Crystallography, Mineralogy and Economic Geology PDF

BGYCT – 133 Indira Gandhi National Open University CRYSTALLOGRAPHY, School of Sciences MINERALOGY AND ECONOMIC GEOLOGY...

BGYCT – 133 Indira Gandhi National Open University CRYSTALLOGRAPHY, School of Sciences MINERALOGY AND ECONOMIC GEOLOGY Crystallography and Mineralogy Vol. 1  BGYCT – 133 CRYSTALLOGRAPHY, Indira Gandhi National Open University School of Sciences MINERALOGY AND ECONOMIC GEOLOGY Block 2 MINERALOGY UNIT 4 Minerals: The Building Blocks of Rocks 89 UNIT 5 Classification of Minerals 119 UNIT 6 Rock-Forming Minerals-I 139 UNIT 7 Rock-Forming Minerals-I 159 Glossary 177 79 Course Design Committee Prof. Vijayshri Prof. M. A. Malik Prof. K. R. Hari Former Director Department of Geology School of Studies in Geology & School of Sciences University of Jammu Water Resources Management IGNOU, New Delhi Jammu, J & K Pt. Ravishankar Shukla University Prof. V. K. Verma (Retd.) Prof. D. C. Srivastava Raipur, Chhattisgarh Department of Geology Department of Earth Science Prof. S.J. Sangode University of Delhi, Indian Institute of Technology Roorke Department of Geology Delhi Roorkee, Uttarkhand Savitribai Phule Pune University Prof. Pramendra Dev (Retd.) Prof. L. S. Chamyal Pune, Maharashtra School of Studies in Earth Sciences Department of Geology Dr. K. Anbarasu Vikram University M.S.University of Baroda Department of Geology Ujjain, MP Vadodara, Gujarat National College Prof. P. Madhusudhana Reddy (Retd.) Prof. H. B. Srivastava Tiruchirapalli, Tamilnadu Department of Geology Centre of Advanced Study in Geology Faculty of Geology Discipline Dr. B.R. Ambedkar Open University Banaras Hindu University School of Sciences, IGNOU Hyderabad Varanasi, UP Dr. Meenal Mishra Late Prof. G. Vallinayagam Prof. Arun Kumar Dr. Benidhar Deshmukh Department of Geology Department of Earth Sciences Kurukshetra University Manipur University Dr. Kakoli Gogoi Kurukshetra, Haryana Imphal, Manipur Dr. M. Prashanth Prof. J. P. Shrivastava Prof. (Mrs.) Madhumita Das Dr. Omkar Verma Centre of Advanced Study in Geology Department of Geology University of Delhi, Delhi Utkal University Bhubaneshwar, Odisha Block Preparation Team Course Contributors Content and Language Editor Dr. Meenal Mishra (Units 6 & 7) Dr. Benidhar Deshmukh (Units 4 & 5) Prof. J. P. Shrivastava School of Sciences School of Sciences CAS in Geology IGNOU, New Delhi GNOU, New Delhi University of Delhi, Delhi Transformation: Dr. Benidhar Deshmukh Course Coordinators: Dr. Meenal Mishra and Dr. Benidhar Deshmukh Audio Visual Materials Dr. Amitosh Dubey Dr. Meenal Mishra and Dr. Benidhar Deshmukh Producer, EMPC, IGNOU Content Coordinators Production Mr. Rajiv Girdhar Mr. Sunil Kumar Mr. Hemant Kumar A.R. (P), MPDD, IGNOU A.R. (P), SOS, IGNOU S.O. (P), MPDD, IGNOU Acknowledgement: Ms. Savita Sharma for assistance in preparation of CRC and some of the figures. Decwember, 2019 © Indira Gandhi National Open University, 2019 ISBN: Disclaimer: Any material adapted from web-based resources or any other sources in this block are being used only for educational purposes only and not for commercial purposes and their copyrights rest with the original authors. All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from the Indira Gandhi National Open University. Further information on the Indira Gandhi National Open University courses may be obtained from the University’s office at Maidan Garhi, New Delhi-110 068 or the official website of IGNOU at www.ignou.ac.in. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by the Registrar, MPDD, IGNOU. Printed by : Hi-Tech Graphics, D-4/3, Okhla Industrial Area, Phase-II, New Delhi-110068. 80 BGYCT-133: CRYSTALLOGRAPHY, MINERALOGY AND ECONOMIC GEOLOGY Block 1 Basic Concepts of Crystallography Unit 1 Crystal Properties Unit 2 Crystal Symmetry Unit 3 Crystal Systems Block 2 Mineralogy Unit 4 Minerals: The Building Blocks of Rocks Unit 5 Classification of Minerals Unit 6 Rock-Forming Minerals-I Unit 7 Rock-Forming Minerals-II Block 3 Optical Mineralogy Unit 8 Polarising Microscope Unit 9 Optical Properties of Minerals Unit 10 Optical properties of Rock-Forming Minerals Block 4 Economic Geology Unit 11 Ore and Ore Deposits Unit 12 Processes of Ore Formation Unit 13 Metallic Minerals Unit 14 Non-Metallic Minerals Unit 15 Coal and Petroleum 81  BLOCK 2: MINERALOGY You have studied about crystals, their symmetry and crystal systems in Block-1 of this course. Crystals are minerals in a crystallised form and minerals are the building blocks of rocks. Minerals have fascinated human since prehistoric period and have been used as tools, ornamental stones and colouring agents. The curiosity of learning more about the minerals has evolved as a branch of geology dealing with the study of minerals, their physical and optical properties, mode of occurrence and formation, called mineralogy. Minerals are studied to unravel Earth’s geological history. Also, study of minerals occurring in a particular geological environment is of significance as it helps geologists to locate potential mineral deposits. Hence, it is important to learn about minerals here prior to studying rocks in the next course to identify different kinds of rocks and know how they are formed. In this block, you will learn about minerals, physical properties useful in their identification, their chemical and structural classification, major rock forming mineral groups, and physical properties of common rock-forming mineral groups. Unit 4 Minerals: The Building Blocks of Rocks introduces you to minerals and their importance. It also discusses about physical properties of minerals that are used to identify them. You will also have a brief idea about gemstones in this unit. As its title suggests, in Unit 5 Classification of Minerals, you would get introduced to the basis of mineral classification and their evolution. It first discusses about chemical classification of minerals and structural classification of silicates. Classification of common rock forming mineral groups such as olivine, garnet, pyroxene, amphibole, mica, feldspar, feldspathoid, and quartz are also discussed. Rock-forming minerals are essential components of rocks occurring in the Earth's crust which are discussed in the next two units. Physical properties of common rock-forming silicate minerals such as muscovite, biotite, augite, hypersthene, olivine, garnet, hornblende and kyanite are discussed in Unit 6 Rock- Forming Minerals-I. Physical properties of common rock-forming silicate minerals such as quartz, feldspar (orthoclase, plagioclase, microcline), nepheline, chlorite, epidote and calcite are discussed in Unit 7 Rock-Forming Minerals-II. Expected Learning Outcomes After studying this block, you should be able to:  get acquainted with minerals, its characters, importance and physical properties;  discuss chemical classification of minerals and structural classification of silicates;  explain classification of common rock-forming mineral groups, and  describe physical properties of common rock-forming minerals. Your familiarity with the diagnostic properties of these common rock-forming minerals would help you to identify them on the basis of their physical properties. In the next block you would learn about optical properties of the minerals. We wish you all success in this endeavour! 82  UNIT 4 MINERALS : THE BUILDING BLOCKS OF ROCKS Structure_________________________________________________ 4.1 Introduction 4.4 Gemstones Expected Learning Outcomes 4.5 Summary 4.2 Mineral 4.6 Terminal Questions Definition and Characters 4.7 References Uses 4.8 Further/ Suggested Readings 4.3 Physical Properties of Minerals 4.9 Answers Depending upon Light Depending upon Atomic Structure and State of Aggregation Based on Specific Gravity Based on Senses Depending upon Forces 4.1 INTRODUCTION You have been introduced to crystals and their symmetry in the Block 1 of this Course, which are minerals in a crystallised form. Now you know that crystals of different minerals have characteristic form or habit that is the reflection of their atomic structure. An external expression of the atomic lattices of a mineral is the development of crystal faces. 