Record 2014/55 | GeoCat 79032
Exploring Minerals and Crystals Teacher notes and student activities
Education and Outreach Editor: Katy Tomkins
APPLYING GEOSCIENCE TO AUSTRALIA’S MOST IMPORTANT CHALLENGES www.ga.gov.au
Exploring Minerals and Crystals
Teacher notes and student activities
GEOSCIENCE AUSTRALIA RECORD 2014/55
Education and Outreach Editor: Katy Tomkins
Department of Industry Minister for Industry: The Hon Ian Macfarlane MP Parliamentary Secretary: The Hon Bob Baldwin MP Secretary: Ms Glenys Beauchamp PSM
Geoscience Australia Chief Executive Officer: Dr Chris Pigram This paper is published with the permission of the CEO, Geoscience Australia
© Commonwealth of Australia (Geoscience Australia) 2014
With the exception of the Commonwealth Coat of Arms and where otherwise noted, all material in this publication is provided under a Creative Commons Attribution 3.0 Australia Licence. (http://www.creativecommons.org/licenses/by/3.0/au/deed.en)
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ISSN 2201-702X (PDF) ISBN 978-1-925124-50-7 (PDF) GeoCat 79032
Acknowledgements Original concept and content compiled by Julie M Gunther (1996): Exploring Crystals.
Scientific advice and editing by Alan Whitaker, Anthony Senior, John Pugh, Terry Mernagh, Ian Roach and Science ASSIST
Educational advice by Shona Blewett, Lara Davis, Steve Tatham and editing by Vicki Pow and Jeanette Holland.
Graphics, design and production by Products and Promotion, Geoscience Australia
Cover images: (LHS) Geoscience Australia, photo credit: Katy Tomkins. (top RHS) Wikimedia Commons, photo credit: Alexander Van Driessche. (bottom LHS) Wikimedia Commons, photo credit: Rob Lavinsky. (bottom middle) Geoscience Australia, photo credit: Chris Fitzgerald. (bottom RHS) Wikimedia Commons, photo credit: Rob Lavinsky.
Education Centre Geoscience Australia GPO Box 378, CANBERRA ACT 2601. Phone: +61 2 6249 9673 Fax: +61 2 6249 9926 Email: [email protected]
Bibliographic reference: Geoscience Australia, 2014. Exploring Minerals and Crystals: Teacher Notes and Student Activities. Record 2014/55. Geoscience Australia: Canberra. http://dx.doi.org/10.11636/Record.2014.055
Contents
1 Introduction ...... 1 1.1 Student activities ...... 1 2 Rocks and minerals in history ...... 2 3 Minerals, crystals and rocks – what’s the difference? ...... 3 3.1 How do crystals form? ...... 5 3.2 Extension: Fractional crystallisation ...... 7 4 Identifying minerals ...... 8 4.1 Colour and streak ...... 8 4.2 Lustre ...... 9 4.3 Hardness ...... 10 4.4 Crystal shape ...... 13 4.5 Cleavage ...... 13 4.6 Specific gravity ...... 14 4.7 Habit (natural shape) ...... 14 4.8 Extension: Crystal systems ...... 17 5 Using crystals in Earth Science ...... 18 5.1 Geoscience Australia ...... 18 6 Mesmerising minerals ...... 20 6.1 Calcite ...... 20 6.2 Quartz ...... 20 6.3 Feldspar ...... 21 6.4 Gypsum ...... 21 6.5 Pyrite ...... 22 6.6 Magnetite ...... 22 7 Captivating crystals ...... 23 7.1 Geodes...... 23 7.2 Metallic crystals ...... 23 7.3 Cultivated crystals ...... 24 7.4 Glass ...... 24 7.5 Luminescent crystals ...... 24 7.6 Irradiated crystals ...... 25 7.7 Electric crystals ...... 25 7.8 Pseudomorphs ...... 25 7.9 Solar cells...... 25 8 Gemstones ...... 27 8.1 Definition ...... 27 8.2 The 4 C's ...... 28 8.2.1 Clarity ...... 28 8.2.2 Colour ...... 28 8.2.3 Cut ...... 28 8.2.4 Carat ...... 29
Exploring Minerals and Crystals iii 8.3 Australian mineral and gemstone resources ...... 30 8.4 Famous gemstones ...... 31 8.4.1 Diamonds ...... 31 8.4.2 Opals ...... 31 8.4.3 Sapphire ...... 33 8.4.4 Ruby ...... 33 9 Mineral collections ...... 34 Glossary...... 36 Resources...... 39 Books ...... 39 Websites ...... 39 Table photo credits ...... 40 Exploring Minerals and Crystals – Student activities ...... 43 Crystals around you ...... 44 The unfair ‘build your own crystal’ race...... 45 Cooling rate and crystal size 1 (salol) – a crystal growing activity...... 47 Scientific investigation write-up guidelines ...... 52 Cooling rate and crystal size 2 (alum or copper sulphate) ...... 54 Growing Crystals ...... 56 Crystallisation of magma ...... 64 Make a goniometer ...... 66 Mineral Detective – testing mineral properties ...... 68 Crystal Models ...... 76 This came from that ...... 82 Information collection template ...... 83 Make your own collection ...... 89 Appendix A ...... 90 A.1 Extension: Crystal Systems ...... 90 A.1.1 Cubic (or isometric) ...... 92 A.1.2 Tetragonal ...... 92 A.1.3 Hexagonal/Trigonal ...... 93 A.1.4 Orthorhombic ...... 94 A.1.5 Monoclinic ...... 94 A.1.6 Triclinic ...... 95
iv Exploring Minerals and Crystals
1 Introduction
Rocks and minerals are all around us. We see and use them every day, without even realising it. Minerals are the building blocks of rocks. If you think of a cake representing a rock, the ingredients of the cake are the minerals. There are several different ingredients in a cake and they can be mixed in different proportions, just as there are different minerals in different proportions in a rock.
Note to teachers: Exploring Minerals and Crystals: Teacher notes and student activities has been developed as an Earth Science companion for teaching primary and secondary level Science, in particular Year 8 Earth and Space Science. Please check that the student activities are appropriate for your cohort of pupils.
Listed below are the relevant links to the Australian Curriculum.
Year 8
Earth and Space Science
ACSSU153 Sedimentary, igneous and metamorphic rocks contain minerals and are formed by processes that occur within the Earth over a variety of timescales:
• recognising that rocks are a collection of different minerals • considering the role of forces and energy in the formation of different types of rocks and minerals • recognising that some rocks and minerals, such as ores, provide valuable resources.
Senior Secondary
Unit 1 Introduction to Earth systems
Rocks are composed of characteristic assemblages of mineral crystals or grains that are formed through igneous, sedimentary and metamorphic processes, as part of the rock cycle.
