO AA SP2 Vo | J- 1 SP3 Apa T E A c SP4 o I SP5 > «Am .-— Tao) — T A) Lo - Fig. 3.6: Symmetry elements for a tetragon a) There is one A4 axis of symmetry. b) There are two A2 axes passing through the centres ofthe large sides. c) There are two A2 axes passing through the centres of the long edges. d) There are five planes of symmetry (SP1 to SP5). SP1 is parallel with the base and top of the tetragon and is therefore not visible in the two-dimensional form to the right in d). There is also a centre of symmetry. Download free books at BookBooN.com 25 Minerals and Rocks Crystallography The total symmetry for the shape in Fig. 3.6 is therefore: 1A4 4A2 5SSP C This is the maximum symmetry in the tetragonal crystal system. Tetragonal minerals include zircon (Z1[SiO,)). 3.2 Crystal systems There are 7 crystal systems, each of which has particular minimum symmetry elements (Table 3.1). CRYSTAL SYSTEM MINIMUM SYMMETRY CUBIC 4A3 TETRAGONAL 144 HEXAGONAL 1A6 TRIGONAL 1A3 ORTHORHOMBIC 3A2 MONOCLINIC 1A2 TRICLINIC NO SYMMETRY Table 3.1: Minimum symmetry elements developed in the 7 crystal systems To aid the description of crystals a coordinate system is used whose “0” -point (origin) is located at the centre ofthe crystal. The three axes are called the a-, b- and c-axes. In the orthorhombic system the three axes are perpendicular to each other (i.e. all at 90º to each other) and a, band c are of different lengths (Fig. 3.7). All angles = 90º Fig. 3.7: Crystallographic axes in the orthorhombic system There are three axes at 90º to each other and of unequal lengths (a, b and c) in the orthorhombic crystal system. 26 Download free books at BookBooN.com Please click the advert Minerals and Rocks Crystallography In the tetragonal system the three axes are again perpendicular to each other, but two of them have the same length (Fig. 3.8). The convention is to place the c-axis as the unique vertical axis (the A4 symmetry axis) and the two others are identical with each other (and may be called a;- and a,-axes rather than the a- and b-axes). a, a, All angles = 90º Fig. 3.8: Crystallographic axes in the tetragonal system The c-axis is the A4 symmetry axis. The other two axes are at 90º to c and each other and of equal lengths (a, and a,). FIRST CONTACT Get matched to top business employers via intelligent emails landing in your inbox. PLANET CAREER ADVICE Milkround.com: rated-the Inhabited by insights into 1 graduate recruitment business careers and orbited g paia by application advice. Ai ei AcA o Ric de Tg To rc a MILKROUND SYSTEM Co AE o] Become a business star with |) : E [Moo RT ee ALE LEIA internships, placements, tg graduate jobs & schemes from leading companies. www.milkround.com ' i E Ff á CITA) Download free books at BookBooN.com 27 Please click the advert Minerals and Rocks Crystallography Fig.3.13: Crystallographic axes in the hexagonal system These are arranged in the same way as in the trigonal system. Ria (ii stat Today's job market values ambitious, innovative, perceptive team players. Swedish universities foster these qualities through a forward-thinking culture where you're close to the latest ideas and global trends. hatever your career goals may be, studying in Sweden will give you valuable Skills amd à competitive advantage for your future. mwiw.studyinsweden.se Download free books at BookBooN.com 30 Minerals and Rocks Crystallography 3.3 Crystal classes We have seen that there are 7 crystal systems. Within each system there are several three-dimensional shapes that can show the minimum symmetry required to define the symmetry. This can be illustrated by two very different shapes that both belong to the cubic crystal system. The total symmetry of a cube (Fig.3.9) is 344, 4A3, 642, 9SP, C. The total symmetry of a tetrahedron (Fig.3.5b) is 443, 3A2, 6SP. Both cube and tetrahedron have 4A3 axes of symmetry and therefore belong to the cubic crystal system (Table 3.1). These are two classes of the cubic crystal system. There are a total of 32 crystal classes, but these are beyond the scope of this text. 3.4 Indices of crystal faces Different types of crystal faces are defined according to their relationship to the crystallographic axes. In Fig.3.14 there are 3 crystallographic axes OX, OY and OZ which meet at O (the origin) and are perpendicular to each other (i.e. 90º). ABC is a crystal face which intersects all three axes. n Fig.3.14 there is an additional crystal face (DEF) that also cuts all three crystallographic axes. One of the vital features of crystal faces is that they cut the crystallographic axes at distances that have a simple, whole number ratio to each other. n Fig.3.14 the ratios of the lengths along the axes defined by the two faces are: OD = OA, OE = 20B and OF =",0C. Fig.3.14: Crystallographic parameters See text for explanation. The parameters of face DEF relative to ABC are therefore 1 1 2, 1 1h Download free books at BookBooN.com 31 Minerals and Rocks Crystallography The form of the crystal face ABC (to which other faces are related) varies from crystal to crystal and is called the unit form. There is a system for the notation of crystal faces that was invented by W. F. Miller in 1836. The notations are called Miller indices. Miller indices are the reciprocal of the parameters defined above. The unit form (which cuts all 3 axes) is called (111), as illustrated in Fig.3.15a. Fig.3.15: Miller indices a) The unit form (111) cuts all three axes. b) This face cuts a and b at the same distance as (111) and is parallel with c. Its parameters are (110) and Miller indices (110). c) Parameters (1000); Miller indices (100). d) Parameters (11 1); Miller indices (112) The crystal face in Fig.3.15b cuts the a- and b-axes at the same distance as (111), but is parallel with the c- axis. The parameters for this face are therefore (1100). It is, however, not practical to use co. Miller indices usethe reciprocal: (Mo) 59 Cho) — (10) In Fig.3.15c the crystal face cuts only the a-axis and is parallel with both the b- and c-axes. The parameters of this face are therefore (1000) and its Miller indices are (100). The crystal face in Fig.3.15d has the parameters (11"/,) so that its Miller indices are (112). Note that it cuts the c-axis at half the distance of the unit form (111) in Fig.3.15a, but its Miller indices are (112). A cube has 6 identical crystal faces, all of which cut one crystallographic axis and are parallel with the other two. Since it is useful to be able to identify any individual face of a crystal, a convention is used in which each crystallographic axis has a positive and a negative end (Fig.3.16). The face that cuts the positive end of the a-axis and is parallel with the b- and c-axes is referred to as (100); the face opposite and parallel to this cuts the negative end of the a-axis and is referred to as (100). Download free books at BookBooN.com 32 Minerals and Rocks Crystallography There are 12 faces of the type (210) in the cubic system (Fig.3.19). These combine to form a pyritohedron, so-called because it is commonly developed in pyrite (Fe S,). (021) (021) Fig.3.19: In the cubic system, twelve identical faces of the type (210) combine to form a pentagonal dodecahedron The (210k> form is commonly developed in the mineral pyrite and is also known as a pyritohedron. There are 24 faces of the type (112) in the cubic system (Fig.3.20). These combine to form a trapezohedron. Fig.3.20: There are 24 identical faces in the form (112) in the cubic system which combine to give a trapezohedron The (112%, form is commonly developed in the minerals garnet and leucite. Download free books at BookBooN.com Minerals and Rocks Crystallography In the tetragonal system there are two important types of crystal faces, those parallel with the c-axis (prism faces) and those that cut both the a- and c-axes (pyramid faces) (Fig.3.21). Itis apparent that there are 4 prism faces (110) and/or 8 pyramid faces (111) that must combine to make a tetragonal crystal. The form (001) (ifit is developed) consists of two faces, one at either end of the crystal. This type of face is called a pinacoid. Fig.3.21: Prism, pyramid and pinacoid faces in the tetragonal system A combination of prism and pyramid faces are commonly present in the mineral zircon. There are three pinacoid forms in the orthorhombic system (100), (010) and (001)(Fig.3.22). Faces of the type (110) are prism-faces and (111)-type are pyramids, as in the tetragonal system. FOSS Sharp Minds - Bright Ideas! 