83 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… You will be introduced to rocks in the course BGYCT-135. A rock is composed of some combination of minerals hence minerals are the building blocks of rocks. Geologists study rocks and minerals to understand processes and events that occurred in the geological past at some specific part of the Earth. The specific rocks and minerals occurring in a particular geological environment also help geologists to locate potential mineral/ore deposits of economically important resources. Hence, it is important to learn about minerals prior to studying rocks to identify different kinds of rocks and understand how they are formed. In this unit, you will learn about minerals and their importance to human beings and discuss about characters and physical properties of minerals. We will also get a brief idea about gemstones. Expected Learning Outcomes____________________ After reading this unit you should be able to:  define a mineral;  state significance of minerals to human beings;  list characters of a mineral; and  prepare a list physical properties of minerals and describe them. 4.2 MINERAL You must have played with sand in river or in a beach, which contains various mineral grains. These grains of different colours are actually different minerals. Different colours of these minerals come from the elements present in them. In the first Unit of Course BGYCT-131, you have learnt that mineralogy is the branch of geology that deals with the study of minerals, their structure, composition, occurrence and association. Let us now understand what a mineral is. 4.2.1 Definition and Characters Geologists define mineral as a naturally occurring inorganic solid crystalline substance having definite chemical composition and distinctive physical property. The above definition of mineral contains six different parts which are six characters of a mineral. We will examine these characters here in Table 4.1. Any substance that does not fulfil above criteria is not called as mineral. There are exceptions of petroleum and coal. Petroleum is not solid and does not have any specific chemical composition. Coal is not formed by inorganic process. These two are called minerals because of their economic significance. You will learn about formation of minerals in detail in Block-4 Economic Geology of this course. Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… Table 4.1: Characters of a mineral and their description. Character Description Naturally Formed in nature by some natural process. Substances produced artificially in a occurring laboratory are called synthetic minerals (e.g. zeolite) Solid Only the solids qualify to be called as a mineral. It excludes gases and liquid materials with the exception of the native mercury. H₂ O as ice in a glacier is a mineral but as water it is not. Similarly, gas as a hydrate in ocean floor is considered a mineral but not otherwise Formed by Although, only the inorganic substances qualify to be called as a mineral, it is now inorganic recognised that minerals may also be formed by organisms e.g. calcium carbonate process (i.e. calcite and aragonite) of corals and shells, and such substances are called biogenic minerals. Coal contains durain, vitrian, clrain and fusain as minerals produced by biogenic activity Crystalline Only the solid substances which are commonly crystalline (but not always) i.e. substance having an orderly internal lattice structure [or geometric framework of its atoms (or ions)] can be called as a mineral. Substances that meet the other criteria but lack internal orderly structure are called mineraloids Definite Minerals have a definite (i.e. same) chemical composition that can be expressed by chemical a specific chemical formula (i.e. either fixed or ranges within particular limits) and is composition homogeneous (i.e. compositionally same) throughout its volume. Chemical composition of quartz is expressed as SiO2 as it contains silicon and oxygen in a ratio of 1:2. Although the formula remains definite, but the composition may vary within limit for some minerals, e.g. chemical composition of dolomite mineral is CaMg(CO3)2 as it contains Ca, Mg and CO3 in a ratio of 1:1:2 however, its general chemical composition may be written as Ca(Mg,Fe,Mn)(CO3)2 for the iron and manganese containing varieties Distinctive Characteristic set of physical properties of a mineral is a result of all the above five physical characters. All minerals have some distinctive physical properties (such as colour, property hardness, nature of breakage, etc.) that are used to identify and distinguish them from other minerals. 4.2.2 Uses As we have read in BGYCT-131, minerals are important to us as right from edible salt (i.e. Halite mineral) to ceramic mugs or glass tumbler to the utensils (made up of metals such as steel or aluminium or copper) are all derived from different minerals. The bricks and cement we use to construct our houses and the paints and tiles we use are all have minerals as a major component. You will be wondering to know that different components of the automobiles, computers, mobiles, battery, filament of light bulbs, etc. are made up of various minerals. Even toothpaste, automobile fuels, pencil leads, mirror glass, cosmetics, jewellery and the gems we wear are minerals. So, now you have understood how significant minerals are to us. In fact, we cannot imagine our lives without minerals because they have become integral part of our lives. Watch the following audio / video to know more about minerals and their uses.  Minerals and their uses Link: http://egyankosh.ac.in//handle/123456789/53487 91 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… 4.3 PHYSICAL PROPERTIES OF MINERALS According to the International Mineralogical Association (https://www.ima- mineralogy.org), there are more than 5530 minerals known so far in the Earth’s crust. You must be wondering how we distinguish one mineral from another. Geologists identify these minerals based on their certain characteristic physical properties. Characteristic composition, texture and physical properties of principal rock-forming minerals have been identified. In this section, we shall learn about the physical properties of common rock forming minerals based on which minerals are identified. These minerals have a wide range of physics properties i.e. their ability to absorb or reflect light, conductivity to heat, electricity, etc. As we see in the Fig. 4.1, the granite rock is composed of quartz, hornblende and feldspar. These minerals have their own physical appearances and characteristics. Granite rock Pink Granite rock (polished surface) Quartz Hornblende Orthoclase Orthoclase (Amphibole) (White K-feldspar) (Pink K-feldspar) Fig. 4.1: Two varieties of granite rocks and their major minerals components. Although, it is difficult to determine chemical composition and crystalline structure of a mineral without the use of proper instruments, these two characteristics determine physical properties of a mineral. Physical properties of a mineral are the result of:  how the atoms and molecules are arranged, and also  the strengths of the bonding between the atoms. These physical properties can be observed or measured without changing its chemical composition such as how they:  appear,  bend, break or deform, and  feel. Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… It may appear that physical properties of many minerals are common but when we examine all the physical properties, we find that each mineral has a unique set of physical characters which are helpful to identify them. For our convenience we may group physical properties of minerals under the following:  Properties depending upon interaction of light such as colour, streak, lustre, transparency and luminescence  Properties depending upon atomic structure and state of aggregation such as form, habit, cleavage, fracture, hardness and tenacity  Properties depending upon specific gravity  Properties depending upon certain senses such as feel, taste and odour  Properties depending upon forces such as heat, magnetism, electricity, radioactivity and reaction to acid. We shall discuss about these physical properties in detail in this section. 4.3.1 Depending upon Light There are several physical properties of minerals that depend upon interaction of light with it. These properties are colour, streak, lustre, transparency and luminescence. Let us discuss these properties here. a) Colour Minerals have a typical colour, which is generally the first noticeable and important physical property for their identification. Colour of a mineral is a result of reflection and/or absorption of light from its surface. Some minerals reflect light, while others absorb. There are other minerals which reflect and absorb varying amount of light in different wavelengths. The colour shown by a mineral depends upon the absorption of light in certain wavelength and reflection of others. When a mineral reflects all of the white light it appears white but when it absorbs all and reflects none, it appears black. So, if a mineral reflects light of green wavelength and absorbs light of other wavelengths then it appears green to our eyes. Some minerals appear colourless (such as pure quartz) or white (such as calcite, barite, aragonite, etc.) or light coloured (fluorite, orthoclase, etc.), some in bright colours (jasper, malachite, azurite, etc.), whereas many others appear in dark colour (hornblende, tourmaline, etc.). Although, colour of a mineral is the most noticeable physical property, it is not a reliable property for their identification (except for sulphur and pyrite minerals) as colour varies for most of the minerals. The variation in the colour of minerals is due to the following:  Amount of trace element present within them - Minerals belonging to a single group show different colours e.g. quartz group of minerals with the composition SiO2. While pure quartz is colourless inclusions of trace elements may produce quartz of many different colours. For example, although a quartz is generally colourless or white but presence of trace elements makes it to appear as pale brown (smoky), pale pink (rose 93 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… quartz), pale yellow (citrine), purple (amethyst), black (morion). Similarly, beryl has two varieties namely, emerald (green) and aquamarine (blue). The two colours exhibited by beryl are because of slight variation in trace elemental concentrations.  