Unit 3 Living on Earth: extracting, using and managing Earth resources
The location of non-renewable mineral and energy resources, including fossil fuels, iron ore and gold, is related to their geological setting.
Mineral and energy resources are discovered using a variety of remote sensing techniques and direct sampling techniques.
1.1 Student activities
This booklet contains activity ideas and experiments for teachers and students. The symbol indicates when there is an associated activity that can be found in the second part of the booklet.
For information about this publication or other education resources please visit Geoscience Australia’s Education website at www.ga.gov.au/education or contact the Education Centre.
Exploring Minerals and Crystals 1 2 Rocks and minerals in history
From the beginning humans needed the ability to cut, grind, pierce and pound various materials. Teeth were undoubtedly the first tools to be used for these purposes. However, it would have been hard to skin an elephant or carve a statue using teeth alone, so tools made of wood, bone and stone appeared very early in prehistory, more than a million years ago.
Stone is a generic term used to describe any of the naturally occurring solid materials which make up the Earth. Generally the term stone is not used in geology, instead geologists use the terms mineral and rock. The term mineral is used to describe a naturally occurring substance with a reasonably fixed chemical composition and crystal structure. When discussing minerals, we will often refer to the chemical composition, crystal structure and melting point. The term rock is used to describe a naturally occurring solid aggregate of one or more minerals. When discussing rocks, we will often refer to the minerals present in the rock.
Not all stone is suitable for toolmaking. The key requirements for a cutting tool are that it should be sharp and durable. Minerals are not very suitable for toolmaking because of their crystalline structure. When a crystal is broken, it splits unevenly and does not generally produce a sharp edge. During the Stone Age mankind’s requirement was the need for a hard rock that was not easily scratched. Among hard rocks, the silicates are the most abundant (12% of the Earth's crust) and they were the material of choice for primitive stone tools.
Stone Age people placed a very great value on flint, which could be used to make sharp knives and arrow heads. Flint is a type of cryptocrystalline quartz made up of such minute crystals they are only visible microscopically. In Europe, people found that striking flint against fool’s gold (pyrite) made a spark that could be used to start a fire.
Indigenous Australians had implements such as knives, scrapers, axe-heads and spears often made of stone. They used stone tools for many things including: to make other tools, to get and prepare food, to chop wood, and to prepare animal skins.
2 Exploring Minerals and Crystals
3 Minerals, crystals and rocks – what’s the difference?
Minerals are natural inorganic substances that have a regular crystal structure and distinctive chemical composition. Minerals are composed of combinations of Earth’s 98 naturally occurring elements and have distinct measurable physical and chemical properties and crystalline structures. Mineral compositions can usually be expressed by a chemical formula indicating the elements present and proportions in which they are combined, for example SiO2 is the formula for quartz.
Minerals vary from simple, pure elements like gold, copper, silver, carbon and sulphur to complex mixtures that have a thousand or so known forms. Examples of common minerals in the Earth’s crust are quartz, feldspar, mica and calcite. Minerals may occur as single crystals or collections of crystals.
Crystals are solid objects that have a regular shape due to their component atoms being arranged in a regular pattern. Well-formed crystals usually have straight edges and flat surfaces, called faces, which reflect light and give crystals their sparkling look. However, during the formation of a crystal, the shapes often become distorted, resulting in imperfect crystals. Solids which form crystals are called crystalline. Solids which are not crystalline are called amorphous (morph is the Greek word for shape, so amorphous means shapeless). These substances do not have regular shapes, for example obsidian, also known as volcanic glass. Obsidian cools so quickly that no crystals form at all. Sometimes the same substance may have both a crystalline and amorphous form; silica (SiO2) can form both crystalline quartz and amorphous jasper (Figure 1).
Figure 1: Left – Quartz, a crystalline form of silica and right – jasper, an amorphous form of silica. Source: Wikimedia Commons, photo credits: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 and Stanislav Doronenko.
Activity: Crystals around you
Exploring Minerals and Crystals 3 Rocks are made of a mixture of minerals and often the minerals are in crystalline form. The mineral content of a rock and the size of the mineral crystals determine the physical and chemical properties of a rock. For example, granite forms when magma slowly cools many kilometres below the surface of the Earth. As it cools, crystals of the minerals quartz (white), feldspar (pink or brown) and mica (black) grow. You can see these crystals very clearly with the naked eye although they may not display the regular geometry you associate with crystals. Granite is an igneous rock, it forms from molten material ("ig" is Latin for fire). Most igneous rocks are made of interlocking crystals of a range of sizes.
Basalt is an igneous rock which forms when lava cools on the surface of the Earth. Because the lava cools relatively quickly, crystals do not have time to grow large so are not usually visible to the naked eye (Figure 2). The crystals can be seen in a thin-section under a microscope (see page 18).
Figure 2: Close-up of granite (left) and basalt (right). Granite has larger crystals, while basalt has small crystals only visible through a microscope. Source: Geoscience Australia, photo credits: Chris Fitzgerald.
Activity: The unfair ‘build your own crystal’ race
Activity: Cooling rate and crystal size 1 and 2
4 Exploring Minerals and Crystals
3.1 How do crystals form?
Crystallisation is the process by which crystals are formed. It occurs in many ways:
• Cooling of molten rock to form a crystalline solid. As magmas cool, groups of atoms begin to come together in the chaotic mix and form crystals. The crystals grow as more atoms attach themselves to the initial structure. As the crystals increase in size they meet, causing growth to cease at the points of contact. Growth continues where space is available. Thus the crystals in igneous rocks are irregular in shape and are said to interlock. Common rock-forming minerals that crystallise from molten material include quartz, olivine, mica, hornblende and feldspar. • Crystals can form from a solution when a solvent evaporates. When fluids become a saturated solution (fully loaded) dissolved solids precipitate out of the fluid. Typically this happens as the fluid evaporates, cools down or boils due to changes in pressure. When sodium, chlorine, boron and calcium are dissolved out of rocks they may be carried by rivers to inland seas and lakes which then evaporate, leaving behind mineral deposits of salt, gypsum and boron. • Existing minerals are affected physically by heat and/or pressure (metamorphism) resulting in the growth of new minerals that are stable under the changed conditions. Limestone is a rock made mostly of the mineral calcite. The grains of calcite are normally very small and cannot be seen with the naked eye. If the limestone is heated or subjected to pressure the calcite grains can recrystallise and form larger calcite crystals. When this occurs, the rock is called marble, a metamorphic rock. • Existing minerals can be altered chemically. For example, limestone reacts with fluids from nearby granite intrusions forming new minerals including garnet, wollastonite, pyroxene and amphibole.