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Foss Slangerupgade 69 3400 Hillerod ACRE ER DEErO www.foss.dk Ts ms Download free books at BookBooN.com 36 Minerals and Rocks Crystallography Fig.3.22: Pinacoid faces in the orthorhombic system In the monoclinic crystal system the crystallographic a- and c-axes are not at 90º to each other. The convention is that the b-axis is identical with the AZ axis of symmetry (Fig.3.23). Download free books at BookBooN.com 37 Minerals and Rocks Crystallography A form that is common in the trigonal system is that of a rhomb (Fig.3.25). The most commonly developed crystallographic form is (1071). This is the form of the excellent cleavage in the trigonal mineral calcite. Calcite rhombs are often used to demonstrate double refraction (birefringence). Fig.3.25. A rhombic form is commontly developed in the trigonal system (e.g. calcite cleavage). We have, in general, only dealt with the most simple types of crystal faces: (100), (110) and (111). Many other faces can be developed in crystals. The Miller indices notation for faces of the type that cut a, b and cis (hk1). This includes forms (111), (1123, 4113), (123) etc. The notation (hk0) includes (110), (120), (130), (230) etc. Download free books at BookBooN.com Please click the advert Minerals and Rocks Systematic Mineralogy 4. Systematic Mineralogy 4.1 Silicate minerals About 3600 minerals have been identified. Most of these occur in the Earth's crust. So far about 30 minerals have been mentioned in this text. Some minerals are, of course, more common than others. The crust is dominated by the elements oxygen and silicon. Oxygen forms O” anions and compounds that contain O? are called oxides (hematite Fe;O; and quartz SiO, are oxides). Silicon forms Si” cations. Silicon and oxygen together form an extremely strong complex ion: the silicate ion [SiO,]". Minerals that contain the silicate ion are silicate minerals, these dominate the crust. O? has an ionic radius of 1.32 À whereas Si is a relatively small cation with an ionic radius of 0.42 À. Considering the ions as spheres, 4 large oxygen ions can be packed around one small silicon ion giving a tetrahedral structure. This structure has 4 positive and 8 negative charges, giving a net charge of 4-. Silicate minerals are dominated by the [SiO,]' silicate tetrahedron. [SiO,]" tetrahedra exist independently in some minerals but can share one, two, three or all four oxygen anions in other minerals. This possibility gives a range of silicate structures (Fig. 4.1). Design your o LoR LILA ÇE DE TES] m MAN Diesel — Powering the World Download free books at BookBooN.com 41 Minerals and Rocks Systematic Mineralogy Trpsof | Formula + E Linkage Valeney SO] est eo Example General trends. Separate “o Olivine NESO- | toirahedra | (SO 14 A Mg,[50,] es e 4 Re r Increasingly uble |, open SORO | antrabedra | 807º ns] Fl (Uncommoni) | structures , a” aca Decreasing Brargo | Bent tetrahedron-| (5Opg!” | 13 + 4 Be AL [51,05] density SE | tras Canto + = Lower single t Pra erystallization no | se | so; [13 id Fade + Ss — Ca MgISiO,], 7 , e . Amphibole Double | (50,5 [278 E Tremolite Ot) INO- chains " - + + b c commonty A CaMas present ms. [51,0 ,11,10H), + + é o - Mica vat. Muscovite KAÍAIS Oro) | Increasina envLLO- | Sheets | (4097 [125 fiz a +. KiMg rela! | cations [AISO] egk á (0h), feldspars eg Three- ' rthoclase TECTO- |dimensional 12 Hoa rency ciciad KIAISi;O,] 1 networks Silicon dioxide Quant SD, Fig. 4.1: Types of SiO, linkage in silicate minerals The most simple silicate structure involves individual SiO,-units (nesosilicates). The structure becomes more complex as the number of oxygens shared by adjacent SiO ,-units increases. One oxygen is shared in sorosilicates and two are shared in a ring-structure in cyclosilicates. Two oxygens are shared to form a chain structure in single chain inosilicates, and these chains are linked by shared oxygens in double chain inosilcates. Three of the four oxygens in the SiO,-units are shared to form a sheet structure in phyllosilicates, and all four are shared to form a complex three-dimensional structure in the tectosilicates. As the amount of oxygen-sharing increases and the structure becomes more open the density of silicate minerals generally decreases and there is more room for large ions (e.g. cations Na* and K*; (OH) anions). 4.1.1 Nesosilicates 4.1.1.1 Olivine group The olivine group consists of two end members: forsterite (Fo) Mg[SiO,] and fayalite (Fa) Fe,[SiO,] Download free books at BookBooN.com 42 Please click the advert Minerals and Rocks Systematic Mineralogy Alternatively, crystallization can take place so rapidly that individual olivine crystals cannot change their composition by reaction with the melt. Olivine crystals can therefore become zoned with Mg-rich cores and Fe-rich 1ims. Exchange between Mg? and Fe? in olivines, however, takes place very easily, and zoning is seldom observed. Other mineral groups that show solid solution are well known for zoned crystals, in particular the plagioclase feldspars, as we will see later. Forsterite occurs in some metamorphosed carbonate rocks. To form forsterite requires both Mg and Si. Pure limestone is composed of calcite (CaCO;) - so no olivine can be formed from this. Many limestones are, however, impure and contain both quartz (SiO») and dolomite (CaMg(CO;).). At a temperature of -500ºC these can react together to form forsterite: 2CaMg(CO;), + SiO, = Mg,[SiOs] + 2CaCO; + 200, dolomite + SiO, = forsterite + calcite + CO, Metamorphosed limestone is marble and the rock produced would be forsterite marble. Mgrich olivine cannot exist in equilibrium with quartz because they react together to form a new mineral - Mg-rich orthopyroxene called enstatite (section 4.1.4.1.1): Me,[SiO4] + SO, = 2Mg[SiO;] forsterite + SiO, = enstatite wealthYsiucentcou k Download free books at BookBooN.com 45 Minerals and Rocks Systematic Mineralogy 4.1.1.2 Garnet group Another mineral group which contains independent [SiO,]" tetrahedra is the gamet group. Gamets have a much wider compositional range than olivine and have the general formula R?;R*[SiO,]; where R” = Mg”, Fe”, Ca” or Mn” (divalent cations) and Rº” = AI”, Fe” or Cr” (trivalent cations). The most common garnet, which occurs in metamorphic rocks and has a reddish brown colour, is called almandine (FesAl[SiO,]:). This occurs in metamorphosed Al - rich rocks, the most usual of which are clay - rich sediments which have been heated to over -500ºC. The green garnet in Picture 4.2 is rich in the grossularite (Ca;AL[SiO,]3) end-member. Gamets are cubic and commonly form 12-sided (dodecahedra) or 24-sided crystals (trapezohedra). As garnets are quite hard (H = 7), have no cleavage and are not uncommon they are used as abrasive material for grinding and polishing. Most varieties of gamet are cut as gemstones. Its name comes from the Latin granatus, meaning grain-like. Picture 4.2: Green grossularite-rich garnet showing the dodecahedral form illustrated in Fig. 3.17. 4.1.1.3 Zircon Zircon (Zr[SiO,]) is the main mineral that contains the element zirconium and occurs in small amouwnts in a wide range of rock types - it is a common accessory mineral. Zircon, which forms brownish tetragonal crystals (Picture 4.3), is a very important mineral for age determinations. This is because zircon is an extremely stable mineral and uranium can enter the zircon structure. The radioactive isotope of uranium (e.g. “St decays to radiogenic lead (Pb) at an extremely slow rate (half life = 4.47 x 10º years). Measurement of the amounts of these isotopes allows determination of the time at which the zircon crystallized. The zircon age determination method is the most reliable technique for very old rocks and has provided information on the oldest rocks in the world. Zircon is the main source for the element zirconium that is used in nuclear reactors. Z1O, is extremely refractory and is used to make crucibles for melting platinum at 1755ºC. Download free books at BookBooN.com Minerals and Rocks Systematic Mineralogy Picture 4.3: Zircon crystal illustrating its tetragonal crystal structure with prism and ho types of pyramid faces developed (see also Fig. 3.21). 4.1.1.4 Sphene Sphene (CaTi[SiO,](O,0H,F)) (altematively called titanite) is also a widely developed accessory mineral, especially in granites. It forms brownish wedge-shaped crystals and is the most important Ti-bearing silicate mineral. There are other Ti-bearing minerals, most notably ilmenite (FeTiOs) and rutile (TiO,) but these are oxides and not silicates. 4.1.1.5 Aluminium silicate polymorphs There are three minerals with the composition AbSiOs. Writing the composition in this ways masks the fact that they contain independent silicate tetrahedra; this becomes clear when the formula is written Al,O[SiO,]. The three aluminium silicate polymorphs are: sillimanite - orthorhombic andalusite - orthorhombic kyanite - triclinic Sillimanite forms white, andalusite brown and kyanite pale blue prismatic crystals (Picture 4.4). These occur in clay-tich sedimentary rocks (most clay minerals are Al-rich; for example kaolinite (Al[Si,05|(0H)4) that have been subjected to high temperature and/or pressure (i.e. metamorphism). Clay-rich rocks are collectively called pelites; after metamorphism they are metapelites. It is clear from the PT diagram (Fig. 4.3) that andalusite is only stable at pressure below 4 kbar (equivalent to a depth of 12-14 km), sillimanite only forms at temperatures above 525ºC, and kyanite is the high pressure phase. During metamorphism kaolinite reacts with quartz to form the mineral pyrophyllite (Ab[Si,05](0H)). Download free books at BookBooN.com 47 Minerals and Rocks Systematic Mineralogy 4.1.1.6 Staurolite The mineral staurolite also occurs in metamorphosed Al-rich sedimentary rocks (metapelites) at 500º C (Fig. 4.4). It has a brownish colour and commonly occurs as well-formed monoclinic (actually pseudo-orthorhombic -ie. they appear to be oithorhombic) crystals. Cruciform twins are a characteristic feature of staurolite (Fig. 2.2). Itis named after the Greek word stauros meaning cross, referring to its cruciform twins. Pressure [kbarj 500 Tempesature*C Fig. 4.4: Formation of staurolite and cordierite relative to the Al>SiOs polymorphs during metamorphism. These minerals are very useful in the determination of the pressure-temperature conditions during the metamorphism of clay-rich sediments (metapelites). Cordierite is in section 4.1.3.3. 