Nature and arrangement of constituent atoms - Minerals having the Al, Ba, Ca, K, Na, Sr and Zr atoms as the main components which are either colourless or light in colour whereas minerals having Co, Cr, Cu, Fe, Mn, Ni, Ti and Vi ions are usually dark in colour.  Bonding between the atoms - One of the best examples is of diamond (colourless) and graphite (black). Although, both of them are composed of carbon atoms, owing to difference in the bonding between carbon atoms, remarkable difference in the colour is observed.  Valency of ion - Minerals with Fe2+ are usually green whereas minerals with Fe3+ are yellow, red or brown. The minerals, in which both the Fe2+ and Fe3+ ions are present, appear black.  Thickness of the mineral pieces being observed - Thicker slices of many minerals appear darker whereas their thinner pieces appear light in colour.  Disturbance of Crystallinity – A very few minerals show colour variation within a single crystal, either arranged in regular fashion or different colour bands (as tourmaline) or in patches within a mineral (fluorite). Fig. 4.2: Different colours of quartz to change in the trace elemental concentrations. There are different terms which are used to describe minerals that display certain characteristics related to interaction of light such as idiochromatic and allochromatic:  Idiochromatic mineral - Mineral which has characteristic colour related to their composition e.g. malachite, azurite, Lapis Lazuli. Unit 4 Minerals: The Building Blocks of Rocks ……………………………………………………………………………………………………………………………………………………………………………  Allochromatic mineral - Mineral which shows a range of colours that are dependent on the presence of impurities or inclusions e.g. quartz, beryl, garnet. A few minerals display a character called play of colours. When these minerals are rotated or observed from different directions, they display a changing series of prismatic colours, similar to a rainbow e.g. diamond, quartz and other colourless minerals. b) Streak You have read that although colour is an important property but not the reliable physical property for identification of minerals because it varies due to different factors. Colour of a mineral in its fine powdered form is called streak, which is usually a constant physical property irrespective of the presence of trace elements. Streak of a mineral could be very different than the colour of that mineral. Although, colour of large mineral pieces may vary because of characteristic reflection of light by the trace elements, it would have little influence on the reflection from very small mineral particles. Colour of minerals may vary for all its varieties but their streaks are generally constant or similar. Hence, streak is a reliable physical property for identification of minerals. Remember that you do not need to crush a mineral piece to see its streak rather you can determine it by rubbing the mineral on a piece of unglazed porcelain plate called streak plate (Fig. 4.3a). However, you should note that since the streak plate cannot be used with minerals of hardness greater than seven because streak plate has a hardness of about seven. Streak of non-metallic minerals is generally light in colour or white because their mineral particles reflect most of the light (Fig. 4.3b), whereas metallic minerals have dark coloured streak because their mineral particles absorb most of the light (Fig. 4.3c). Fluorite may be of different colours but its streak is white. Black hematite gives reddish brown streak, whereas gray galena has lead gray and brassy yellow, pyrite has greenish/brownish black streak. Hence, streak is very useful property, especially for identification of metallic minerals. (a) (b) (c) Fig. 4.3: Streak of light and dark coloured minerals: a) Streak plate; b) White streak of calcite (a non-metallic mineral); and c) Cherry red streak of hematite (a metallic mineral). Let us spend five minutes to check our progress prior to leaning next physical property. 95 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… SAQ 1 a) List the characters of a mineral? b) What are the reasons for the variation in the colour of minerals? c) What is the difference between colour and streak of minerals? c) Luster Luster refers to the appearance of mineral surfaces to the combination of scattered and reflected light. It may vary in intensity from splendent (i.e. distinctly reflective as a mirror e.g. quartz) to shining (i.e. indistinctly reflective e.g. hornblende, augite), glistening (i.e. shiny by reflection with a sparkle, e.g. diamond) and glimmering (feebly reflective and intermittent flicker) and also in type from glassy to resinous to silky to waxy. Lustre is always observed and determined on the freshly broken surfaces of a mineral because minerals may chemically weather to a dull lustre with time such as a copper coin or an iron piece. Generally, two types of lustre are recognised i.e. metallic and non-metallic. Let us now understand about the two general types of luster:  Metallic luster - Minerals reflecting light and looking shiny, like metal objects are said to have metallic lustre. These minerals are opaque to light and usually have reflective surfaces with sheen such as steel, gold, silver, brass or copper metals. These minerals usually give black or very dark coloured streak. Common examples of minerals having metallic luster are galena, pyrite (Fig. 4.4a) and chalcopyrite.  Non-metallic luster - If surface of a mineral is not shining then it is said to have non-metallic lustre. In general, such type of minerals are, light coloured and transmit light. For dark coloured minerals, their thin edges/slices would transmit light. These minerals usually give colourless or very light coloured streak. There are different kinds of non-metallic luster, which are discussed in Table 4.2. (a) (b) Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… (c) (d) (e) Fig. 4.4: Different types of luster: a) Metallic luster in pyrite; b) Adamantine luster; c) Vitreous luster in quartz; d) Pearly luster in biotite; and e) Earthy lustre of bauxite. (Photo credit: a-Shanu Shukla) Table 4.2: Different kinds of non-metallic luster and their examples. Non- metallic Description Example luster Adamantine It is a luster displayed by minerals such as Diamond, Zircon, Garnet, diamond and garnet. Usually, it is due to mineral’s Cassiterite, Cerussite (Fig. high refractive index 4.4b) Vitreous This type of lustre is like that of a polished glass Vitreous - Quartz (Fig. 4.4c), and is common. It is displayed by silicates and Emerald, Tourmaline carbonates, sulphates and the halides and other non-metallic minerals such as quartz, feldspars, Sub-vitreous - Hornblende, pyroxenes, etc. Calcite and Augite It is further classified as sub-vitreous when it is less developed, e.g. hornblende and augite) Resinous It is the luster of a resin and glue. Minerals Sphalerite, Opal, Amber, displaying this kind of luster are usually yellow or Sulphur brown in colour Silky It gives silk or satin like luster. It is caused by Asbestos, Satinspar (gypsum), reflection of light from a fine fibrous parallel Serpentine, Malachite aggregate Waxy Minerals with this type of luster appear like a Serpentine, Calcedony paraffin or wax e.g. candle Pearly This type of luster has milky shimmer and Talc, Selenite, Apo-phyllite, resembles to iridescent pearl Brucite, Biotite (Fig. 4.4d) Greasy This type of luster resembles to luster of a grease Nepheline, some milky quartz and thin layer of oil covered materials Earthy (dull) When there is no reflection at all, (e.g. in chalk Chalk, Goethite, Limonite, and clay). This type of luster lacks in the reflection Glauconite (Fig. 4.4e) and appears dull such as dry soil is characteristic of aggregates of very fine-grained material (Compiled and tabulated from Gribble, 1991; and Klein and Dutrow, 2017) 97 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… Minerals with an intermediate luster (i.e. luster is slightly less than the metallic minerals) are said to be sub-metallic e.g. chromite and cuprite. However, it is treated as metallic for the purpose of mineral identification. d) Diaphaneity Besides a mineral’s colour, you may also record clarity of that mineral. Clarity of a mineral depends upon its diaphaneity or degree of transparency i.e. their ability to allow the light to pass (in other words, transmit) through it. Diaphaneity of a mineral also depends upon thickness of the mineral piece. You can determine diaphaneity of minerals by observing it. Based on diaphaneity, minerals can be grouped under the following:  Transparent – When light passes through a mineral and objects seen clearly like a clear glass, e.g. clear quartz (Fig. 4.5a).  Subtransparent – When objects can be indistinctly seen through a mineral.  Translucent – When the mineral cannot be seen through clearly and rather appears foggy because of diffusion of light and internal absorption, e.g. quartz, calcite. It is partly due to thickness and purity of the mineral e.g. pure quartz is transparent, but some varieties and thicker pieces with the presence of large numbers of bubbles are translucent (Fig. 4.5b). Another example is of hematite which is usually opaque, but very small sized pure crystals are translucent.  