Minerals and rocks

Minerals and rocks

(Parte 1 de 4)

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J. Richard Wilson Minerals and Rocks

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Minerals and Rocks © 2010 J. Richard Wilson & Ventus Publishing ApS ISBN 978-87-7681-647-6

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Contents

Contents

Preface 9 1. Introduction 10 1.1 Igneous rocks 1 1.2 Sedimentary rocks 1 1.3 Metamorphic rocks 12 1.4 The Rock Cycle 12

2. Minerals – an Introduction 14 2.1 Definition of a mineral 14 2.1.1 Polymorphs 15 2.2 Properties of minerals 15 2.2.1 Crystal form, growth habit and twinning 16 2.2.2 Cleavage and fracture 18 2.2.3 Lustre 18 2.2.4 Colour 18 2.2.5 Streak 19 2.2.6 Hardness 19 2.2.7 Tenacity 20 2.2.8 Density 20 2.2.9 Other properties 20 what‘s missing in this equation?

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Contents

3. Crystallography 21 3.1 Symmetry 2 3.2 Crystal systems 26 3.3 Crystal classes 31 3.4 Indices of crystal faces 31

4. Systematic Mineralogy 41 4.1 Silicate minerals 41 4.1.1 Nesosilicates 42 4.1.2 Sorosilicates (epidote) 50 4.1.3 Cyclosilicates 51 4.1.4 Inosilicates 53 4.1.5 Phyllosilicates 58 4.1.6 Tectosilicates 61 4.2 Non-silicate minerals 69 4.2.1 Native elements 70 4.2.2 Sulphides 70 4.2.3 Oxides 71 4.2.4 Chlorides and fluorides 73 4.2.5 Carbonates 75 4.2.6 Sulphates 76 4.2.7 Phosphates 76

5. Igneous rocks 7 5.1 Classification of igneous rocks 7

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Contents

5.1.1 Plutonic rocks 78 5.1.2 Volcanic rocks 85 5.1.3 Mineral assemblages 91 5.2 Magma 92 5.2.1 Where does magma come from? 93 5.2.2 The composition of magma 96 5.2.3 Temperature 9 5.2.4 Viscosity 9 5.2.5 Density 100 5.3 Eruption of magma 100 5.3.1 Non-explosive eruptions 100 5.3.2 Explosive eruptions 103 5.4 Volcanoes 105 5.4.1 The shapes of volcanoes 105 5.4.2 Calderas 105 5.5 Plutonic rocks 106 5.5.1 Minor intrusions (dykes and sills) 106 5.5.2 Major intrusions (plutons) 108 5.5.3 Chilled margins 1 5.6 The origin of magma 1 5.6.1 Distribution of volcanoes 1 5.6.2 Origin of basaltic magma 112 5.6.3 Origin of andesitic magma 114 5.6.4 Origin of rhyolitic magma 114 5.6.5 Crystallization of magmas 114

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Contents

5.7 Igneous mineral deposits 122

6. Sedimentary rocks 123 6.1 Introduction 123 6.2 Weathering 123 6.3. Classification of sedimentary rocks 128 6.3.1 Clastic sedimentary rocks 129 6.3.2 Biochemical sedimentary rocks 132 6.3.3 Organic sedimentary rocks 132 6.3.4 Chemical sedimentary rocks 133 6.4 Sedimentary structures 134 6.4.1 Layering (bedding) 134 6.4.2 Surface markings 135 6.4.3 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 7.1 Introduction 142 7.2 Metamorphism – causes and effects 143 7.2.1 Heat 143 7.2.2 Pressure 143 7.2.3 Water 144 7.2.4 Differential stress 144