Activity: Growing crystals
Minerals will form well developed crystals under natural conditions favourable to crystal growth. However, if the growing crystals interfere with each other they are often poorly formed and distorted. It should be noted that the mineral is still described as crystalline, even though the crystals are imperfectly formed. Even imperfect crystals have an ordered atomic structure and definite chemical composition, despite not having perfectly formed crystal faces.
Activity: Crystallisation of magma
Exploring Minerals and Crystals 5 Minerals with the same chemical composition may have completely different crystal structures. These are known as polymorphs and the difference in structure can influence the physical properties of the mineral. For example, carbon can form diamonds, which are the hardest of all minerals because of their very strong crystal lattice structure. A different polymorph of carbon is graphite, which is very soft and malleable due to weak bonds between the ‘layers’ of atoms (Figure 3).
Figure 3: Uncut diamond and graphite mineral sample (top) with their respective molecular structures (bottom). a) Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. b) Source: Wikimedia Commons, photo credit: Itub. c) Source: Geoscience Australia, image credit: Chris Fitzgerald. d) Source: Wikimedia Commons, image credit: Itub.
6 Exploring Minerals and Crystals
3.2 Extension: Fractional crystallisation
When a molten substance cools to form a crystalline solid, minerals with the highest melting points crystallise first; as this happens the composition of the remaining melt changes (Figure 4).
Figure 4: While cooling, a basaltic magma changes in composition as different minerals crystallise from the melt. 1: olivine crystallises; 2: olivine and pyroxene crystallise; 3: pyroxene and plagioclase crystallise; 4: plagioclase crystallises. Source: Wikimedia Commons, image credit: Woudlouper.
The petrologist Norman Bowen (1887–1956) carried out decades of rock melting experiments in the early 1900s and developed Bowen’s Reaction Series. This lists common rock forming minerals in the sequence that they crystallise from a cooling melt (Figure 5). For example, quartz crystallises at the lowest temperature, therefore last.
Figure 5: This diagram depicts the sequence of mineral crystallization from magma with cooling. Source: Geoscience Australia.
Exploring Minerals and Crystals 7 4 Identifying minerals
Mineralogists are scientists who specialise in the study of minerals, both on Earth as well as those from beyond the Earth, such as rock samples from the Moon or meteorites.
Many mineral names end in ‘ite’. This suffix is derived from the Greek word lithos (from its adjectival form -ites), meaning rock or stone.
Every mineral has two different names:
• Mineralogical, e.g. pyrite, quartz.
• Chemical, e.g. iron sulphide FeS2 or silicon dioxide SiO2.
In addition, well-formed crystals of some minerals will also have a gemmological name, e.g. peridot is clear, gemstone quality olivine.
Some minerals are valuable to us and some are not. It’s useful to be able to identify those we want to use. Most identification is done by assessing the properties of the mineral, observing its features and undertaking a few simple tests. The properties most commonly used in identification of a mineral are colour, streak, lustre, hardness, crystal shape, cleavage, specific gravity and habit. Most of these can be assessed relatively easily even when a geologist is undertaking fieldwork.
4.1 Colour and streak
The colour of minerals can be deceiving. Minerals with the same chemistry can have different colours, usually caused by minor impurities in the crystal. Quartz can be clear, white, pink, purple, grey, red, yellow, green, brown and even black (Figure 6). The mineral corundum (aluminium oxide – Al2O3) is colourless when pure but when it contains chromium and iron it is a red colour and is called a ruby. When it contains iron and titanium it is blue and is called a sapphire.
Figure 6: Varying colours of quartz. From left to right - clear quartz on haematite, amethyst (purple quartz), smoky (black quartz), citrine (yellow quartz) and rose (pink quartz). Source: Wikimedia Commons, photo credits: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
Scientists use the streak rather than the colour of a mineral as a more reliable identification method. The streak is the colour of the powered mineral and is often found by scratching the sample on an unglazed white tile (streak plate). Even with the presence of impurities, the colour of the streak remains consistent.
8 Exploring Minerals and Crystals
Pyrite (fool's gold) is a mineral which has a streak colour very different from the colour of the mineral. This is handy when trying to determine if a sample is real gold or fool's gold. Pyrite will leave a black powder if it is scratched on a white tile, whereas gold will leave a yellow/gold smear (Figure 7).
Figure 7: Streak plates showing a black pyrite streak (top) and a yellow gold streak (bottom). Source: Geoscience Australia, photo credits: Chris Fitzgerald.
4.2 Lustre
Lustre or sheen describes how light is reflected from a mineral’s surface. It indicates what the mineral surface looks like, disregarding its colour. The two basic divisions of lustre are metallic and non- metallic. A number of other words are often used to describe non-metallic lustre – glassy, earthy, pearly, greasy, dull, adamantine (diamond-like), silky and resinous. For example, galena is metallic, haematite is earthy, quartz is vitreous (glassy) and sphalerite is resinous (Figure 8).
Figure 8: Sample of galena showing metallic lustre (left) and sphalerite with resinous lustre (right). Source: Geoscience Australia, photo credits: Chris Fitzgerald.
Exploring Minerals and Crystals 9 4.3 Hardness
Minerals can also be identified by comparing their relative hardness. If a substance is able to scratch another substance, it is harder. A standard hardness scale, called Mohs Scale of Hardness, is named after the scientist Friedrich Mohs (1773-1839) (Table 1). It is simply a relational scale from 1 to 10, where 1 is the softest value and 10 is the hardest. For example, quartz with a hardness of 7 will scratch feldspar which has a hardness of 6. Diamond, hardness 10, will scratch every other mineral.
There is a general link between hardness and chemical composition via a mineral’s crystal structure. The stronger the chemical bonding the harder the mineral, as more energy is required to break the chemical bonds. However, hardness can change depending on which bonds are broken within the crystal structure (i.e. the direction in which it is scratched). For example, when kyanite is scratched in one direction it exhibits a hardness of 4 to 5 but when scratched in a perpendicular direction it exhibits a hardness of 6 to 7.
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Table 1: Mohs scale of hardness with common objects for comparison.
Hardness Mineral or common testing items Photo 1 Talc
Mg3(Si4O10)(OH)2
Pencil lead (graphite) C
2 Gypsum
CaSO4
Fingernail
3 Calcite
CaCO3
Australian Silver Coin (75% copper, 25% nickel)
4 Fluorite
CaF2
Iron nail Fe
5 Apatite
Ca5(F,CL,OH)(PO4)3
Exploring Minerals and Crystals 11 Hardness Mineral or common testing items Photo Glass
SiO2 (+Na or Li)
6 Orthoclase feldspar
K(AlSi3O8)
Steel knife blade
7 Quartz
SiO2
Knife sharpener
7.5-8 Beryl
Be3Al2(SiO3)6
8 Topaz
Al2(SiO4)(F,OH)2
9 Corundum (sapphire/ruby)
10 Diamond
12 Exploring Minerals and Crystals
4.4 Crystal shape
The shape that a mineral will take as it crystallises reflects the internal arrangement of its atoms (Figure 9). Some crystal shapes are particularly recognisable. Many perfect crystal shapes can be described as geometric shapes such as a cube (pyrite, halite), octahedron (magnetite, diamond, fluorite) or dodecahedron (garnet).