Opaque – When the mineral is impervious to light. It means no light passes through mineral. You cannot see through the mineral, e.g. quartz (Fig. 4.5c), tourmaline, hornblende and metallic minerals such as galena and hematite, which are always opaque even in thin sections. (a) (b) (c) Fig. 4.5: Different varieties of quartz mineral: a) Transparent; b) Translucent; and c) Opaque. e) Luminescence Some minerals emit light at low temperature and are visible in dark. Luminescence is the emission of light by a mineral that is not the direct result of incandescence (i.e. the mineral has not been heated). There are two types of luminescence:  Fluorescent minerals: Luminescent during exposure to UV light, X-rays or cathode rays, e.g. sheelite (yellowish green glow) and scapolite (yellowish orange glow)  Phosphorescent minerals: Luminescent continuously even after the existing rays are cut off, e.g. diamond and ruby. Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… 4.3.2 Depending upon Atomic Structure and State of Aggregation Atomic structure and state of aggregation determine several physical properties of minerals such as form, habit, cleavage, fracture, hardness and tenacity. We shall discuss about these characters here. a) Crystal Forms We all have seen some beautiful crystals of a mineral in a variety of shapes. We also know that crystals grow and take their shape from their tiny building blocks, called as unit cells. Each unit cell has an identical atomic arrangement/ structure. The characteristic geometric shape of a crystal that is formed by intersecting flat outer surfaces (i.e. crystal faces) is called its form. Minerals grow into a definite crystal form only under favourable environmental conditions such as:  availability of constituents in the mineral solution for growth,  enough space for the crystals to grow, and  non-obstruction by other solids. Form of a mineral denotes whether the mineral is crystallised or non- crystallised. Gribble, (1991) describes following terms associated with the form or crystal characters of a mineral:  Crystallised: It refers to minerals with well developed crystals, e.g. rhombohedral calcite.  Crystalline: It refers to a confused aggregate of imperfect crystal grains interfering with each other during the growth.  Cryptocrystalline: It refers to mineral with traces of crystalline structure such as poorly developed crystal faces. Such structures can only be observed using microscope.  Amorphous: It refers to the complete absence of crystalline structure, e.g. obsidian. It occurs rarely. In the Block-1 of this course, you have read about the seven crystal systems (Table 4.3) in which minerals crystallise. Table 4.3: Crystal systems and the minerals which crystallise in these systems. Crystal systems Minerals Galena, garnet, pyrite, halite magnetite, flourite, Cubic leucite, Tetragonal Rutile, cassiterite, zircon Hexagonal Aplite, beryl Trigonal Calcite, quartz, tourmaline Orthorhombic Sulphur, barite, olivine, topaz Monoclinic Gypsum, augite, hornblende, orthoclase Triclinic Axinite, rhodonite 99 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… On the basis of degree of development of crystal faces and forms, crystals are grouped into following three groups:  Euhedral crystals – Although, it is rare, but when crystals grow unhindered, they have well developed and clearly defined crystal faces thus crystal forms are recognised.  Subhedral crystals – Such crystal types are more common because most of the time crystals grow together resulting in crystals with deformed crystal faces and imperfect crystal forms. Such types of crystals are imperfect. Subhedral crystals have enough faces developed thus their forms are recognised.  Anhedral crystals - When minerals have no crystal faces developed thus called as anhedral crystals. Based on matching of their crystal faces, crystals are classified into following forms (Farndon and Parker, 2009):  Isometric forms - When crystals have various numbers of matching faces e.g. tetrahedron (4 faces), octahedron (8 faces) (Fig. 4.6a&b).  Non-isometric forms - When crystals have non-matching faces e.g. rhombohedron, dipyramid (Fig. 4.6c&d). (a) (b) (c) (d) Fig. 4.6: Crystal forms: Isometric forms: a) tetrahedron; b) octahedron - Non- isometric forms; c) rhombohedron; and d) dipyramid. b) Mineral Habits Habit of a mineral is its general shape and pattern in totality. Individual crystal and aggregate tends to form under a given set of environmental conditions. Such type of character is helpful in identification of minerals. Habit can be either individual or group of crystals (aggregates). We shall discuss about these two types and their habits. Individual crystals may have several habits (patterns and shapes) as given in Table 4.4. Similarly, combination of two or more crystals (i.e. crystal aggregates) produces habit as given in Table 4.5. You may note that when there is no distinctive pattern of a mass of crystals as observed in specimen (due to tight intergrowth), it is called massive. Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… Table 4.4: Habits of individual crystals. Habit Crystal Characteristics Illustration Acicular Fine needle like crystals, e.g. natrolite Bladed Resembles to a blade of a knife, e.g. kyanite Consists of clumps of stringy, or hair like fibres or Fibrous thread like structure, e.g. satin-spar (gypsum), and asbestos Foliated or Consists of thin and separable plates or lamellae foliaceous or leaves, e.g. mica minerals Consists of separable plates or leaves, e.g. Lamellar wollastonite Elongated crystals in one direction, like net, e.g. Prismatic pyroxene, amphiboles Network of small crystals developed in a cross Reticulated mesh pattern, e.g. inclusion of rutile needles in or rutilated quartz Scaly In small plates, e.g. tridymite Tabular Broad, flat and thin crystals e.g. feldspar (Text compiled and tabulated from Gribble, 1991 and Klein and Dutrow, 2017) 101 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… Table 4.5: Habits of crystal aggregates. Habit Characteristics of Crystal Aggregates Illustration Almond shaped aggregates, e.g. zeolites in Amygdaloidal basalt cavities Spherical aggregation, like bunch of grapes, Botryoidal e.g. azurite, chalcedony Aggregates making slender columns usually Columnar and parallel, e.g. calcite forming stalactite, Stalactitic stalagmite; beryl, tourmaline Concretionary Spherical, ellipsoidal or irregular masses, e.g. and nodular flint Generally found in minerals deposited in Dendritic and crevasses or narrow planes. It resembles to arborescent tree branches or a river system, e.g. psylomelane Evenly sized coarse and fine grained aggregates, e.g. chromite and olivine. Granular Resembles to a lump of sugar, hence also called saccharoidal Lense like, flattened balls or pettets, e.g. Lenticular many concretionary and nodular minerals Mutually intersecting spheroidal surface but Mammiliated larger than botryoidal, e.g. malachite Radiating or Fibres arranged around a central point, e.g. divergent barite and in many concretions Consisting of small spheroids or ellipsoids that Oolitic resembles to tiny fish eggs, e.g. oolitic hematite, chamosite Similar to oolitic but comparatively larger Pisolitic spheroids, e.g. bauxite Mineral aggregates in which radiating crystals terminate in rounded masses with kidney Reniform shaped surface, larger than botryoidal, e.g. kidney iron ore (hematite) Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… Fibers radiating from a centre producing star Stellate like shape, e.g. wavellite Wiry or Like many hair like or threadlike filaments or filiform a twisted wire, e.g. native copper and silver It is a cavity in rock that is lined with mineral Geodic or but not completely filled e.g. agate drusy (Text compiled and tabulated from Gribble, 1991 and Klein and Dutrow, 2017) c) Cleavage and Parting Form and habit depend upon the state of aggregation. Now we shall learn about the physical properties that depend upon the internal atomic structure. When you hit a mineral, it tends to break (cleave) along its weakest points/ lines/ planes. When hammered, most of the minerals tend to break in a systematic way along planes of weakness. When a mineral breaks along a definite plane surface it is said to possess a cleavage. So, cleavage is the tendency of a mineral to break/split in a systematic way. Cleavage plane is determined by internal atomic structure of the mineral i.e. by the type and strength of the chemical bonds between the atoms. The cleavages (i.e. planes of weakness) represent parallel layers between rows or sets of planar atoms, where the atomic bonds are weaker than the adjacent layers of the atoms. Since, cleavage is closely related to crystalline form and atomic structure; cleavage planes are parallel to either a particular face or to a set of faces representing a crystal form. Different minerals break in different ways and show different types of cleavages. Hence, cleavage is used as a diagnostic physical property for identification of minerals. Many a times, cleavage planes in mineral specimens are identified easily. Sometimes, the cleavage planes are not visible, however the mineral still cleave along the weak planes because the cleavage surface may be microscopic. Cleavage is defined using following two sets of criteria:  Ease in obtaining: If we can easily obtain cleavage and distinguish cleavage planes then the cleavage is considered as an excellent or perfect. If the mineral has obvious cleavage planes but we can obtain them with some difficulty then it is called good. If we can obtain cleavage with difficulty and some of the planes are difficult to distinguish, then the cleavage is called as imperfect.  