4.1.1.7 Topaz Topaz (AL[SiO,](OH.F).) is an orthorhombic mineral that is often used as a gem stone. It occurs in granitic rocks. The best crystals are found in granitic pegmatites. A pegmatite is a very coarse grained igneous rock. 4.1.2 Sorosilicates (epidote) There are not many important sorosilicates (at least not important for us at present). The epidote group of minerals, however, contains both [SiO,]“ and [Si,0,]é units and has a complicated chemical composition in which Ca?*, Al and Fe”” are involved. Epidote (with the composition Ca(AL Fe”) O[SiO,][Si0;](OH)) is a greenish mineral (Picture 4.5) which occurs as a relatively low temperature (typically 200-400ºC) alteration product in many rocks types (where it commonly occurs in cracks and veins) and as a metamorphic mineral. It is an essential mineral in “greenschists” which are metamorphosed basalts. Download free books at BookBooN.com 50 Minerals and Rocks Systematic Mineralogy Picture 4.5: An aggregate of green prismatic epidote crystals. 4.1.3 Cyclosilicates 4.1.3.1 Beryl Beryl (Be: Al,[SicO1s]) is the "type" ring silicate in which the hexagonal symmetry reflects the six-membered 1ings of SiO, tetrahedra in the structure (Fig. 3.13). It is a green mineral (bright green varieties are known as emerald; bluish green as aquamarine). Beryllium forms a very small cation (Be?”) which does not enter the structure of the common rock-forming minerals during fractional crystallization of magmas. Be?” is therefore concentrated in the residual magma that will also be enriched in other small cations (e.g. boron, lithium). The same applies to very large cations (e.g. thorium, uranium) and large anions (e.g. fluorine, hydroxyl ((OH)-groups), chlorine). Late stage magmatic fluids are therefore volatile-rich and can crystallize relatively rare minerals that contain small (or large) cations. The mineral beryl therefore occurs in late stage granitic 10cks, commonly in very coarse-grained rocks called pegmatites. These can contain very large individual crystals (in extreme cases up to several meters long). Beryl is typically associated with quartz, K -feldspar, Li-mica, tourmaline (a boron-mineral) and other "exotic" minerals. 4.1.3.2 Tourmaline This is another ring-structured silicate mineral with [Sis01s]-units. The structure contains boron in BO;- groups as well as Na, Mg, Al, Fe, Li and (OH)-groups. The colour of tourmaline varies with its composition and many varieties are used as gemstones. The most common, Fe-rich varieties, are black; Li-rich ones are green; brown, green and red types also occur. Colour zoning is common. Tourmaline forms trigonal crystals that commonly have a triangular cross-section (Picture 4.6). Since tourmaline contains the small boron cation (B*) it occws (like beryl) in late stage granitic rocks, especially pegmatites. Tourmaline crystals are commonly striated parallel with the c-axis. Download free books at BookBooN.com 51 Please click the advert Minerals and Rocks Systematic Mineralogy Picture 4.6: Black tourmaline crystal where the trigonal form is obvious. WWYW.STUDYINSWEDEN.SE Today's job market values ambitious, innovative, perceptive team players. Swedish universities foster these qualities through a forward-thinking culture where you're close to the latest ideas and global trends. b Whatever your career goals may be, studying in Sweden will give you valuable Swedish Institute skills and a competitive advantage for your future. www.siudyinsweden.se Download free books at BookBooN.com 52 Minerals and Rocks Systematic Mineralogy Other clinopyroxenes include aegirine which has the composition NaFe[Si,Os], where Fe here is Fe”” to achieve valancy balance with Na” and [Si,06]”; it is dark green and occurs in Na-rich igneous rocks. Compositions intermediate between aegirine and augite occur; these are (logically enough) called aegirine- augite. Another end-member is jadeite (NaAI[Si,Os]) which forms under high pressure. Compositions intermediate between jadeite and augite are called omphacite and are noteworthy for their occurrence in eclogites, which are the metamorphic equivalents of basalts formed under very high pressures and relatively low temperatures (in subduction zones). Eclogites consist dominantly of two minerals: green omphacite and 1ed-brown garnet. These very attractive, dense, rocks are quite rare. Pyroxenes can often be distinguished from amphiboles by their cleavage (Fig. 4.7) or their crystal outlines (Picture 4.7). Both groups of minerals have prismatic cleavage, but pyroxenes break into fragments with square or rectangular cross sections (i.e. 90º between the cleavage planes) whereas amphiboles break into diamond-shaped cross sections (60º between the cleavage planes). a b (110) (10) Fig. 4.7: Pyroxenes and amphiboles can often be distinguished by their cleavage. The angle between cleavage surfaces in pyroxenes is close to 90º whereas in amphiboles the angle is close to 60º/120º. 4 Picture 4.7: Crystal shape can sometimes be used to distinguish between pyroxenes and amphiboles. Individual crystals of pyroxene (left) and amphibole (right) have been cut through to illustrate their cross- sectional shapes. The angles between prismatic crystal faces in pyroxenes are close to 135º whereas in amphiboles they are close to 60º or 120º. Download free books at BookBooN.com 55 Please click the advert Minerals and Rocks Systematic Mineralogy 4.1.4.2 Amphiboles Like the pyroxenes, some amphiboles are orthorhombic but most are monoclinic. The compositional variation of amphiboles is expressed by the general formula: Ao aB2Cs[TsO»|(OH)» A=Na K; B=Ca, Na, Mg, Fe”, C=Mpg, Fe”, Fe”, AI, T=Si, Al The A-site is for relatively large cations; this site is empty in some amphiboles. The B -site is slightly larger than the C-site. Note three points: a) large cations (K”) may be present; b) Al” substitutes quite extensively for Si””, c) The presence of (OH) groups. These features reflect the fact that the amphibole structure is fairly "open", in contrast to more compact structures like those of olivine and garnet (silicates with independent SiOy-tetrahedra), as is shown in Fig. 4.1. Distinction between amphiboles and pyroxenes can commonly be achieved using the angle between cleavage surfaces (Fig. 4.7) or crystal outlines (Picture 4.7). 4.1.4.2.1 Orthorhombic amphiboles The most common orthorhombic amphibole is anthophyllite (Mg;[Sis0»>](OH),) in which the A-site is empty and both B- and C-sites are occupied by Mg. Anthophyllite is a grey-brown mineral that forms elongate prisms that occur in aggregates. It occurs in metamorphosed, olivine-rich ultramafic rocks. ramaahairendde oticon PEOPLE FIRST Download free books at BookBooN.com 56 Minerals and Rocks Systematic Mineralogy 4.1.4.2.2 Monoclinic amphiboles Two end-members in the monoclinic amphiboles are tremolite (CaMgs[Sis0»](OH)) and actinolite (CayFes[Sis0»](OH)-). Tremolite is colomless to pale green and typically occurs as a result of the metamorphism of Ca- and Mg-bearing carbonate sediments (dolomites). Compositions intermediate between tremolite and actinolite occur as a result of the metamorphism of the most common pyroxene, augite. They also occur as one of the green minerals in metamorphosed basaltic rocks known as greenschists. The other green minerals in greenschists are epidote and a mineral with a layered structure called chlorite. The Na- feldspar, albite, is also present in greenschists. The most widespread amphibole is called hornblende that has a complex and variable composition: NapiCa(Mg.Fe”! Fe” ADa[(Si ADsOno] (OH) Homblende (Fig. 4.8) is a dark green to black mineral that occurs in many different rock types. mn igneous 1ocks it may be the only hydrous mineral in, for example, basalts and gabbros, but is more common in intermediate and acidic (SiO,-rich) types like granites. Homblende is an important mineral in metamorphic 1ocks, especially in the metamorphosed equivalents of basaltic rocks known as amphibolites. These consist dominantly of hornblende and a feldspar mineral (plagioclase) and form at higher temperatures than greenschists. Fig. 4.8: Illustration of a typical hornblende crystal. The angle between the a and c axes is