Figure 9: Visualisation of a diamond crystal. Source: Geoscience Australia, photo credit: Chris Fitzgerald.
4.5 Cleavage
When crystals break, they split along straight faces called cleavage planes (Figure 10) which are weak due to the atomic structure of the crystal.
Figure 10: Common types of cleavage present in minerals. Source: Wikimedia Commons, image credit: Michael Rygel.
Exploring Minerals and Crystals 13 Mica has one cleavage plane which allows it to break into flat sheets; calcite (CaCO3) has regular cleavage at 60° and 120° resulting in a beautiful rhombic crystal (Figure 11) whereas iron pyrite (FeS2) has 90° cleavage resulting in cubic crystals known by old time miners in Western Australia as “Devil’s dice”.
Figure 11: Calcite rhombohedra up to 5 cm long, Wessels Mine, South Africa. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
Quartz does not have any planes of weakness so does not cleave (split along planes). When it breaks we say that it fractures, leaving glassy conchoidal edges. Stone Age people knew that quartz behaved in this way and learnt to use the fracture to make quartz tools such as knives and arrow heads. Rocks containing lots of silica, such as chert or obsidian, were also used for stone tools and are often called flint.
Activity: Make a goniometer
4.6 Specific gravity
Specific gravity is a measure of the density of a mineral. Imagine a tennis ball and a cricket ball. Although they are about the same size, the cricket ball feels much heavier. This means the cricket ball has a higher specific gravity (is more dense) then the tennis ball, which has a lower specific gravity (is less dense).
Specific gravity compares the weight of a mineral to the weight of the same volume of water. Water has a specific gravity of 1. Gold has a specific gravity of about 19. This means that a certain volume of gold would weigh 19 times more than the same volume of water. If a mineral floats, its specific gravity is less than 1. Precious stones such as diamond, zircon and rubies are easily distinguished by differences in specific gravity.
4.7 Habit (natural shape)
Habit refers to the way a mineral looks when it is found in its natural state (Table 2). It is the characteristic shape in which a mineral commonly grows. For example, although individual magnetite crystals are cubic, the magnetite mineral itself usually grows into a lumpy globular shape. Any particular mineral may display more than one habit.
14 Exploring Minerals and Crystals
Table 2: Mineral habits
Habit Description Common Example(s) Photo Acicular Needle-like, slender Natrolite and/or tapered Rutile Gypsum
Columnar Long, slender prisms Apatite often with parallel Quartz growth Calcite Hematite
Cubic Cube shape Pyrite Galena Halite
Dodecahedral Dodecahedron, 12- Garnet sided
Foliated or lamellar Layered structure, Mica (layered) parting into thin sheets
Globular or Grape-like, Malachite botryoidal hemispherical masses Hematite Pyrite Magnetite
Exploring Minerals and Crystals 15 Habit Description Common Example(s) Photo Hexagonal Hexagon shape, six- Quartz sided
Octahedral Octahedron, eight-sided Diamond (two pyramids base to Fluorite base)
Prismatic Elongate, prism-like. Tourmaline Beryl Azurite
Rosette or lenticular Platy, radiating rose-like Gypsum (desert rose) aggregate Barite Hematite
Stellate Star-like, radiating Pyrophyllite Aragonite
Activity: Mineral detective
16 Exploring Minerals and Crystals
4.8 Extension: Crystal systems
There are six fundamental ways atoms can be arranged in crystals to form the six different crystal systems.
As crystal systems should only be discussed with senior secondary or university students the relevant information can be found in Appendix 1.The system with the greatest symmetry (cubic system) is described first. The other systems are organised so that the one with the least symmetry is described last. The activity ‘Crystal Models’ could be used with primary and high school aged students to discuss three dimensional shapes.
Activity: Crystal models
Exploring Minerals and Crystals 17 5 Using crystals in Earth Science
5.1 Geoscience Australia
Crystallography is the study of the structure and properties of crystals. One of Geoscience Australia’s roles is to produce geological maps. In order to produce maps, Geoscience Australia scientists must identify the rock types which are located in the area they are mapping. This is where crystallography is very useful. Crystallography can be used to help classify rocks by identifying the crystals and the minerals they are made from.
The pictures below show thin-sections of rocks (Figure 12). A thin-section is a section cut from a rock, which has been glued onto a glass slide, then ground down until it is so thin that light can pass through it (only 3/100 of a millimetre — or 30 microns). The different shapes and colours represent different types of minerals.
Figure 12: Left – A photomicrograph of a thin-section of granite under cross polarised light. The width of the view is approximately 4 mm. Main minerals are quartz, potassium feldspar, plagioclase and biotite. Source: Wikimedia Commons, photo credit: Siim Sepp. Right - A photomicrograph of a garnet-grunerite-quartz rock thin-section under cross polarised light. Main minerals are garnet, quartz and amphibole. Source: Geoscience Australia, photo credit: Nathan Daczko.
Geologists can identify rocks and minerals using these thin-sections and a petrographic microscope. This kind of microscope has two polarising filters, one above, and the other below the stage where the thin-section is placed. When the polarising filters are arranged so that they are parallel, the light coming through the thin-section is said to be ‘plane’ light and appears like normal daylight. When the
18 Exploring Minerals and Crystals
polarising filters are crossed at 90 degrees to each other, the light coming through the thin-section is said to be ‘cross polarised’ and each mineral will exhibit characteristic interference colours (Figure 13).
Figure 13: Left - Photomicrograph of a volcanic sand grain in plane-polarised light. Right - Photomicrograph of a volcanic sand grain in cross-polarised light. Source: Wikimedia Commons, photo credit: Qfl247.
Crystals can also help determine the age of rocks. Microscopic grains of zircon, which are commonly found in igneous rocks, can be dated using high-tech instruments. One such instrument is the Australian-invented Sensitive High Resolution Ion Micro Probe (SHRIMP) (Figure 14).
Zircons may contain tiny amounts of the element uranium. Over time, the uranium decays to lead at a known rate. The SHRIMP measures and analyses the amount of uranium and lead isotopes (isotopes are forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei) in samples and using the known decay rates of the uranium isotopes an age for the sample can be calculated (for more information about radioactive dating, refer to Geoscience Australia’s Geological TimeWalk Booklet).
Figure 14: View of the Geoscience Australia SHRIMP instrument. Source: Geoscience Australia.