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7.3 Types of metamorphic rocks 145 7.3.1 Non-foliated metamorphic rocks 146 7.3.2 Foliated metamorphic rocks 146 7.3.3 Types of protolith 149 7.4 Grades of metamorphism 150 7.4.1 The progressive metamorphism of shale 151 7.4.2 Index minerals and metamorphic zones 155 7.4.3 Metamorphic facies 157 7.4.4 Geothermal gradients 158 7.5 Environments of metamorphism 158 7.5.1 Burial metamorphism 158 7.5.2 Blueschist facies and eclogite facies metamorphism 159 7.5.3 Regional metamorphism 160 7.5.4 Thermal metamorphism 161 7.5.5 Dynamic metamorphism 162 7.5.6 Metamorphism at mid-ocean ridges 162 7.6 Where do metamorphic rocks occur? 163

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Preface

Preface

The first version of this text was written to serve as lecture notes for a first term geology course in “Minerals and Rocks” at Aarhus Universityin Denmarkin 2003. In Aarhus this course is accompanied by a general “Introduction to Geology” course that presents, for example, the structure of the Earth, plate tectonics and paleontology. These topics are therefore not treated here, and some knowledge of the Earths structure and plate tectonicsis assumed.

After a brief introduction to the Rock Cycle, this text presents the physical properties of minerals and an introduction to crystallography. The most important rock-forming minerals are then dealt with in a systematic way, followed by the three main rock groups –igneous, sedimentary and metamorphic.

A lecture course in Minerals and Rocks must be accompanied by a parallel sequence of practical classes where, for example, crystal structures, minerals and rock types dealt with in theory are demonstrated in practice.

J. Richard Wilson June 2010

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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 SiO2and 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 FeS2.

Picture 1.2: Pyrite cube.

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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:

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 Earths mantle. Magma formed in the mantle rises towards the surface because it has a lower density than the surrounding mantle rocks. If the magma reaches 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 rises 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 formof 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) becomeconsolidatedand form compact rocks –sedimentary rocks. An example is consolidated clay that is called shale.Other sedimentary rocks form as a result of the precipitationof minerals from water;rock saltis 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.

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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 atemperature in the vicinity of 1200°C. Sedimentaryrocks close to the chamber will be heated more than those further 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 becauseit has been heated as a result of its proximity to an igneous rock -it has become thermallyor 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.

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Introduction

Fig. 1.1: The Rock Cycle

The cycle does not always follow the central circular course. Sedimentary rocks can be weathered and eroded and form new sediments. Metamorphic rocks can likewise form sedimentary rocks. Rocks can be metamorphosed more than once. Igneous rocks can be metamorphosed. While some igneous rocks are formed as a result of the solidification of melts produced by high-grade metamorphism, most are formed as a result of the partial melting of the mantle.

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Minerals – an Introduction

2. Minerals–an Introduction

2.1 Definition of a mineral

Amineralis a naturally formed solid; liquids and gases are therefore excluded. A mineral hasaspecific composition. It may be a naturally occurring element e.g. gold (Au), copper (Cu) or diamond (C). Minerals are, however, usually compounds e.g. quartz (SiO2), pyrite (FeS2) or olivine (Mg,Fe)2[SiO4] in which the ratio of metal (Fe + Mg) to silicate group [SiO4] is 2:1. Minerals possess a characteristic crystal structure and therefore exclude non-crystalline materials such as glass. For example, quartzis a mineral. It is a common naturally formed phase with a simple composition: SiO2. It forms elongate, 6-sided crystals that terminate at a point(Picture 1.1). Quartz is an essential component of, for example, granitewhich is a rock.

Granite consists of several minerals. It is dominated by feldspar and quartz (light coloured minerals) but also contains a minor amount of dark minerals.

ROCKSare composed of MINERALS To learn about rocks we have to start by understanding minerals.

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Minerals – an Introduction

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/cm3

DIAMOND–cubic crystals, hardest known mineral, density 3.51 g/cm3

Diamonds occur in nature in rocks that were formed at extremely high pressure. To convert graphite to diamond in the laboratory requires a pressure of ca. 25 kilobars (25.0 times atmospheric pressure) at low temperature, increasing to 100 kb at about 2500°C (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.

K graphite diamond

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 HCl is a commonly used chemical property.

(Parte 1 de 4)

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