not 90º. There are many other amphiboles, but the only one we will consider here is called glaucophane (NaMg3Al[Sis0,2](OH)-). This amphibole is blue and occurs in basaltic rocks that have been metamorphosed at relatively high pressures and low temperatures - called blueschists. Download free books at BookBooN.com 57 Minerals and Rocks Systematic Mineralogy 4.1.5.4.1 Muscovite Muscovite is the most common colourless mica (Picture 4.8) with the composition KAL[AlSi,0,](OH),. Note that Al occurs in two sites - one replacing Si (with coordination number (CN) = 4) and one slightly larger site, linking the [AlSis010] layers together (CN = 6). The large site occupied by K has CN = 12. Fluorine (F) can occur replacing some of the (OH) - groups. Picture 4.8: Flakes of brownish-black biotite (top) and colourless muscovite (bottom) illustrating their perfect basal cleavage. Muscovite is a soft mineral (H = -2) which forms elastic flakes with G = — 2.9. It occurs in some granites and granitic pegmatite where it can form meter-sized plates. It is common in metamorphosed clay-rich sediments called mica schists. Here the muscovite defines a foliation that develops in response to pressure during metamorphism. Large muscovite flakes used to be used instead of glass in Russia when it became known as “Muscovy-glass”, which is probably how it got its name. A Li-bearing variety of mica which is pink to purple in colour is known as lepidolite. 4.1.5.4.2 Biotite The composition of biotite (dark mica) is similar to muscovite except that the Al with CN = 6 is replaced by Mg and Fe. This gives K(Mg,Fe);[AlSi,O|(OH,F),. The structure is similar to that of muscovite, forming pseudohexagonal flakes. It is slightly harder (H = 2.5-3) and denser (G = 2.9-3.4, increasing with Fe-content) than muscovite. Mg-rich varieties are called phlogopite and are brown, whereas Fe-rich varieties (biotite) are black (Picture 4.8). Biotite occurs in small quantities in many plutonic rocks i.e. in rocks that crystallized slowly from magma at some depth below the surface of the Earth. Its composition shows that (OH)-groups are essential for its formation i.e. the magma must be hydrous. The amount of H,O that can be dissolved in magma depends to some extent on the confining pressure. Magma at the surface of the Eath (= lava) cannot contain much water in solution. Hydrous minerals are therefore not common in volcanic rocks but can form in plutonic rocks. This explains why, for example, micas are common in granite but rare in its volcanic equivalent called rhyolite. Download free books at BookBooN.com 60 Minerals and Rocks Systematic Mineralogy Biotite is also very common in many metamorphic rock types and is a major component of mica schists. Books of biotite are common in granitic pegmatites. It is named after a French physicist, J. B. Biot. 4.1.5.5 Chlorite The chlorite group of minerals has the composition (Mg,Fe, Als [(Si, Al)O1](OH)s. Like other layer silicates, chlorite has a perfect basal cleavage and is fairly soft (H = 2 - 2.5). Chlorites are usually green (it is named from the Greek chloros meaning green) and are formed by the alteration of other silicate minerals that contain Mg and Fe (e.g. olivine, augite, hornblende, biotite). These mineral reactions, which take place at temperatures in the range -100-500ºC, require the presence of a hydrous phase. Chlorite is a widespread “late-stage” mineral. For example, it commonly fills “holes” in volcanic rocks called vesicles that are formed as a result of the escape of a gas phase from the magma at low pressure. Itis a common vein-filling mineral in many rock types. It is a major component of greenschists (basalts metamorphosed at 300-500ºC), together with, amongst other minerals, epidote. 4.1.6 Tectosilicates The silicate structures in this group are based on a 3-dimensional framework of SiO,-tetrahedra in which all the four comer O” anions are shared with neighbowring tetrahedra. When all tetrahedra have Si at their centres, the O? anions are all valency-satisfied and the SiO, unit is therefore electrically neutral. SiO, is, of course, the composition of quartz. No other compositions would be possible if it were not for the fact that some of'the Si cations can be replaced by Al”. This gives rise to a wide variety of minerals, including most importantly the feldspars. Framework silicates (mostly feldspars and quartz) make up about 64% of the continental crust and are therefore very important minerals in geology. The proportions of quartz and feldspar are used to classify most igneous rocks, as we will see later. 4.1.6.1 Quartz Quartz, SiO», is trigonal and forms prismatic 6-sided crystals (Picture 1.1). It defines hardness = 7 on Mohs” scale and G = 2.65. It has a vitreous (glassy) lustre. Colourless crystals are the most usual but many coloured varieties occur. Quartz has a conchoidal fracture i.e. curved fracture surfaces. The names commonly given to some of the coloured, coarsely crystalline varieties, have been mentioned in section 2.2.4. Quartz also occurs in microcrystalline varieties that appear to be amorphous. Their crystalline nature is only revealed by powerful microscopes or X-ray studies. The general term for microcrystalline varieties of quartz is chalcedony. It is commonly deposited from aqueous solutions and is frequently found lining or filling cavities in rocks. Colour and banding give many varieties: carnelian - Ted chalcedony chrysoprase - green chalcedony agate - layered with different colours. Many agates sold commercially have been aitificially coloured. Moss agate has moss-like pattems. onyx - alayered variety in which the layers are planar and parallel flint and chert - greyto black compact varieties Download free books at BookBooN.com 61 Please click the advert Minerals and Rocks Systematic Mineralogy fossilised wood - has commonly been silicified (replaced by microcrystalline quartz) opal - hasthe composition SiO,.nH,O and is of the few amorphous minerals Quartz is in fact only one of several polymorphs of SiO,. Two naturally occurring high temperature polymorphs are tridymite and cristobalite. Two high pressure polymorphs are coesite and stishovite. Coesite is formed from quartz by, for example, meteorite impact. Stishovite has been formed as a result of the extremely high local pressure produced by underground atomic explosions. Quartz is an extremely widespread mineral in the continental crust. It is the main component of yellow beach sand. It is an essential component of many igneous and metamorphic rocks. Under their breakdown by weathering processes, quartz survives and is therefore a major component of many sedimentary deposits. 4.1.6.2 Feldspars The feldspars form an extremely important group of minerals. Their compositions can be expressed in terms of three end members involving the cations K*, Na'and Ca”: K[AIS,O] | - — ORTHOCLASE (Or) Na[AISi,Os] - ALBITE (Ab) Ca[ALSbOs] - — ANORTHITE (An) Download free books at BookBooN.com 62 Please click the advert Minerals and Rocks Systematic Mineralogy Na-rich feldspar Homo- geneous (albite)(B) E alkali E feldspar K-rich feldspar | É a (orthoclase(C) HIGH LOW TEMPERATURE TEMPERATURE Fig. 4.13: Development of perthite in alkali feldspars. Homogeneous alkali feldspar at high temperature (A in Fig. 4.10A) splits into two phases during cooling (B and Cin Fig. 4.10B and Fig. 12). In Paris or Online International programs taught by professors and professionals from all over the world BBA in Global Business MBA in International Management / International Marketing DBA in International Business / International Management Main International Education Main Cross-Cultural Communication Main Foreign Languages Innovative — Practical — Flexible — Affordable it: www.HorizonsUniversity.org Admissionshorizonsuniversityorg Call: 01.42.77.20.66 www.HorizonsUniversity.org Download free books at BookBooN.com 65 Minerals and Rocks Systematic Mineralogy Picture 4.9: Alkali feldspar with well-developed perthite structure. The pinkish host phase is rich in orthoclase (C in Figs. 4.12 & 4.13) whereas the whitish veins are rich in albite (B in Figs. 412. & 4.13). Orthoclase and microcline have perfect (001) and good prismatic (010) cleavage, and readily form roughly 1ectangular-shaped cleavage fragments. They define hardness = 6 on Mohs scale and have G 2.57. Their colour is usually white to pale yellow or grey. Pink to red varieties are due to the presence of minute flakes of hematite (Fe,O;). Green microcline is known as amazonite. Twinning is frequently developed; the most common type is called Cailsbad twinning. Microcline and/or orthoclase are essential components of many igneous rocks such as the plutonic rock types granite and syenite. Their volcanic equivalents (rhyolite and trachyte respectively) contain the high temperature polymorph sanidine. Microcline and/or orthoclase are also important in many metamorphic 10cks, particularly in gneisses. K-feldspar is widely used as a component in the manufacture of ceramics. 