Exploring Minerals and Crystals 19 6 Mesmerising minerals
6.1 Calcite
Calcite is a common mineral made of calcium carbonate (CaCO3). Most samples of calcite do not show this trigonal shape because the calcite has broken along its cleavage planes. Broken calcite forms perfect rhombohedrons. Some clear calcite samples (known as Icelandic Spars) have the ability to split light passing through them (double refraction) producing a double image (Figure 15). Rocks composed of calcite include limestone and marble. These may be quarried and used as building stone and for statues, or ground up and heated to make cement. Other uses for calcite are acid neutralization, soil conditioner, whitening pigment, animal feed supplement and antacid for upset stomachs!
Figure 15: Icelandic Spar showing double refraction. Source: Geoscience Australia, photo credit: Chris Fitzgerald.
Fossils of trilobites have been found to have eyes made of calcite crystals. They had lenses which focussed light onto clusters of sensory cells lying below them. The resulting image was put together a lot like a picture on your computer screen, with each lens producing one pixel of the whole.
6.2 Quartz
Quartz is one of the most common minerals in the Earth's crust. It is made of silicon dioxide (SiO2), also known as silica. Pure quartz forms hexagonal shaped crystals. Quartz has no cleavage, so when it breaks it does not form straight edges. Quartz can be many different colours including yellow, black, purple, pink or clear. The colours are due to small amounts of impurities in the crystal. For example, amethyst is purple quartz, the colour resulting from small amounts of iron. Rose quartz is coloured by small amounts of titanium. Quartz is very similar in composition and has approximately the same hardness as glass. In fact, glass is made by melting sand (silicon dioxide) together with limestone (calcium carbonate) and soda (sodium bicarbonate). The molten liquid is then cooled very quickly so that the atoms are not able to form into regular arrangements and form crystals.
Many sand grains are made of quartz. This is because quartz is very resistant to weathering. All rock fragments break down physically or chemically, but quartz grains are resistant to weathering and slowly wear down, meaning that they accumulate, eventually forming deposits such as beaches. Quartz is the most common mineral used in glassmaking.
20 Exploring Minerals and Crystals
6.3 Feldspar
Feldspars are an important group of rock-forming minerals that make up as much as 60% of the Earth's crust. Feldspars crystallize in igneous rocks and are also present in many types of metamorphic rocks. Feldspars are also found in some sedimentary rocks.
Feldspar is a common raw material used in glassmaking and ceramics. In glassmaking, feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, parts of the feldspar act as a flux, lowering the melting temperature of a mixture. In Earth science and archaeology, feldspars are used for a variety of dating techniques.
In October 2012, the Mars Curiosity rover analyzed a rock from the surface of Mars and found it had a high feldspar content.
6.4 Gypsum
Gypsum is a white, beige or colourless mineral made of calcium sulphate (CaSO4). Blackboard chalk is made from gypsum. If it occurs as a silky, fibrous form it is commonly called "satin spar". In arid areas, gypsum can occur in a flower-like form, typically opaque, with embedded sand grains called desert rose.
Gypsum is used to make “gyprock” or plasterboard which is commonly used to line the walls in buildings. The Montmartre gypsum quarries near Paris are famous for providing the material. This is where the name "Plaster of Paris" comes from. Gypsum is also used as a soil conditioner. The largest crystals in the world are made of gypsum. They were found in the Naica caves in Mexico and measure up to 12 metres in length (Figure 16).
Figure 16: Gypsum crystals of the Naica cave, Mexico. Note person for scale. Source: Wikimedia Commons, photo credit: Alexander Van Driessche.
Exploring Minerals and Crystals 21 6.5 Pyrite
Pyrite is commonly called fool's gold. It is made from iron and sulphur and has the chemical formula FeS2. It has a shiny yellow appearance, slightly resembling gold. Pyrite crystals are usually cubic shaped, but they can also be 12 sided (dodecahedrons) or 8 sided (octahedrons). While the mineral is yellow/gold in colour, it is much harder than gold and when crushed to a powder it is black. The word pyrite comes from the Greek for ‘fire’, because when the mineral is struck with a hammer, sparks are produced. Pyrite is commonly used to produce sulphuric acid, an important industrial reagent.
6.6 Magnetite
Magnetite is a mineral which is rich in iron. It has the chemical formula Fe3O4. The presence of the iron makes the mineral heavy and magnetic. It usually forms 8 sided (octahedral) crystals - like a double pyramid. Magnetite commonly forms massive granular shapes and appears as if it is not crystalline at all. The mineral was named after the Magnesia region of Thessaly, Greece, the home of the Magnetes (an ancient Greek tribe) and an important centre of iron production. Magnetite is an important ore of iron and is also used as a heavy medium for producing clean coal, as an abrasive (emery paper and in water jet cutters), as toner in photocopiers and laser printers, as a paint pigment and as aggregate in high-density concrete.
22 Exploring Minerals and Crystals
7 Captivating crystals
7.1 Geodes
Geodes are formed when a cavity or space forms in a rock and this space is lined with crystals (Figure 17). The hollows usually form in igneous rocks within trapped gas bubbles. Over time, fluids carrying dissolved minerals flow through the rock and precipitate crystals in the cavity by evaporation or boiling. Crystals stop growing when fluid movement ceases or if the pathway is sealed. This is how thunder eggs, agate and amethyst are commonly formed.
Figure 17: Goethite-Quartz geode (left). Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Agate-Quartz geode (right). Source: Wikimedia Commons, photo credit: Rachel.
7.2 Metallic crystals
Metals have a crystalline structure which is almost impossible to see with the naked eye. One place where the structure is more obvious is on the surface of galvanised iron. If you look carefully at the surface of galvanised iron, you may be able to see the crystals of zinc where the iron has been coated. When nickel-iron meteorites are cut, polished and etched with acid, you can see the most amazing interlocking crystals, referred to as Widmanstatten texture (Figure 18).
Figure 18: Interlocking crystals (Widmanstatten texture) in meteorite from Delegate, New South Wales. Source: Geoscience Australia’s collection R23393, photo credit: Lara Davis.
Exploring Minerals and Crystals 23 7.3 Cultivated crystals
Modern society relies heavily on artificially grown crystals. They are used in the electronics industry for making computer chips and synthetic crystals are used in jewellery. Crystals can be grown under controlled conditions to have required properties suitable to specific applications. Artificial gemstones and rods for lasers are grown by high temperature freezing. Diamond grit is grown at high pressure and quartz is grown from hot water solutions. Crystalline coatings for lenses and optical devices are grown from vapour or solution.