Direction of the cleavage surfaces: We have learnt that cleavage planes are parallel surfaces of weak chemical bonding between lattices. Each set of parallel cleavage planes is called a cleavage direction. There could be more than one set of cleavage planes present in a crystal with each different set of cleavage planes having an orientation relative to the crystalline structure. Some minerals have one direction of cleavage whereas others may have two, three, four or six. In such cases, the cleavage types are termed (based on the shape formed by the cleavage surfaces) as cubic, rhombohedral, octahedral, dodecahedral, basal or prismatic. These two sets of criteria are defined specifically by the angles of the cleavage lines as indicated in the Table 4.6. Some examples are shown in Fig. 4.6. 103 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… Table 4.6: Different types of cleavages. Cleavage Cleavage Description Illustration Types Directions Basal One direction Cleavage occurs along planes between parallel to basal sheets or multiple sheets (layers) of atoms plane of the because there are weak bonds along the mineral sheets. Cleavages is parallel to basal plane of the mineral, e.g. mica minerals, which split apart like pages of a book Prismatic Two directions Cleavages occur between the stacked parallel to the chains of Si-O atoms. Mineral cleaves by prismatic faces breaking off thin, vertical, prismatic intersecting at or crystals off of the original prism, e.g. near 90° orthoclase (90°), plagioiclase (at 86° & 94°) and pyroxene (augite) (at 87° & 93°) Prismatic Two directions Cleavages occur parallel to the prism parallel to the zones of the minerals, e.g. amphibole pris-matic faces (hornblende) (at 56° & 124°) but not inter- secting at 90° Cubic Three directions Minerals with this cleavage break into parallel to the small cubes and shapes made of cubes. faces of the cube Cleavages are parallel to all three pairs of at 90° to one cube faces and cleavage directions are at another 90° to one another, e.g. rock salt (halite), galena Rhombo- Three directions Minerals with this cleavage split along hedral parallel to the three cleavage planes giving them faces of the 'diamond' shape called a rhombohedron, rhombo-hedron e.g. calcite but not at 90° to one another Octa- Four directions Minerals with this cleavage break into hedral parallel to the shapes made of octahedrons and parts of faces of the octa-hedrons, e.g. diamond and fluorite. octahedron Four main cleavages intersect at 71° and 109° to form octahedrons, which split along hexagon shaped surfaces; may have secondary cleavages at 60° and 120° Pyramidal Four directions Type of cleavage that occurs parallel to parallel to the the faces of a pyramid, e.g. sheelite pyramidal faces Dodeca- Six directions Minerals with this cleavage break into hedral parallel to the shapes made up of dodecahedrons and faces of the parts of dodecahedrons, e.g. Sphalerite dodecahedron intersecting at 60° and 120° (Simplified from Boger et al, 2015) Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… Fig. 4.7: a) One direction of cleavage in muscovite; b) Three planes giving rise to rhombohedral cleavage in calcite; c) Two planes of cleavages in orthoclase at right angles to each other; and d) Two planes of cleavages in hornblende at an angle of 24° and 156°. Parting is similar to a very poor cleavage but it refers to breaking along planes of structural weakness due to crystal defects. It is generally not very recognisable. You may have some confusion between cleavage and crystal form. While both of them give rise to flat planes, the reasons are different. Some minerals may have both the cleavage and crystal form such as in calcite, fluorite, halite, but some may have only cleavage e.g. muscovite and a few may have only the crystal form e.g. quartz. You can distinguish them by remembering that minerals with cleavage will always break in the same direction or set of directions, forming flat planes or stair-step pattern on their surfaces, whereas minerals with crystal form will not break in any particular direction and form irregular surface after breakage. d) Striations Some minerals such as quartz, tourmaline, feldspar, garnet and pyrite have hairline grooves or furrows on the cleavage planes or crystal faces which is also useful for their identification (Fig. 4.8). They are very fine parallel lines or furrows on cleavage planes or crystal faces and called as striations, which form due to crystal structure and growth patterns. Plagioclase feldspars (i.e. albite and labradorite) commonly exhibit striations on one cleavage plane as the calcium content of the feldspar increases. You can clearly identify striations in a 105 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… mineral by slightly rotating it back and forth in the light and observing change in the reflection due to striations. (a) (b) (c) Fig. 4.8: Striations in a) tourmaline; b) and c) quartz. Let us spend 5 minutes to check our progress before we proceed to the next physical property. SAQ 2 a) What is a crystal form? b) What are the habits of individual crystal and crystal aggregates. c) What is a cleavage? d) What is a striation? Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… e) Fracture Cleavage is a plane of weakness along which the minerals breaks and the broken surface is smooth and flat. When you break a mineral in random direction(s) other than the cleavage plane(s), the broken surface would have a typical characteristic feature, which is known as fracture. It describes characteristics of a broken surface. Unlike, the cleavage planes which are smooth and flat, fracture surface is generally rough or uneven. There are different terms which are used to describe various types of fractures as given in Table 4.7 and as shown in Fig. 4.9: Table 4.7: Different terms used to describe various types of fractures. Type of Description Illustration fracture Fracture surface is a curved (concave or convex) parting surface having shell-like lines or arcuate ridges. It is similar to the smooth curved surface produced when a glass is Conchoidal broken. It is developed in the minerals that are homogeneous and equally strong in all directions e.g. pure quartz, natural glass (obsidian), opal Fracture surface is rough and irregular with minute Uneven elevations and depressions, e.g. milky quartz, anhydrite and most minerals Fracture surface is flattish (not cleavage), e.g. chert, Even magnesite Fracture surface is jagged with sharp points or edges e.g. cast iron, native copper, kyanite. You should be careful Hackly while handling such minerals because the sharp points or edges may catch on your figure Fracture surface is similar to the broken wooden surface. It is produced by intersecting good cleavages or partings e.g. hornblende. It occurs in finely acicular minerals and in Splintery minerals with relatively higher hardness in one direction than the other two directions e.g. chrysotile serpentine, kyanite Fracture surface is thin and elongated and separates into Fibrous soft fibers, like cloth, e.g. asbestos. It is produced by crystal forms or intersecting cleavages Fracture surface is similar to the broken children's clay. It is Earthy generally found in massive and loosely consolidated minerals e.g. limonite (Text compiled and tabulated from Gribble, 1991; and Klein and Dutrow, 2017) 107 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… (e) Fig. 4.9: Fractured surface of minerals: a) Conchoidal fracture in quartz; b) Splintery fracture in kyanite; c) Hackly fracture in native copper; d) Uneven fracture in hematite; and e) Earthy fracture in kaolinite. You can clearly recognise cleavage planes and fracture surfaces. You now know that cleavage planes are parallel, smooth and flat, whereas fracture surface(s) is generally rough or uneven and never occur in parallel sets. Further, when you rotate a broken piece of a mineral crystal in bright light, there would be periodic reflective flashes of light from its cleavage planes, whereas no such reflective flashes of light occur if there is no cleavage. f) Hardness Hardness is one of the most important diagnostic properties of minerals. It is the resistance offered by a smooth surface of a mineral against its scratching so it might also be said to be its “scratchability”. Hardness of minerals depends upon atomic structure and density of ions in the mineral. Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… Friedrich Mohs, a German mineralogist developed a relative scale of mineral hardness in 1812 by arranging minerals in the order of their increasing relative hardness. The quantitative scale is known as the Mohs’ scale of hardness, which is a set of 10 minerals of known arbitrary hardness (Table 4.8 and Fig. 4.10). The softest mineral (talc) has hardness of 1 and the hardest mineral (diamond) has hardness of 10. Higher numbered (i.e. the harder) minerals can scratch the lower-numbered (i.e. the softer) minerals because the forces that hold the crystals together can be broken by the harder mineral. The scale provides a standard to which all other minerals can be compared. However, you should note that this is a simplified relative hardness scale and the increase in hardness of the minerals from 1 to 9 is approximately linear but hardness of mineral at 10 (i.e. diamond) has been estimated as four times higher than the mineral at 9 (i.e. corundum). Table 4.8: Mohs’ scale of hardness of minerals and hardness of common objects. Common objects that can be used Minerals with similar Hardness Mineral to measure relative hardness of hardness minerals 1 Talc Graphite, clays Finger nail (2.2) Sulphur, halite, muscovite, 2 Gypsum May vary from person to person chlorite Copper coin (2.9) Barite, biotite, native 3 Calcite Brass (wood screw, washer) (3.5) copper, gold, silver Siderite, dolomite, 4 Fluorite Wire (iron) nail (4.5) aragonite, malachite Steel nail, Steel Knife blade (5-6.5) Limonite, serpentine, 5 Apatite depending on the steel quality kyanite (along length) Glass plate (~5.5) Leucite, nepheline, Orthoclase sodalite, amphibole, 6 Steel file (~6.5) feldspar pyroxene, epidote, pyrite, rutile, hematite Garnet, kyanite (across 7 Quartz length), olivine, Streak plate (~7) casseterite, andalusite 8 Topaz Beryl Emery sandpaper 9 Corundum Knife sharpener 10 Diamond (Compiled and tabulated from Gribble, 1991; and Klein and Dutrow, 2017) You can determine hardness of a mineral by comparing its hardness with other minerals or common objects. In general, minerals are grouped into two classes:  Soft minerals (those with hardness 5.5 or less) that are softer than the glass plate (i.e. they can be scratched by glass), and  Hard minerals (those with harness >5.5) that are harder than the glass plate (i.e. they can scratch the glass). 109 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… Soft minerals can be easily scratched by a knife blade or steel nail and do not scratch glass. Hard minerals cannot be scratched by a knife blade or steel nail and easily scratch glass. You should take precautions while testing hardness. You need to carefully note the kind of noise a mineral makes when it is scratched on another mineral and also the powder produced as a result of scratch. Further, some minerals may display different hardnesses in different scratch directions. A significant difference in the hardness is noticeable in kyanite and calcite. Kyanite shows hardness of 5 parallel to the length, but 7 across the length. Fig. 4.10: Minerals in the Mohs’ hardness scale: first row from left are talc, gypsum, calcite, fluorite and apatite; second row from left are orthoclase feldspar, quartz, topaz, corundum and diamond. d) Tenacity The way a mineral offers resistance (or deforms) when it is subjected to crushing, bending, breaking or tearing, is known as tenacity. In other words, it is cohesiveness of the mineral. Tenacity varies from one mineral to another; hence it is used as a useful physical property for mineral identification. The terms used to describe tenacity are given in Table 4.9. Table 4.9: Terms used to describe tenacity of minerals and their examples. Term Description Example Brittle Such minerals are broken or crushed Galena, hematite, easily into powder with hammer sulphur, iron pyrite, apatite, fluorite Malleable Such minerals can be hammered out into Native gold, silver, thin flat sheets copper Elastic Such minerals or their thin plates or Mica laminae can bent and return to their original position after removal of the pressure Flexible Such minerals bent, but do not return to Talc, chlorite, their original position even after removal selenite of the pressure Sectile Such minerals can be cut with a knife Graphite, steatite, gypsum Ductile Such minerals can be drawn into thin Gold, silver, copper wires (Tabulated from Gribble, 1991) Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… 4.3.3 Based on Specific Gravity Specific Gravity (SG) determines relative density of minerals. It is a constant feature for each of the minerals. It depends upon the chemical composition of a mineral and also the packing of atoms in its crystal structure. Usually, the minerals composed of higher atomic weight elements usually have higher specific gravity such as minerals rich in Mg and Fe. Tighter the packing of atoms and heavier is the mineral e.g. diamond and graphite have same chemical composition, but diamond has higher specific gravity of 3.5 due to closely packed structure. Therefore, two minerals of the same size may have different weights. However, specific gravity may slightly vary within a mineral because of impurities present in its structure. Most of the minerals with a metallic luster are heavy. Galena is known for its high specific gravity. The specific gravity of a mineral determines how heavy it is by its relative weight to water. The specific gravity is expressed upon how much greater the weight of the mineral is to an equal amount of water. The specific gravity of a body is the ratio of weight of the body to that of the equal volume of water at 40o C. Water has a specific gravity of 1.0. Minerals with a specific gravity value < 2 are considered light, between 2 and 4.5 average, and > 4.5 heavy. If a mineral has a specific gravity of 3.5, it is 3.5 times heavier than the water. There are several methods employed for determination of specific gravity of different kinds of minerals. Selection of a method depends usually upon the size and character of the specimen. Commonly used methods are listed in Table 4.10. The specific gravity is very useful in the identification of minerals. You should note that to determine specific gravity, selection of mineral specimens is important. Ideally, pure specimens, which are homogeneous and devoid of any crack or cavity, are considered suitable for the purpose. However, such specimens are not easily found hence generally, specimens having a volume of about one cubic centimetre are used. 4.3.4 Based on Senses Sensory tests such as feel, taste and odour may be diagnostic in identification of some minerals. We shall learn about them here. a) Feel How a mineral feels in our hand can be used to identify some minerals along with other physical properties to identify them. This property of minerals can be sensed by handling of minerals by bare hands. Some minerals have their own characteristics feel such as:  Soapy – Some minerals such as talc gives sense of soapy feeling.  Greasy - Some minerals such as graphite gives feel of a grease when touched.  Smooth and rough – Some minerals feel smooth to touch and some others rough to touch such as chromite. 111 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… Table 4.10: Commonly used methods of determining specific gravity. Method Description Useful for/to It is a simple method to judge specific gravity of one mineral relative to another. This is done by holding equal To differentiate sized pieces of two minerals in different hands and metallic minerals Hefting feeling the difference in weight between the two. The from non-metallic mineral feeling heavier has a relatively higher specific minerals gravity than the other It is used for approximate and faster determination of specific gravity. It is obtained by half filling a graduated By cylinder with water, placing the previously weighed Large number of measuring specimen into the cylinder, and noting the increase in the pieces of the displaced volume. SG is determined by weight (in grammes) of same mineral water mineral in air divided by the increase in volume (in millilitres) Specimen is suspended by a thread from one arm of the Fragments of Chemical balance and immersed in a beaker of water. SG is minerals about balance determined by dividing the mineral’s weight in air by the the size of a difference between its weights in air and water walnut The apparatus consists of a long graduated beam which Walkers’s is pivoted near one end and counterbalanced by a heavy Large specimens steelyard weight suspended from the short arm It is similar to the chemical balance method. However, instead of determining the absolute weight of the specimen, values proportional to the weights in air and in Jolly’s spring water are determined. Specimen is suspended by a Very small balance thread from one arm of the balance and immersed in a specimens beaker of water. SG is then determined by dividing the mineral’s weight in air by the difference in weights in air and in water A small glass bottle containing a known volume of water Porous or friable Pycno-meter is used which is fitted with a stopper having a fine minerals, mineral or specific opening. Specific gravity is determined by dividing weight grains gemstones gravity bottle of the mineral by the weight of distilled water displaced and liquids by it in the bottle used Determined by comparing specimens to liquids of known specific gravity, which are of relatively high densities, hence called heavy liquids e.g. bromoform or tetrabromoethane (SG-2.89) and methylene iodide (SG- 3.3). Heavier minerals sink and the lighter ones float in For separation of the liquid. SG of a liquid can be adjusted to a value at mineral mixtures Heavy liquids which a mineral will neither float nor sink. By knowing SG into their pure of the liquid we can determine SG of the mineral. components Diffusion column is used for small samples, Berman Torsion microbalance for very small samples, and Westphal balance for small amount of liquids (Compiled and tabulated from Gribble, 1991; and Klein and Dutrow, 2017) Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… b) Taste Minerals soluble in water can be identified by their taste. However, testing this property in classroom/laboratory could be dangerous because some minerals are poisonous. So, you should not put minerals in your mouth or on the tongue. Gribble (1991) has used following terms for minerals based on taste:  Saline – Some minerals such as halite (NaCl) taste salty which is common salt.  