4.1.6.2.2 Plagioclase feldspar There is complete solid solution in the plagioclase series, like in olivine (section 4.1.1.1). Anorthite is the high-temperature end member (melts at -1560ºC) and albite the low-temperature (melts at 1118ºC) onein a cigar-shaped phase diagram. Individual plagioclase feldspars are given specific names (Fig. 4.10). Expressed in terms of the % anorthite (An) end member these are: Ano.10 ALBITE Anio-30 OLIGOCLASE AM30.50 ANDESINE Ansomo LABRADORITE Anjo.90 BYTOWNITE Anoo-100 ANORTHITE Download free books at BookBooN.com 66 Minerals and Rocks Systematic Mineralogy One of the consequences of fractional crystallization (i.e. when crystals and melt do not keep in equilibrium during the crystallization of magma) is that zoning can develop. This is uncommon in olivines but is very common in plagioclase feldspars. This is because when plagioclase changes composition during reaction with the melt it requires a coupled reaction involving Na! + Si” = Ca? + AI”. Not only does some calcium become replaced by sodium in a site with coordination number 6-8, but in order to maintain electronic neutrality, some aluminium has to be replaced by silicon in the tetrahedral site (CN =4). This is a slow process, and often results in incomplete reaction so that compositional zoning of plagioclase crystals results with Ca-rich cores and Na-rich margins. All members of the plagioclase series are triclinic (Fig. 4.14). Crystals are commonly tabular parallel to (010). Repeated twinning (also called multiple or polysynthetic twinning) parallel with (010) is extremely common and is sometimes visible in hand specimen. The hardness of plagioclase feldspars is close to 6; the density increases with Ca-content from 2.62 for albite to 2.76 gm/cm? for anorthite. Plagioclases can be colowless, white or grey. A beautiful play of colours (called labradorescence) is seen in some plagioclase crystals (Picture 4.10). Fig. 4.14: Illustration of a perfect crystal of albite. All members of the plagioclase series are triclinic. Plagioclase feldspars are even more widely distributed than alkali feldspars. The classification of igneous rocks isto a large extent based on the proportions of plagioclase to alkali feldspar. Amongst volcanic rocks plagioclase is essential in, for example, basalt that is the most common rock type of all (ocean floors are formed of basalt, usually below a thin layer of sediments). Plagioclase is therefore also a major component of the plutonic equivalent of basalt, called gabbro. The composition of plagioclase in igneous rocks varies with the temperature of formation. n keeping with the phase diagram, Ca-rich plagioclases form at higher temperatures than Na-rich ones. For example, gabbros typically contain labradorite-andesine whereas granites typically contain oligoclase. Plagioclase feldspars are also important in many metamorphic rocks. Download free books at BookBooN.com 67 Minerals and Rocks Systematic Mineralogy 4.2.1 Native elements Gold, silver, copper and platinum all occur naturally in the form of native metal elements, but most of them are rare! Native non-metallic minerals include the two polymorphs of carbon, diamond and graphite (section 2.1.1), and the element sulphur. Native sulphur forms soft (H = 2), yellow, orthorhombic crystals where volcanic gases have been active. 4.2.2 Sulphides The sulphides form an important group of minerals that include the majority of the ore minerals. They (nearly) all have metallic lustre and high densities. 4.2.2.1 Galena Galena (PbS) forms cubic crystals, has perfect cubic cleavage, is fairly soft (H = 2.5) and has high G (7.5). Its colour and streak are dark grey. Galena often contains some silver in its structure and is an important ore for both Pb and Ag. 4.2.2.2 Sphalerite Sphalerite (also known as zinc blende) (ZnS) is cubic and forms tetrahedral crystals with perfect cleavage. H =3.54 and G 4. Its lustre is non-metallic (it is sometimes adamantine) and it is commonly yellowish brown to black. It is often found together with galena. 4.2.2.3 Pyrite Pyrite (FeS,) is cubic and generally forms cubes or 12-sided crystals called pyritohedra (Fig. 3.19). Cube faces are commonly striated (Fig. 4.15). Cubes can grow together in an interpenetrating fashion. It is quite hard for a sulphide mineral (H = 6-6.5). G=5. It has a very metallic lustre and is pale brassy yellow. The streak is black. It is sometimes referred to as “fools gold” (section 2.2.5). It is the most widespread sulphide mineral and occurs as an accessory mineral in many rock types. Fig. 4.15: Pyrite crystals commontly form perfect cubes. The cube-faces may show characteristic striations. The striations on opposite faces have identical orientations. Download free books at BookBooN.com Minerals and Rocks Systematic Mineralogy 4.2.2.4 Chalcopyrite Chalcopyrite (CuFeS,) is tetragonal but usually occurs in a massive form.H=3.54,G=4.2.Ithasa metallic lustre and is brass-yellow, often with a tarnished appearance. It has a greenish-black streak. It is one of the most important ores of copper. Like pyrite it is sometimes called “fools gold”. 4.2.3 Oxides 4.2.3.1 Corundum Corundum (ALO:) is hexagonal and defines H =9 on Mohs” scale (Picture 4.11). Its colour varies widely. Gem varieties include ruby (red) and sapphire (blue). Powdered corundum is used as an abrasive. It occurs in silica-poor igneous and metamorphic rocks. Picture 4.11. Corundum forms 6-sided (hexagonal) crystals. The reddish variety illustrated here is used commercially as ruby. 4.2.3.2 Hematite Hematite (Fe,O3) is hexagonal and typically forms thin tabular crystals. Two common habits of hematite are kidney-like (reniform) (Picture 4.12) and micaceous or platy (specular). It is reddish-brown to black with H =-S and G = 5.26. Crystals have a metallic luster, but other varieties may be dull. It has a reddish-brown streak. Hematite is formed by the oxidation of other Fe-bearing minerals (note that hematite contains exclusively Fe”) and commonly imparts a reddish colowing to rocks. It is the most important iron ore for the manufacture of steel. Download free books at BookBooN.com 71 Please click the advert Minerals and Rocks Systematic Mineralogy Picture 4.12. Hematite sometimes forms kidney-shaped aggregates. fp graf EL E pa ETR e Rega ERR «perience, expertise, and creativity, Wé Edo] Knowledge Engineering DARREN AE TEAR LATER as a Download free books at BookBooN.com Minerals and Rocks Systematic Mineralogy 4.2.5 Carbonates 4.2.5.1 Calcite Calcite (CaCO:) is a trigonal mineral with a wide range of forms (more than 300 different varieties have been described). Rhombohedral cleavage is perfect, as illustrated in Fig. 3.25. Calcite definesH=3; 6G=2.71. Calcite is usually white, but many coloured varieties occur. It is readily identified by its crystal form, hardness, birefringence and reaction with dilute HCI (section 2.2.9.). Calcite is an extremely widespread mineral. It is the main component of limestone. Metamorphosed limestone (marble) is also composed of finely crystalline calcite. It occurs in many igneous and metamorphic rocks as an accessory mineral, and is found together with many ore minerals. The vast majority of igneous rocks are dominated by silicate minerals, but one very rare type is dominated by carbonates - these rocks are known as carbonatites. A very important property of calcite is its very high double refraction (birefringence) (Picture 4.14). The minerals magnesite (MgCO;) and siderite (FeCO;) are related to calcite. Picture 4.14. Calcite has extremely high birefringence. This is evident here in that a single line appears to be double when viewed through a transparent calcite crystal. 4.2.5.2 Aragonite Calcite is in fact one of two minerals with the composition CaCO;. The other polymorph is aragonite which is orthorhombic. The shells of molluscs are largely formed of aragonite even though calcite is the stable phase at low temperatures and pressures. 4.2.5.3 Dolomite Dolomite CaMg(CO;), is compositionally related to calcite and is also trigonal. WithH =3.5 -4itis somewhat harder than calcite and only reacts very slowly with cold HCI. It occurs in sedimentary rocks, paiticularly in dolomitic limestones. The metamorphic equivalents of these can contain a wide variety of Mg- and/or Ca-bearing minerals like forsterite, diopside, tremolite and talc. Download free books at BookBooN.com 75 Minerals and Rocks Systematic Mineralogy 4.2.6 Sulphates 4.2.6.1 Gypsum Gypsum (CaSO,.2H,0) forms monoclinic crystals which are usually tabular on (010). Swallowtail twins are common, as illustrated in Fig. 2.2a. It has a perfect (010) cleavage and defines H = 2. G -2.3. Gypsumis colowless, white or grey; it can be transparent. Crystalline gypsum is also known as selenite. Satin spar is a fibrous variety; alabaster is a fine-grained massive variety. Gypsum is a widely distributed mineral in sedimentary rocks and is formed by the evaporation of seawater, usually together with halite - rock salt. The attractive aggregates known as “desert roses” are composed of gypsum. Gypsum also occurs together with many ore minerals in veins. 