7.4 Glass
Glass is not crystalline. Glass is the result of a material cooling too quickly to allow crystallisation. The atoms in glass do not get a chance to form any sort of regular pattern and they remain in a state of disorder. However, over a very long time, the atoms in glass may start to gather into some form of order. When this happens, glass is said to be devitrified, meaning it has lost its glassy qualities and has started to crystallise. There is no known naturally occurring glass over 300 million years old because after such a long time the atoms have rearranged into some crystalline order. Snowflake obsidian is volcanic glass that has started to devitrify, forming small crystalline snowflakes.
7.5 Luminescent crystals
Some crystals have the ability to give off light. This is called luminescence. There are two types of luminescence: fluorescence and phosphorescence. The process of fluorescence involves absorption of high energy light which is then reemitted at a lower energy (Figure 19).
Figure 19: A portion of the electromagnetic spectrum showing the visible light spectrum and the invisible infrared and ultraviolet ends. Fluorescence is the property of some materials to absorb electromagnetic radiation of one wavelength (high energy e.g. ultraviolet light) and emit radiation of another (usually lower energy e.g. visible light). Source: Geoscience Australia.
Aragonite is an example of a fluorescent mineral. When aragonite is placed under ultraviolet (UV) light it glows pink, yellow, white or blue (Figure 20). Phosphorescent minerals, however, will continue to emit light for a short time after the UV light is removed. Fluorite can be both fluorescent and phosphorescent.
Figure 20: Aragonite, CaCO3 under white (left) and shortwave UV-light (right). Source: Wikimedia Commons, photo credit: H. Zell.
24 Exploring Minerals and Crystals
7.6 Irradiated crystals
Natural radiation from the sun over millions of years can change quartz from a colourless quartz crystal to a brown smoky quartz crystal. This is called irradiation. Irradiation is frequently used to enhance the appearance of gemstones. Beware when you are purchasing precious gemstones. Aquamarine may be a topaz crystal which has been treated to change the colour.
Crystals which change colour in this way are known as photosensitive. Sunlight can change zircon from red to colourless and some violet fluorites become colourless when exposed to natural light.
7.7 Electric crystals
Some crystals are able to generate a charge when they are compressed or bent. This phenomenon is called piezoelectricity. Quartz crystals have this property (Figure 21). When attached to an electric current they vibrate at a certain frequency. Because of this we can use quartz crystals to keep accurate time in watches. Record players also use piezoelectric crystals. The needle picks up vibrations created by the grooves in the record, which are then translated into an electric field through a piezoelectric crystal. This signal is then amplified and transmitted to the speakers. Quartz crystals are also used in ultrasound generators and many electronics, including clocks, microphones, loudspeakers, inkjet printers and motors.
Figure 21: This image shows the parts found inside an ADSL modem/router. Number 12 is a quartz crystal. Source: Wikimedia Commons, photo credit: Mike1024.
7.8 Pseudomorphs
Pseudomorphs are created by mineral substitution. The original substance is gradually removed and simultaneously replaced by another. A common example of this is petrified wood, in which all the cellulose fibres are replaced by silica. The chemistry of the mineral changes, but the structure stays the same. Similarly, pyrite (FeS2) may be replaced during weathering by hematite (Fe2O3) or goethite (FeO.OH), which keeps the same outward shape.
7.9 Solar cells
The sun's light contains energy. Usually, when light hits an object the energy turns into heat, like the warmth you feel while sitting in the sun. But when light hits certain materials the energy turns into an electrical current instead, which we can then harness for power.
Exploring Minerals and Crystals 25 A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity. Most solar technology uses large crystals made out of silicon which produce an electrical current when struck by light (Figure 22). The unit of electrical energy is called a Watt. On average the Earth receives between about 150 and 300 Watts per square metre (W/m2) from the Sun; this is enough to instantaneously power 15 to 30 small compact fluorescent lights. Over the course of a cloud-free day this equates to an energy harvest of between 3.5 and 7 kiloWatthours per square metre (kWh/m2). This is enough energy to power a (very) small house.
Figure 22: Part of a silicon solar cell 100x100 mm. Source: Wikimedia Commons, photo credit: Armin Kübelbeck http://creativecommons.org/licenses/by-sa/3.0/deed.en
Activity: This came from that
26 Exploring Minerals and Crystals
8 Gemstones
8.1 Definition
Gemstones are minerals that possess beauty and durability and have a rarity that makes them desirable. Most gemstones are harder than quartz (7) and cannot be scratched by the blade of a knife. The test of a good gemstone is its resistance to wear and tear. Colour, clarity and the absence of impurities and cleavage are also critical to the final assessment of a crystal to be of gemstone quality.
Gemstones are incredibly rare. There are over 4000 different minerals, yet only 130 are considered gemstones and of these less than 50 are frequently used. The rarest and most valuable of all are diamond, emerald (green beryl), ruby (red corundum) and sapphire (blue corundum) (Figure 23). Gemmologists use the optical properties of crystals (e.g. refraction and birefringence) to identify gemstones.
Figure 23: Well-known gemstones, clockwise from top: sapphire, ruby, emerald, amethyst and diamond. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
Unusual geological conditions are required to create gemstones, which is why they are so rare. Gemstones are often found in igneous rocks. Pegmatite, an intrusive igneous rock, may concentrate rare minerals to form gemstones such as beryl, ruby, sapphire, tourmaline and topaz. Intense metamorphism may create garnet, emerald, jade and lapis lazuli.
Exploring Minerals and Crystals 27 8.2 The 4 C's
Gemstones are valued according to four different criteria: clarity, colour, cut and carat (weight or size).
8.2.1 Clarity
Clarity is the quality most prized in gemstones. A perfect gemstone is a flawless, transparent crystal that sparkles brilliantly as it reflects light internally. Sometimes crystals contain inclusions which are impurities that distort the appearance of the gemstone. Some gemstones, such as star sapphires, pink diamonds and rutilated quartz are valued even more because of these inclusions (Figure 24). Inclusions can be used to identify if a gemstone is naturally formed or synthetically made. Clarity ranges from Internally Flawless to Imperfect.
Figure 24: Anatase inclusions in quartz. Anatase gemstone inclusions are 2 mm across and make this specimen rare and valuable. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
8.2.2 Colour
Bright and intense colour will increase the value of a gemstone. Colourless beryl is only moderately valued, but emerald (green beryl) is one of the world’s most valued stones. Jade, turquoise and lapis lazuli have rich green and blue colours making them hugely sought after.
Many gemstones acquire differing colours due to trace elements contained in the stones. Peridot (gemstone olivine) is commonly green, but can vary from pale lemon to dark olive green. Colourless diamonds are usually the most highly valued, however, diamonds tinted blue or pink are sometimes more valuable because they are so rare; many of the diamonds from the Argyle diamond mine in Western Australia are pink or champagne coloured, increasing their value (see below).