Alkaline – Potash and soda taste alkaline.  Cooling – Nitre or potassium chlorate give cooling taste.  Astringent or puckering – Green vitriol (hydrated iron sulphate) gives astringent taste and alum gives sweetish astringent taste.  Bitter – Some minerals such as sylvite (KCl) and epsom salt (hydrated magnesium sulphate) tastes salty and bitter.  Sour – Sulphuric acid tastes sour. c) Odour Most minerals do not have any odour, however when they are rubbed, struck, heated or breathed upon they leave some odour which could be a diagnostic property for those minerals. Gribble (1991) has used several terms to describe those odours as given in Table 4.11. Table 4.11: Terms used to describe odours of minerals. Odour Description Example Alliaceous Some minerals like arsenic compounds give odour Arsenopyrite (Garlic) that of garlic upon heating and grinding Some minerals such as selenium compounds give Horse raddish Selenium minerals odour of decaying horse-radish upon heating When iron pyrite is struck or some sulphides are Pyrite when struck, Sulphurous heated they give rise to odour of burning sulphur chalcocite upon heating Sphalerite when Some minerals such as certain varieties of quartz or Fetid scratched, heating, limestone give odour of rotten eggs when heated or (rotten eggs) rubbing of geodes of rubbed agate, quartz Argillaceos or When some minerals are breathed upon they give Clay minerals clayey (Musty) odour of clay 4.3.5 Depending upon Forces Physical properties based on forces such as heat, magnetism, electricity and radioactivity can also be used in identification of some minerals. We will learn about these physical properties in this subsection. a) Heat It has been recognised that minerals behave differently on heating. Some minerals melt at lower temperatures, whereas other minerals melt at much higher temperatures at the atmospheric pressure. So they have their specific 113 Block 2 Mineralogy ……………………………………………………………………………………………………………………………………………………………………… fusibility i.e. the temperature or amount of heat that is required to melt or liquefy a mineral. Wolfgang Xavier Franz Ritter von Kobell, a German mineralogist suggested a scale of fusibility which is used in mineralogy to define the approximate relative fusibility (temperature of fusion) of different minerals. The scale consists of following six minerals arranged according to approximate temperature of fusion:  Stibnite (525° C)  Natrolite (965° C)  Almandine garnet (1,200° C)  Actinolite (1,296° C)  Orthoclase (1,300° C); and  Bronzite (1380° C). b) Magnetism Some minerals display the property of magnetism (i.e. attraction or repulsion of magnetic materials to the mineral) and for these minerals, it could be a diagnostic property. Generally, the iron bearing minerals display magnetism; however it is not the case always. Also the degree of magnetism displayed by a mineral does not necessarily depend on the iron content. Minerals may vary from nonmagnetic to weakly magnetic to strongly magnetic. Although, it can be difficult for you to determine different types of magnetism, but it is worth knowing that there are distinctions made. Based on the type of magnetism displayed by minerals they can be grouped under the following:  Diamagnetic (non-magnetic) – Minerals having no attraction for magnetic field e.g. quartz, calcite and most minerals.  Paramagnetic – Minerals that are drawn to a magnetic field as long as the magnetic field is present. Paramagnetic minerals can be further classified as:  Strongly magnetic/ferromagnetic – These minerals are most magnetically active, e.g. magnetite, native iron.  Moderately magnetic: These minerals are not so magnetically active, e.g. Ilmenite, siderite, hematite, chromite.  Weakly magnetic: These minerals are least magnetically active, e.g. tourmaline, monazite, some hematite. You have read in the course BGYCT-131 that past magnetic events are useful in reconstructing geological history. Magnetic minerals record the direction of the Earth’s magnetic field through time and hence are very important. You may also note that the magnetic property of minerals is utilised to separate ore minerals from waste materials. c) Electricity Some minerals exhibit distinct electrical property i.e. the capacity to conduct electricity which is useful for their identification. Mostly, the minerals with metallic luster such as native metals (e.g. native copper, silver, gold) and Unit 4 Minerals: The Building Blocks of Rocks …………………………………………………………………………………………………………………………………………………………………………… sulphides except sphalerite (which has non-metallic luster) are good conductors of electricity. Depending upon their capacity for conducting electricity, minerals can be categorised as non-conductor, semiconductor and conductor. It has been observed that some minerals develop an electrical charge either  when they are subjected to stress i.e. piezoelectric, e.g. quartz, or  when they are heated i.e. pyroelectric, e.g. tourmaline. c) Radioactivity Uranium and thorium minerals (e.g. uraninite, pitchblende, thorianite, autunite) contain elements which continuously undergo radioactive decay reaction. In the process, radioactive isotopes of U and Th (such as U238, U235 and Th232) form various daughter elements and large energy is released in the form of alpha and beta particles and gamma radiation. This released radiation can be measured using instruments called Geiger - Muller counters, scintillometers and radon detectors. d) Solubility in Acid or Reaction to Acid Another diagnostic property of some minerals is that they undergo a change when a drop of dilute hydrochloric acid (HCL) is applied on their fresh surfaces from a dropper bottle. This test is particularly useful for identification of carbonate minerals such as calcite, aragonite, strontianite, etc. which show bubbles or effervescence. However, other carbonate minerals such as dolomite, rodochrosite, magnesite and siderite show effervescence in hot HCL. There are some varieties of minerals known as gems, which are considered as precious due to their certain characteristics and rarity. We shall have a brief idea about them in the next section. 4.4 GEMSTONES Most of the mineral crystals are dull and quite small but a few of them are rich in colours and sparkle. When such beautiful stones are hard enough to cut and fashion into jewellery, they are called gemstones. In other words, gems are minerals with an ornamental value, and are distinguished from non-gems by their beauty, durability, and usually, rarity. Of the number of minerals known, only about 130 mineral species are considered as gem minerals, and the gem minerals which are frequently used as gems are less than 50. The rarest and most valuable of all the gems are diamond, emerald (green beryl), ruby (red corundum) and sapphire (blue corundum). Gem minerals are often present in several varieties, and so one mineral can account for several different gemstones; for example, ruby and sapphire are both corundum and composed of Al2O3. Some of the less rare ones, such as pearl (aragonite), turquoise, lapis lazuli (lazurite), garnet, topaz, tourmaline, peridot (gem olivine), acquamarine (blue beryl), chrysoberyl, opal (chalacedony), jadeite are known as semi- precious stones. Gemstones are very rare because they form under rare geological conditions for example:  In volcanic pipes such as diamonds in kimberlite and lamproite (rock) pipes 115 Block 2 Mineralogy ………………………………………………………………………………………………………………………………………………………………………  In pegmatite rock, which in the last stages of magma intrusions concentrate rare minerals to form gems such as beryl, rubies, sapphires, tourmalines, topazes and many others  Due to intense metamorphism may create gems such as garnets, emeralds, jades, lapis lazuli, etc. Gemstones are valued based on their beauty, durability and rarity and are often assessed in terms of following four Cs:  Clarity - We have read about clarity in the previous section. It is considered as the most valued property for gems. Gems, which are flawless transparent crystals, are considered as the perfect gems that sparkle brilliantly due to internal reflection e.g. diamond,  Colour - We have also learned about the colour as the first noticeable property of minerals. In the case of gems which are opaque, but have vivid colours are also prized e.g. jade, turquoise, lapis lazuli, etc. As you have studied, presence of trace elements brings wide range of colours in gems. While colourless diamond is most valued in comparisons to coloured diamonds, coloured varieties of beryl (i.e. emerald) is more valued than the colourless beryl.  Cut - Besides the colour and clarity, cut is also important for gems because gems are cut to bring all its sparkle and colour. Hence it is important for gems to be tough enough to be used in jewellery.  