4.2.6.2 Barytes Barytes (BaSO,) usually forms colouless or white tabular oithorhombic crystals with perfect (001) cleavage. It has a notably high density for a non-metallic mineral (G = 4.5) and H =3 - 3.5. Barytes occurs together with a variety of ore minerals in veins. 4.2.7 Phosphates 4.2.7.1 Apatite Apatite (Cas(PO,)s(F,CLOH)) is a hexagonal mineral which commonly forms long prismatic crystals. It is usually greenish in colour. It has hardness = 5 and can just be scratched with a knife. G -3.18. Apatite is a very widely distributed accessory mineral but also occurs in large deposits, sometimes in pegmatites where it can form large green hexagons. Download free books at BookBooN.com 76 Minerals and Rocks Igneous rocks 5. Igneous rocks Petrology is the study of rocks that are naturally occurring aggregates of minerals. As we have seen, there are three main types of rocks: igneous, sedimentary and metamorphic. Here we are concerned with the study of igneous rocks. 5.1 Classification of igneous rocks Igneous rocks can be classified according to many criteria, such as texture, crystal or grain size, colour, mineralogy, chemical composition, mode of occurrence, and genesis. Since it must be possible to give a rock a name in the field, a first order classification cannot be based on chemical composition since this first requires chemical analysis. Genetic classifications should be avoided as far as possible, but this is not always easy. For example, the main subdivision of rocks into sedimentary, igneous and metamorphic is genetic but cannot be avoided. Igneous rocks are classified by their content of essential minerals ie. those that make up the bulk of the rock - this is known as the mode of the rock. Minor amounts of accessory minerals are not considered. As we will see later, this is fairly straightforward for coarse grained rocks but can be very difficult for very fine grained ones - and impossible for glassy rocks. Two terms used in connection with grain size are: phaneritic: individual crystals can be distinguished with the naked eye. aphanitic: most of the individual crystals cannot be distinguished with the naked eye. Igneous rocks are divided into two main groups on the basis of their field relations or on their grain size. PLUTONTC - crystallized at depth. Phaneritic. Average crystal or grain size > Smm (coarse); 1 - 5mm (medium); 0.5 - Imm (fine grained). VOLCANTIC - extruded at the surface of the Earth. Aphanitic. Grain size < 0.5mm (very fine grained). Itis not unusual for igneous rocks to have large crystals of one or more mineral(s) in a finer grained groundmass (also called the matrix). m this case it is the grain size of the matrix that is used. For example, a 10ck with a black, fine grained matrix may contain cm-sized pyroxene crystals. The rock was extruded from a volcano as lava and is volcanic, despite the presence of some large crystals which crystallized in a magma chamber before eruption. Large crystals in a finer grained matrix are called phenocrysts. The presence of phenocrysts in a finer grained matrix is referred to as a porphyritic texture. Download free books at BookBooN.com 77 Minerals and Rocks Igneous rocks The following notes refer to the numbered fields in Fig. 5.1. The most important names are in italics. 10. 11. 12. 13. 14. . Rocks with > 60% foid minerals (amongst the light minerals) are rare and are called fodolites (e.g. Igneous rocks with > 60% quartz amongst the light minerals are very rare. Rocks with > 90% are quartzolites. With 60-90% quartz the term quartz-rich is used as a prefix to the appropriate name from field 2, 3, 4 or 5. e.g. quartz-rich granodiorite. Alkali feldspar granites are, as the name implies, granitic rocks which are rich in alkali feldspar and poor in plagioclase. The granite field is the largest one in the QAP triangle and granites are very common in the continental crust. Note that granites can contain 20-60% quartz (ignoring dark minerals) and that the 1atio of alkali feldspar to plagioclase can vary from 9:1 to —-1:2 (actually 35:65). Fields 2, 3 and 4 are all “granitic rocks”. The term “granite” sensu strictu is exclusively used for rocks that lie in field 3. Granodiorite is the name given to rocks that are more plagioclase-rich than true granites. They can be considered as having a composition essentially intermediate between granites (field 3) and diorites (field 10) Rocks dominated by quartz and plagioclase are tonalites. Rocks that are dominantly composed of alkali feldspar are alkali feldspar syenites. If there is 5-20% quartz the term quartz alkali feldspar syenite is used. With up to 10% feldspathoid mineral (e.g. nepheline) the rock is e.g. a nepheline-bearing alkali-feldspar syenite. Syenite (see point 6 for quartz syenite and nepheline-bearing syenite). Monzonite (see point 6 for quartz and nepheline-bearing types). Monzodiorite or monzogabbro (see point 6 for quartz and nepheline-bearing types). For distinction between monzodiorite and monzogabbro see point 10. This field is used for three rocktypes - diorite, gabbro and anorthosite. Anorthosites consist of > 90% plagioclase. Distinction between diorite and gabbro is usually based on the composition of the plagioclase in the rock. Gabbro contains plagioclase with An-so; diorite has An.so. This is, of course, difficult to determine in hand specimen! Diorites are usually more felsic than gabbros. Diorites typically contain andesine (plagioclase in the range Anzo.s0) + hornblende + biotite + clinopyroxene. Gabbros contain, for example, labradorite (plagioclase Anso.79) + clinopyroxene + orthopyroxene + olivine. Gabbroic rocks are further classified according to their dark minerals (section 5.1.1.1.1). Rocks in this field are foid syenites. Ifthe foid (= feldspathoid) mineral is, for example, nepheline, the rock is called a nepheline syenite. Foid monzosyenite e.g. nepheline monzosyenite. Foid monzodiorite or foid monzogabbro. e.g. nepheline monzodiorite. Foid gabbro or foid diorite e.g. nepheline gabbro. nephelinolite). The term nephelinite seems more obvious but is used for equivalent volcanic rocks. Not all these rock types are equally common. The most important types are rocks of field 10 (particularly gabbros and diorites), granitic rocks (fields 2, 3 and 4) and various syenitic rocks (fields 6, 7 and 11). The intermediate 1ock types (monzo-) and F-rich types occur less frequently. Quartz-bearing rocks lie in the upper (QAP) triangle and are silica-oversaturated. Rocks that lie on the AP boundary are silica-saturated. Those that lie in the APF triangle contain a foid mineral and are silica- undersaturated. Download free books at BookBooN.com so Please click the advert Minerals and Rocks Igneous rocks 5.1.1.1.1 Gabbroic rocks Gabbroic rocks, which lie in field 10 in the QAPF classification, consist dominantly of plagioclase, clinopyroxene (typically augite), olivine and orthopyroxene (commonly hypersthene). Some special names are used for rocks belonging to the gabbro family (Table 5.1). Main minerals Rock name clinopyroxene + plagioclase gabbro orthopyroxene + plagioclase norite olivine + plagioclase troctolite Table 5.1. Rocks belonging to the gabbro family Intermediate types are quite common. For example, a rock consisting of clinopyroxene + oithopyroxene + plagioclase is called a gabbronorite. One consisting of olivine + clinopyroxene + plagioclase is an olivine gabbro, and so on. Prefixes are commonly used depending on the colour index (M). These are mela- (short for “melanocratic”) when M is in the range 65-90 e.g. melatroctolite; meso- when M is 35-65 e.g. mesonorite; and leuco- when M is 10-35 e.g. leucogabbro. Today's job market values ambitious, innovative, perceptive team players. Swedish universities foster these qualities through a forward-thinking culture where you're o the latest ideas and global trends. N E studying in Sweden will RETO Download free books at BookBooN.com 81 Minerals and Rocks Igneous rocks The “meso-” prefix is not often used since it refers to the “normal” modal composition. Note that if M > 90 the 10ck classifies as ultramafic, and if M < 10 in field 10 (i.e. plagioclase > 90%) the rock is an anorthosite. The prefixes mela- and leuco- are used for all rock types which are relatively enriched in dark or light minerals respectively. e.g. melasyenite, leucogranite. 5.1.1.1,2 Charnockitic rocks A special nomenclature is used for plutonic rocks that contain orthopyroxene. This is because these rocks have a special mode of occurrence and tend to be found related to one another. We have already encountered one rock type that belongs to the charnockitic family - norite. The term charnockite is used for hypersthene granite. Other chamockitic rocks types include mangerite (= hypersthene monzonite) and jotunite (= hypersthene monzonorite). Charnockitic rocks typically occur associated with large bodies of anorthosite in which the dominant mafic mineral in orthopyroxene. 