8.2.3 Cut
Most gemstones which are used for jewellery have been cut or faceted (Figure 25). Natural diamond crystals have quite a dull greasy appearance and a double pyramid (octahedral) shape. When the diamond is faceted, more light is reflected and refracted from the crystal making the gemstone sparkle. The most common way diamonds are faceted is called the brilliant cut. This cut allows light to reflect off 58 different surfaces.
The specific angles between cut faces are unique for each type of gemstone. If cut incorrectly gemstones will have less sparkle and consequently be poorer quality. The diagram below show some common gemstone cuts.
28 Exploring Minerals and Crystals
Figure 25: Common gemstone cuts. Source: Geoscience Australia.
8.2.4 Carat
The term carat refers to the weight of a gemstone. One carat = 200 milligrams (1/5 of a gram). The pictures below show the approximate size of diamonds of different weights in carats (Figure 26).
Figure 26: Approximate size of diamonds of different weights. Source: Geoscience Australia.
Using specific gravity we can tell the difference between similar looking gemstones. Diamond has a specific gravity of 3.52 and a cubic zirconia, which looks very similar, has a specific gravity of 5.80. A two carat diamond is larger than a two carat cubic zirconia and very much more expensive.
Exploring Minerals and Crystals 29 8.3 Australian mineral and gemstone resources
Australia, with its long geological history, includes some of the world’s oldest rocks and has a wide range of mineral and gemstone varieties (Figure 27). Some minerals have brought Australia wealth and fame. Gold is one of the most significant of these. During Australia’s gold rush days some of the largest gold nuggets ever found were located in Australia. From 1851 to 2012, Australia produced 12 675 tonnes of gold (at A$45 million a tonne), which was 7.3% of worldwide production (174 000 tonnes).
Figure 27: Gemstone occurrences and mines in Australia. Source: Geoscience Australia.
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8.4 Famous gemstones
8.4.1 Diamonds
Diamonds are the hardest type of gemstones. The largest diamond ever found was 3106 carats (621 grams) and was called the Cullinan. It was found in South Africa and was presented to Edward VII in 1907. The Cullinan was cut into nine large and 100 smaller stones, forming part of the British Crown Jewels.
Australia is the world's largest supplier of natural diamonds, producing 30 million carats per year, but only 5% of these diamonds are of gemstone quality. The rest are used for industrial purposes, such as masonry drill bits and saws and powders for polishing gemstones.
The Argyle Diamond Mine in Western Australia is the world’s largest producer of natural coloured diamonds. The Argyle Diamond Mine was commissioned in 1985 and has produced more than 791 million carats of diamonds. Argyle produces diamonds in a range of colours including white, champagne and blue, but is renowned as the world’s only consistent producer of rare pink diamonds (Figure 28). Argyle produces more than 90% of the world’s supply of pink diamonds.
Figure 28: Pink diamond. Source: Wikimedia Commons, photo credit: Roy Fuchs.
Diamonds have also been mined at Ellendale in Western Australia, in northern New South Wales and more recently in the Northern Territory from the Merlin deposit.
8.4.2 Opals
In Australia, precious opal is found in Cretaceous age sandstones and mudstones (Figure 29) that were deeply weathered under unusual climatic conditions.
Figure 29: Opal in matrix from Queensland. Source: Wikimedia Commons, Photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
Exploring Minerals and Crystals 31 This weathering released silica into groundwater, where it slowly hardened into a gel and settled into layers of sub-microscopic spheres (Figure 30). These spheres produce the amazing colours of opal as the different sphere diameters create various sizes of voids between the spheres approximately equal to the wavelengths of light. Red opals are rare as red light’s wavelength is longer than blue or green, so red would only be created if the opal contained larger silica spheres.
Figure 30: Idealized molecular structure of precious opal: an orderly array of silica spheres. Source: Wikimedia Commons, Photo credit: Dpulitzer.
Australia's national gemstone is the opal and Australia supplies around 95% of the world’s precious opal from large deposits in New South Wales, South Australia and Queensland (at the edges of the Great Artesian Basin). The most famous is black opal from Lightning Ridge in New South Wales (Figure 31).
Figure 31: Black opal from Lightning Ridge, New South Wales. Source: Geoscience Australia, photo credit: Chris Fitzgerald.
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8.4.3 Sapphire
Sapphires are found in a large variation of colours. Blue sapphires are the most common but they can also be green, golden yellow, purple and orange. The latter colours are rarer and command higher prices. Sapphires can sometimes contain crystal needles producing a star reflection. These are called star sapphires (Figure 32).
Figure 32: Blue star sapphire. This is the 182-carat (36.4 g) Star of Bombay, housed in the National Museum of Natural History, Washington D.C. Source: Wikimedia Commons, photo credit: Mitchell Gore.
Sapphire crystals can grow to large sizes. The Star of India, at 563 carats is the most famous star sapphire in the world. It was supposedly discovered more than 300 years ago in Sri Lanka where excellent sapphires are still to be found in deposits of sand and gravel left by ancient rivers. Industrialist and financier J. P. Morgan presented the sapphire to the New York Museum of Natural History in 1900. Today, the Star of India is one of the most renowned objects in all of the Museum's collections.
Australia has been the source of up to 70% of the world’s sapphires, with New South Wales accounting for more than half the production. Sapphire is mined in the New England region and north west of Goulburn in New South Wales and at Anakie in Queensland.
8.4.4 Ruby
Ruby and sapphires are both made of corundum (aluminium oxide – Al2O3). However, ruby deposits are uncommon as they depend on the presence of the rare element chromium. Chromium gives ruby its red colour. Ruby is usually found in metamorphic rocks, such as those in the Harts Range in the Northern Territory. However, a more transparent, gemstone-quality ruby comes from a few basaltic areas in eastern Australia where it has been brought up from underlying metamorphic rocks by volcanic action. Rubies are mined from the same places as sapphires, at Anakie in Queensland and northwest of Goulburn in New South Wales. Ruby is also found as star rubies due to the same reason as star sapphire.
Exploring Minerals and Crystals 33 9 Mineral collections
Collections such as the National Mineral Collection at Geoscience Australia help us understand our environment and form part of Australia’s cultural heritage. The National Mineral Collection is an impressive selection of some 15 000 mineral and 90 meteorite specimens.
Some of the mineral and meteorite specimens were collected by Geoscience Australia scientists while undertaking fieldwork. Others were donated or bequeathed by amateur collectors, purchased or exchanged. A number of collections belonging to the National Museum of Australia are held at Geoscience Australia as part of the National Mineral Collection.
Approximately 600 specimens from the National Mineral Collection are on permanent display at Geoscience Australia in Canberra (Figure 33). Geoscience Australia is located on the corner of Jerrabomberra Avenue and Hindmarsh Drive, Symonston ACT and is open to the public from Monday to Friday between 9am and 5pm.