Carat - Size of the gems is also prized e.g. larger stones are the most valued. The word carat is believed to have been derived from the carob tree, seed of which were used in past to weigh gems. These seeds are known for their constant weight. It forms the basis of a standard weight called carat i.e. about a fifth of a gram. There are several famous gems in the world. In India, one of the most famous gems in India is the Kohinoor. 4.5 SUMMARY Let us now summarise what we have learnt in this unit. In this unit, we have learnt that  a mineral is a naturally occurring inorganic solid crystalline substance having definite chemical composition and distinctive physical property  minerals form through the processes of crystallisation, evaporation/ precipitation, alteration and metamorphism  minerals are very significant to us. Right from the edible table salt to the utensils, toothpaste, building materials, computers and cosmetics we use, are all either the minerals or the products derived from them  minerals have certain physical properties based on which we can identify them  physical properties of minerals are their characteristics that we can observe such as i) shape and pattern of the crystals and their growth individually and in aggregates (form, habit), ii) its interaction with light and appearance of its fresh surface (color, luster, clarity, transparency, luminescence), iii) its resistance to scratching (hardness), iv) nature of bending, breakage or deformation under stress (cleavage, fracture, tenacity), v) density of packing of atoms (specific gravity), vi) response of our senses to them (feel, taste, odour), and vii) its response to forces (heat, magnetism, electricity, radioactivity), and Unit 4 Minerals: The Building Blocks of Rocks ……………………………………………………………………………………………………………………………………………………………………………  gems are rare minerals which are formed under unusual geological conditions. 4.6 TERMINAL QUESTIONS 1. List the basis of grouping physical properties of minerals? 2. What are the physical properties of minerals which are used to identify them? 3. List any five types of lusters of non-metallic minerals. 4. How many sets of cleavages are found in minerals? 5. List any five types of fractures of minerals. 6. List any four types of habits of both individual crystal and crystal aggregates. Audio/video material based questions  Can you categorise minerals based on their usage?  List the house hold objects where minerals are used?  What is the significance of minerals in our life? 4.7 REFERENCES  Bates, R.L. and Jackson, J.A. (eds.) (1987) Glossary of Geology. American Geological Institute, Alexandria, VA, 788 p.  Boger, J.L., Boger, P.D. Carlson, R.J., Frye, C.I. and Hochella, Jr. M.F. (2015) Mineral properties, identification and uses, Laboratory 3 in Laboratory Manual in Physical Geology, 10th Edition, Edited by Busch, R.M., Pearson, Delhi.  Dana, J.D. and Ford. (1962) A Text book of Mineralogy, Asia Publishing House, New Delhi.  Farndon, J. and Parker, S. (2009) The Complete Illustrated Guide to Minerals, Rocks & Fossils of the World, Anness Publishing, London.  Gribble, C.D. (1991) Rutley’s Elements of Mineralogy, 27th Edition. CBS Publishers and Distributors, Delhi.  Klein, C and Dutrow, B. (2017) The Manual of Mineral Science, 23rd Edition, Wiley, Delhi. 4.8 FURTHER/SUGGESTED READINGS  https://www.britannica.com/science/mineral-chemical-compound/Classification-of- minerals  https://www.britannica.com/science/mineral-chemical-compound/Crystal-habit-and- crystal-aggregation  https://geology.com/minerals/crystal-habit/  http://www.geologyin.com/2019/10/crystal-habits-and-forms.html  www.mindat.org/minerals.php  https://www.ima-mineralogy.org/  https://en.wikipedia.org/wiki/Crystal_habit  http://cnmnc.main.jp/imalist.htm 117 Block 2 Mineralogy ………………………………………………………………………………………………………………………………………………………………………  http://earthsci.org/mineral/rockmin/mineral/minerals.html  Mahapatra, G.B. (2012) A Textbook of Geology, CBS Publishers, New Delhi  Singh, P. (2013) Engineering and General Geology, S.K. Kataria & Sons, Delhi. 4.9 ANSWERS Self Assessment Questions 1 a) Naturally occurring, solid, formed by inorganic process, crystalline substance, definite chemical composition and distinctive physical property. b) Amount of trace element present within them, nature and arrangement of constituent ions, bonding between the atoms, valency of ion, thickness of the mineral pieces being observed, disturbance of crystallinity. c) Colour of the minerals is a result of reflection and/or absorption of light from its surface whereas streak is the colour of its fine powder. 2 a) Crystal form is the characteristic geometric shape of a crystal that is formed by intersecting flat outer surfaces. b) The general shape and pattern in totality that its individual mineral crystals and aggregates tend to produce under a given set of environmental conditions of their formation is known as habit of the mineral. c) Cleavage is the tendency of a mineral to break/split in a systematic way which is determined by internal atomic structure of the mineral. d) Striations are the very fine parallel lines or furrows on cleavage planes or crystal faces. Terminal Questions 1. The basis of grouping physical properties of minerals are i) shape and pattern of the crystals and their growth individually and in aggregates, ii) their interaction with light and appearance of its fresh surface, iii) resistance to scratching, iv) nature of bending, breakage or deformation under stress, v) density of packing of atoms, vi) response of our senses to them, and vii) their response to forces. 2. The physical properties of minerals which are used to identify them are form, habit, color, luster, clarity, transparency, luminescence, hardness, iv) cleavage, fracture, tenacity, specific gravity, feel, taste, odour, heat, magnetism, electricity, radioactivity and reaction to acid. 3. Adamantine, vitreous, resinous, silky, waxy, pearly, greasy and earthy (dull). 4. One, two, three, four and six. 5. Concoidal, uneven, even, hackly, splintery, fibrous and earthy. 6. Individual crystals: acicular, bladed, fibrous, foliated or foliaceous, lamellar, prismatic, reticulated, scaly and tabular. Crystal aggregates: amygdaloidal, botryoidal, columnar and stalactitic, concretionary and nodular, dendritic and arborescent, granular, lenticular, mammiliated, radiating or divergent, oolitic, pisolitic, reniform, stellate, wiry or filiform and geodic.  UNIT 5 CLASSIFICATION OF MINERALS Structure_____________________________________________ 5.1 Introduction 5.6 Summary Expected Learning Outcomes 5.7 Terminal Questions 5.2 Basis and Developments of Mineral 5.8 References Classification Schemes 5.9 Further/ Suggested Readings 5.3 Chemical Classification of Minerals 5.10 Answers 5.4 Structural Classification of Silicates 5.5 Rock Forming Minerals Group Olivine Garnet Pyroxene Amphibole Mica Feldspar Feldspathoid Quartz Carbonates 5.1 INTRODUCTION You have been introduced to minerals and their physical properties in Unit 4 of this Block and about crystals (i.e. minerals in crystallised form) in Block1 of this course. You have also read in the Unit that minerals have definite chemical composition and their crystals have characteristic form or habit, which are governed by their atomic structure. In this unit, you will learn about classification of minerals which are based primarily on their chemical composition and also on internal crystal structure. We will first introduce chemical classification of minerals and then about structural classification of silicates. We will then discuss about classification and characteristics of common rock forming mineral groups. Unit 5 ……………………………………………………………………………………………………………….………………………………………………………..Classification of Minerals You will be introduced to megascopic properties of commonly occurring rock forming mine the next two units of this block i.e. Units 6 and 7. Expected Learning Outcomes___________________ After reading this unit you should be able to:  identify basis of mineral classification and major developments in classification schemes;  describe chemical classification of minerals;  discuss structural classification of silicates; and  list out the commonly occurring rock forming mineral groups and their important characteristics. 5.2 BASIS AND DEVELOPMENTS OF MINERAL CLASSIFICATION SCHEMES As we have read earlier, there are ~5530 minerals known so far and many are discovered every year. We have also read that each mineral has unique chemical composition however, despite the distinct chemical identity; there are many minerals which appear quite similar to other minerals. Some minerals are also known to take same crystal shape as another mineral. It is not an easy task to study these many thousands of minerals individually, so to make sense of them and for convenience to study them, scientists have tried to organise/classify such a large set of minerals into certain groups. There are different ways by which minerals have been classified such as on the basis of their:  physical properties,  chemical properties,  chemical composition, and  internal crystal structure. The two parameters which are most important for mineral classification are the chemical composition and internal crystal structure because these