5.1.1.2 M > 90% (Ultramafic rocks) Ultramafic rocks are usually dominated by one or more of the minerals olivine, orthopyroxene and clinopyroxene (Fig. 5.2). Ultramafic rocks with > 40% olivine are peridotites. Various types of peridotite are dunite (> 90% olivine), harzburgite (olivine + orthopyroxene), wehrlite (olivine and clinopyroxene), and lhercolite (olivine +two pyroxenes). Rocks with > 60% pyroxene are pyroxenites. Pyroxenites are divided into 6 types: orthopyroxenite (> 90% orthopyroxene), clinopyroxenite (> 90% clinopyroxene), websterite (two pyroxenes), olivine orthopyroxenite, olivine clinopyroxenite and olivine websterite. Common accessory minerals in ultramafic rocks are garnet or spinel. OLIVINE Wentite PERIDOTITES Olivina Olivine orthopyroxenise x prox ENITES orTHO N onte PYROXENE Orio- Clino. pyroxenite tiweaite Fig. 5.2: Classification of ultramafic rocks composed of olivine, orthopyroxene and clinopyroxene. Ultramafic rocks are defined as containing more than 90 vol. % dark (mafic) minerals. Download free books at BookBooN.com 82 Minerals and Rocks Igneous rocks 5.1.2 Volcanic rocks Volcanic rocks have, by definition, an aphanitic groundmass but their classification is more complicated than that of plutonic rocks. As we have seen, the latter are classified according to the minerals present. This is often difficult or impossible to determine for volcanic rocks. When it is possible a QAPF double triangle is used, but in many cases (e.g. when a rock is extremely fine grained) a chemical analysis is required and the so-called TAS classification is used (TA = total alkalies NO +K,0; S = silica). Many volcanic rocks are formed by explosive activity, resulting in the formation of fragmentary rocks - the so-called pyroclastic rocks (pyro = heat; clast = fragment). This is also taken into consideration in the naming of volcanic rocks. Ultramafic volcanic rocks are extremely uncommon. Ultramafic plutonic rocks are formed by the separation of mafic minerals (commonly olivine and/or pyroxenes) from magmas. The magma itself is not ultramafic. 5.1.2.1 QAPF double triangle for volcanic rocks This classification (Fig. 5.3) can only be used if the mineral mode can be estimated. In many cases it is based exclusively on the phenocryst phases present. The numbers of the fields are the same as those for the plutonic rock classification (Fig. 5.1). thyolite alkali feldspar trachyte NX N basanite/ tephrite = phenolitic basanite/teprite phonolitic — = tephritio fordite foidite Fig. 5.3: The QAPF double triangle for the classification of volcanic rocks. This classification cannot be used if the rock is too fine grained to allow mineral identification. It may be based on phenocrysts (if present). When this classification cannot be used it is necessary to use the TAS diagram (Fig. 5.4). Download free books at BookBooN.com 85 Minerals and Rocks Igneous rocks Comments on the individual fields in Fig. 5.3 are given below. The most important rock names are in italics. 1. 2. 3. There are no volcanic rocks in this field. The term alkali feldspar rhyolite corresponds to alkali feldspar granite. Rhyolite is the volcanic equivalent of granite. 4&5. Fields 4 and 5 are covered by dacite which is the volcanic equivalent of both granodiorite and 7. 8. tonalite. Alkali feldspar trachyte. With minor (5-20%) quartz the term quartz alkali feldspar trachyte is used; with minor foid (0-10%) the term e.g. nepheline-bearing alkali feldspar trachyte is used. Trachyte is the volcanic equivalent of syenite. See point 6 for quaitz- and foid-bearing varieties. Latite is the volcanic equivalent of monzonite. See point 6 for subtypes. 9 & 10. These fields contain the large majority of volcanic rocks - basalts and andesites. These are 1. 12. 13. 14. broadly the volcanic equivalents of gabbros and diorites respectively. Distinction can be made using the colour index (basalt has M > 35; andesite M < 35). The plagioclase composition is difficult to apply because andesites commonly contain phenocrysts of calcic plagioclase (labradorite or bytownite) and compositional zoning is commonly developed in the plagioclase in volcanic rocks. The TAS classification is very widely used for dark volcanic rocks (which includes basalts and andesites). Phonolites ave the volcanic equivalents of foid syenites e.g. nepheline syenites. In volcanic rocks leucite (K[AlSi,06]) may be an important foid mineral. Tepluitic phonolites are relatively rare. Phonolitic basanites and phonolitic tephrites are relatively rare. For distinction between them see point 14. Basanites and tephrites belong to this field. They are separated by the amount of olivine present. Basanite has > 10% olivine; tephrite has < 10% olivine. Because these rocks are very fine grained it is not possible to determine the amount of olivine optically (even with a microscope) and the amount of olivine is estimated on the basis of the chemical composition of the rock which is recalculated in terms of simple mineral proportions - this is called the normative composition of a rock (hypothetical mineral composition of a rock calculated from its chemical composition) and is not to be confused with its modal composition (volume % minerals actually present). This field is subdivided into three subfields: 15a: Phonolitic foidite e.g. phonolitic nephelinite 15b: Tephritic foidite e.g. tephritic leucitite 15c: Foidite e.g. nephelinite Volcanic rocks commonly have holes formed by escaping expanding gas during solidification. These are called vesicles and the rock is vesicular. Vesicles are commonly filled by later material; they ave then referred to as amy gdales and the rock is amygdaloidal. Download free books at BookBooN.com 86 Minerals and Rocks Igneous rocks Some special terms are used for glassy rocks: OBSIDIAN | Massive glass. Commonly black. The most common type of obsidian is thyolitic. PERLITE Peaily grey glass with concentric curved fractures called a perlitic texture. PUMICE Lightweight, highly vesicular, frothy, glassy rock. The most common types of pumice are rhyolitic or phonolitic. Pumice can usually float on water. 5.1.2.2 Chemical classification using the TAS diagram Volcanic rocks show a great range of variation in chemical compositions. This variation is, however, far from random and shows a series of rational pattems. Two of the most important and useful chemical parameters in volcanic rocks are SiO; and Na,O + K,0O (total alkalies). The vast majority of volcanic rocks have between 35 and 75% SiO, and 1-16% total alkalies. It is clear that a fine grained basalt consisting essentially of augite (Ca(Mg,Fe)[Si,0s]), calcic plagioclase (Ca[ Al,Si,0s] — Na[AlSi;0s]) and magnesian olivine ((Mg,Fe)[SiO,]) will be relatively poor in SiO, and Na,O + K,0. A fine grained rhyolite, on the other hand, consisting essentially of quartz (SiO,), potassium feldspar (K[AlSi,0s]) and sodic plagioclase (Na[AlSi:0s] — Ca[ Al,Si,Os]) will be relatively enriched in SiO,, K,O and Na,0. The chemical compositions of rocks are therefore closely related to their mineralogy. The TAS system in Fig. 5.4 is widely used for the classification of volcanic rocks when a mineralogical classification is difficult to apply. mn addition to volcanic rock names, most of which are also used in Fig. 5.3, some important terms used in the description of volcanic rocks are shown in Fig. 5.4. These are based solely on the SiOrcontent of the rock. This is the total amount of the chemical component silicon dioxide (silica) and is NOT the amount of quaitz that is a mineral with the composition SiO,. A basalt does not contain quartz but has 45-52% SiO, (which is mainly present in the silicate minerals augite, olivine and plagioclase or in basaltic glass). Volcanic rocks are divided into four groups on the basis of their SiO,-content (Table 5.2). WSIO, <45 ultrabasic 45-52 basic 52-63 intermediate >63 acid Table 5.2. Subdivision of igneous rocks on the basis of their SiO;-content. Note in Fig. 5.4 that basalts are basic, andesites are intermediate and rhyolites are acidic. The terms acid, intermediate, basic and ultrabasic are used for plutonic and some metamorphic rocks as well as volcanics. Gabbros are therefore basic, granites are acidic. Download free books at BookBooN.com 87 Minerals and Rocks Igneous rocks 5.1.2.3 Volcaniclastic rocks During the eruption of a volcano lava may be extruded, gas is vented, and fragments of volcanic rocks are ejected into the air (or into water if the eruption takes place subaqueously). The lavas are igneous rocks and have been considered in sections 5.1.2.1. and 5.1.2.2. The gas is dispersed and leaves no record. The ejected material, collectively known as tephra, will be deposited around the volcano as a subaerial or subaqueous deposit. Cleaily such deposits are intermediate between true igneous and true sedimentary rocks. Sediments composed of fragments of volcanic material are called volcaniclastic deposits. The size of volcanic fragments is fundamental (Table 5.3). <2mm 2-64mm > 64mm volcanic ash lapilli block, bomb sedimentary clay, silt, sand granule, pebble cobble, boulder Table 5.3. Names of volcaniclastic rocks are based on particle size. Comparison is made with sedimentary deposits. Blocks are angular clasts (fragments) produced by the breaking-up of solid rock. Bombs are fragments that were partly molten when they were ejected and were streamlined into smooth forms as they solidified during flight through the air. Volcanic fragments may also be classified according to their composition: vitric - glassy lithic - rock fragments crystalline - consists of crystals Ash-sized clasts are usually vitric (glass shards) or crystal, whereas blocks are usually lithic. Clasts of volcanic origin may still be extremely hot when they are deposited so that they become welded together. Ignimbrites ("fiery cloud" rocks) are deposited from one of the most violent types of volcanic eruption - pyroclastic flows. These fragmental volcanic rocks are usually SiO,-rich (dacitic or thyolitic) and commonty contain flattened glassy fragments (called "flames"). Ignimbrites are commonly welded. Volcanic deposits that have become consolidated are classified according to the size of the fragments. ash tuff lapilli tur volcanic breccia rock composed entirely of ash-sized fragments rock composed entirely of lapilli-sized fragments rock composed entirely of blocks Intermediate types occur. Agglomerate is a term sometimes used for unsorted volcaniclastic deposits with a mixture of blocks, bombs, lapilli and ash that accumulated near the vocanic vent. Download free books at BookBooN.com 90 Minerals and Rocks Igneous rocks It is common practice to combine the different type of terms e.g. lithic lapilli tuff, vitric tuff breccia, crystal tuff, andesite breccia, rhyolitic pumice etc. 5.1.3 Mineral assemblages As we have seen above, most igneous rocks are classified according to their mineralogy. Rocks that consist entirely of glass cannot, of course, be classified on a mineralogical basis. Ultramafic rocks are classified using mafic minerals whereas the QAPF double triangle uses felsic minerals. Apart from ultramafic rocks, plutonic and volcanic rocks only differ in terms of grain size and texture; their mineralogical variation is essentially similar. The relationship between mineral assemblage (i.e. all the important minerals in a rock) and rock composition is illustrated in Fig. 5.5. basalt andesite dacite rhyolite jvoLcanic peridotite | gabbro | diorite |granodiorite| granite [ruronic — Colour of rock becomes increasingly dark €— —» — Increasing SiO; % —> Fig. 5.5: Variations in mineral assemblages in plutonic and volcanic rocks. This diagram only applies to rocks in the QAP part of the QAPF double triangle. Several main features of Fig. 5.5. are: a) The names of equivalent plutonic and volcanic rocks are: volcanic basalt andesite dacite rhyolite plutonic peridotite gabbro diorite granodiorite granite Note that there is no volcanic equivalent to peridotite. Such a rock does exist (it is called komatiite) but it is so rare that we will not consider it further in this introductory text. a) The SiO, content increases from left to right from < 45% for the ultramafic (and ultrabasic) peridotite to > 63% for the felsic, acidic granite/rhyolite. b) The colour index (Y% mafic minerals, M) decreases from left to right. c) Peridotite consists of > 90% olivine + pyroxene and up to 10% Ca-rich plagioclase feldspar. d) Gabbros/basalts consist dominantly of Ca-rich plagioclase and pyroxene + olivine. e) Diorites/andesites are dominated by plagioclase and hornblende + pyroxene. f) Granodiorites/dacites contain quartz, Na-rich plagioclase and hornblende + K-feldspar + biotite. Download free books at BookBooN.com 91 Please click the advert Minerals and Rocks Igneous rocks g) Granites/thyolites are dominated by quartz and K-feldspar with Na-rich plagioclase and minor mica (biotite + muscovite) and hornblende. Micas are, however, very rare in volcanic rocks. h) Plagioclase becomes progressively poorer in the anorthite (Ca[Al,Si,0s]) component and richer in the albite (Na[AlSi;0s]) component from left to right. i) All the mafic minerals (olivine, pyroxene, amphibole, biotite) become progressively more Fe-rich and Mg-poor from left to right. This systematic relationship is not coincidental. Basaltic magma, which is by far the most common, changes in composition during crystallization in magma chambezs. This process, which is called fractional crystallization, will be dealt with later. 5.2 Magma All igneous rocks are formed by the solidification of molten rock matter - magma. Magmas are generated by the partial melting of deep crustal or upper mantle rocks. Magma reaches the surface of the Earth through, for example, a volcano. Molten rock coming, out of a volcano is called lava. Magmas that reach the surface form volcanic (or extrusive) rocks; those which crystallize at depth are known as plutonic (or intrusive) rocks. f'calendar www.1cale: Download free books at BookBooN.com 92 Please click the advert Minerals and Rocks Igneous rocks Oceanic MA crust Es AT >| EM Tu es Conti | I aí Es E = ontinenta! ol o E 26 crust I EE Se EB E 8 5 = Magma Ú Asthenosphere Asthenosphere E Partial melting Fig. 5.8: Subduction zone at the margin between oceanic and continental lithospheric plates. Basalt formed at mid-oceanic ridges becomes metamorphosed by circulating seawater. Hydrous minerals like chlorite, hornblende and serpentine form. During subduction these minerals release water (and turn into other, dry minerals). The water enters the overlying mantle wedge. The solidus temperature of the peridotite is lowered by the water, and it starts to melt. Design your o LoR LILA ÇE DE TES] m MAN Diesel — Powering the World Download free books at BookBooN.com 95 Minerals and Rocks Igneous rocks Temperature ("C) 500 1000 1500 T Al T A Crust 4 1 Melting 20/- GN t t curve for a 1 deybasah 1yy 0 NI 1 Moho = E à 1 g = f g E í y Am â a N Mantle 5 sof- V Melting À 1 i00]- — cuvefor 4 1 E wetbasalt 4 y 1 h 1 1 Fig. 5.9: Pressure — temperature diagram showing the influence of water on the melting curve (solidus) for basalt and the geothermal gradient. The presence of water lowers the solidus temperature of rocks. 5.2.2 The composition of magma As we have seen when dealing with the classification of igneous rocks, three common types of volcanic rock are basalt, andesite, and thyolite. Dacite has a composition intermediate between andesite and rhyolite. These are formed from basaltic, andesitic, dacitic and ihyolitic magmas. n the TAS diagram (Fig. 5.4) itis apparent that basalt represents a basic (45-52% SiO») magma, andesite is intermediate (52-63% SiO») and dacite and rhyolite are both acidic (> 63% SiO,) magmas. The chemical compositions of magmas (and 10cks) are expressed in terms of major (> 0.1% oxide) and trace (< 0.1% or 1000 ppm = parts per million) elements. Major elements are expressed as % oxide; traces elements as ppm element. Some elements are “major” in one magma/rock and only occur in trace quantities in another (e.g. phosphorus is “major” in basalt, andesite and dacite and “trace” in rhyolite in Table 5.4). Table 5.4 illustrates that magmas and igneous rocks consist mainly of only a few (eight) major oxides (SiO, + ALO; + Fe,O; + FeO + MgO + CaO + Na,O + K,0). The remaining oxides (TiO>, MnO, P,Os and H,0) are usually present in only small amounts. The total should, of course, be close to 100%. This is a check that the analysis is reliable. Values between 98.5 and 100.5 are acceptable. This + is accepted because of analytical error giving some + on all the elements. Some trace elements may be present in higher concentrations than normal, and there are other elements/oxides that can be present that are not normally determined (sulphur, carbon dioxide etc.). Download free books at BookBooN.com 96 Minerals and Rocks Igneous rocks BASALT ANDESITE DACITE RHYOLITE SiO, 49.60 57.94 65.01 72.82 Tio, 1.84 0.95 0.58 0.28 ALOs 15.84 16.67 15.91 13.47 Fe,05 3.79 2.50 2.43 1.48 Feo 713 4.92 2.70 111 MnO 0.20 0.12 0.09 0.06 Mgo 6.99 3.91 1.58 0.39 Cao 9.70 6.78 4.32 1.14 Na,0 2.91 3.54 3.79 3.65 KO 0.51 1.76 217 4.50 P,Os 0.95 0.29 0.15 0.07 Ho 0.35 1.15 1.20 1.10 Total 99.81 99.94 99.93 100.07 MgO/Feo 0.98 0.79 0.59 0.35 CaO/Na,0 3.33 1.92 1.14 0.31 Table 5.4. Chemical compositions of typical basalt, andesite, dacite and rhyolite From basalt to rhyolite there are several important geochemical trends: * SiO, increases e mafic oxides (TiO,, MgO, FeO, Fe,03, MnO) decrease * Cao decreases e | alkalies (Na,O and K,0) increase e PO, decreases e the MgO/Feo ratio decreases e the CaO/Na,0 ratio decreases The mafic oxides are those that enter Mg-Fe minerals like olivine, pyroxenes, amphiboles, magnetite and ilmenite. The MgO/FeoO ratio decreases because early-crystallizing mafic minerals are Mg-rich. The CaO/Na,0 ratio decreases because early-crystallizing plagioclase is Ca-rich and Ca enters eaily-formed clinopyroxene. We will see why these compositions are related in this way later on. 97 Download free books at BookBooN.com
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