Figure 33: Part of the National Mineral Collection at Geoscience Australia. Source: Geoscience Australia, photo credit: Lara Davis.
Other exceptional mineral and gemstone collections can be seen at:
• The Crystal Caves - Atherton, Queensland A privately run museum and gift shop located in north Queensland. It has a wonderful large amethyst geode on display. • Terrestrial - The Ted Elliott mineral collection - Georgetown, north Queensland Run by the Etheridge Shire council this display has fine specimens put together by collector Ted Elliott OAM. • Emmaville Mining Museum - Emmaville, NSW A great mining and mineral museum in northern NSW that features many high quality local
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specimens of cassiterite, quartz and beryl. They also have a number of others pieces displayed from private collections. • Albert Kersten Mining and Mineral Museum - Broken Hill, western NSW Contains rare Broken Hill minerals, historical displays and memorabilia. A very large silver nugget and the ‘Silver tree’ sculpture are a highlight. • The Crystal Kingdom - Coonabarabran, NSW A privately run museum and gift shop located in central NSW. It has many large specimens of local minerals on display. The area was renowned for zeolites such as stilbite, stellarite and heulandite in the past. • Australian Mineral and Fossil Museum – Bathurst, NSW Located in the central NSW historic town of Bathurst it showcases the Warren Somerville mineral and fossil collection. It features Australian and overseas mineral specimens, as well as large fossil, including dinosaur, exhibits. • Australian Museum - Sydney, NSW The oldest museum in Australia, it contains wonderful specimens from the earliest days of mining, including the world's best cerussite and large molybdenite specimens. It houses the Albert Chapman collection – one of Australia's finest mineral collections and displays which includes great Broken Hill and Australian pieces. • University of Wollongong - Wollongong, NSW Howard Worner Collection - A comprehensive collection of mineral, rock and fossil specimens donated by Professor Warner to the University of Wollongong. • The Museum of Victoria - Melbourne, Victoria Displays fine classic Australian and overseas specimens that have been assembled since 1854. It has large and fine crystal pieces on display including large gold nuggets. • Queen Victoria Museum and Art Gallery - Launceston, central Tasmania Excellent historical exhibits and Tasmanian mineral collection. • West Coast Heritage Centre - Zeehan, western Tasmania Displays include mining machinery and a great mineral display of west coast minerals such as crocoite and cerussite. Core of the collection is the Frank Mihajlowits collection put together by the ex-owner of the Adelaide Crocoite mine. • The Tasmanian Museum and Art Gallery - Hobart, Tasmania Fine collection of Tasmanian mineral specimens and interesting Antarctic mineral and meteorite displays. • South Australian Museum - Adelaide, South Australia The museum houses a fine collection of Broken Hill minerals and has an excellent educational display of historic and classic locality specimens. • The Western Australian Museum - Perth, Western Australia The museum has a good collection of meteorites with many types well displayed and described.
Activity: Make your own collection
Exploring Minerals and Crystals 35 Glossary
Amorphous Non-crystalline; no regular crystal structure. Atom Smallest unit or building block that is required to make a molecule or substance. Basalt Dark coloured, fine-grained volcanic igneous rock, low in silica content, composed largely pyroxene, and olivine and feldspar. Birefringence The splitting of a single ray of light into two rays (also referred to as double refraction). Birefringent gemstones have two different refractive indices; this makes the optical phenomenon very useful for gemstone dealers to correctly identify certain man-made fakes from real gemstones.
Calcite Common mineral composed of calcium carbonate (CaCO3). Carat A unit used to measure the mass of gemstones: 1 carat = 200 mg. Clarity A measure of internal defects of a gemstone (inclusions). Cleavage Planes of weakness along which a mineral splits when broken. Coherent light Light all of the same wavelength and all in phase (i.e. in step with each other). Crystal shape The shape that a crystallising mineral will take reflects the internal arrangement of its atoms and molecules. Crystal structure The arrangement of atoms or molecules in a material, creating a lattice exhibiting order and symmetry. Crystal A solid mineral enclosed by symmetrically arranged planes. Crystalline Having the structure and form of a crystal. Crystallisation The process by which crystals are formed. Crystallography The study of crystalline solids, including their growth, shape and atomic structure. Element A pure chemical substance consisting of a single type of atom distinguished by its atomic number which is the number of protons in its atomic nucleus. Faceted When a crystal is cut with flat surfaces, it is said to be faceted. Feldspar A common rock forming mineral, it occurs in two main varieties: orthoclase (potassium-rich) and plagioclase (sodium- or calcium-rich). Fluorescence Process by which a material absorbs high energy light (such as UV or X-rays) and re-emits lower energy light (often in the visible spectrum). Gemstone Minerals that possess beauty and durability, usually because they are relatively hard. Gemstone have a rarity that makes them desirable. Geode Rocks which have a hollow space inside them into which crystals have grown. Granite Common igneous rock usually composed of the minerals quartz, feldspar and biotite mica or hornblende. Granite is made of large crystals that grew slowly as magma cooled deep underground. Grains Particles of sediment ranging in size from tiny bits of clay to sand to enormous boulders. Sediments can be transported then deposited by water, wind or ice and this will wear the grains down to smaller sizes.
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Habit The natural shape that a mineral forms - it may not be the same as the crystalline structure of the mineral. Hardness Useful property to help us identify different minerals. When discussing hardness, Mohs scale of hardness ranges from 1-10; if a substance is able to scratch another substance, it is relatively harder and sits higher up the scale. Icelandic Spar Very pure, transparent variety of calcite. It occurs in large easily cleavable crystals that split into rhombohedrons, and is remarkable for its double refraction. The best specimens are from Iceland. Igneous Rocks formed from molten material (magma or lava). Inclusions Impurities inside a crystal. Ion An atom or molecule with a net electric charge due to the loss or gain of one or more electrons. Irradiation Process by which an object is exposed to radiation (most commonly from the Sun). Isotope Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei and hence differ in atomic mass but not in chemical properties. Lustre Lustre or sheen describes how light is reflected from a mineral’s surface. Magma Molten rock. Metamorphism The process of one rock changing to another rock because of heat and/or pressure. Mica A silicate mineral containing iron and magnesium. It forms flat sheets and has a shiny appearance and can be black or colourless. Mineral A naturally occurring substance with a reasonably fixed chemical composition and crystal structure. All minerals have a crystalline form but not all crystals are made of rock-forming minerals (e.g. sugar). Molecule Contains a number of atoms held together by bonds and are the smallest complete unit of a substance i.e. an individual molecule of water contains three separate atoms held together by two bonds. Phosphorescence Emitting light without significant heat by slow oxidation of phosphorous. Photosensitive Crystals which change colour after being exposed to radiation. Photovoltaic cell A device which converts solar radiation into electricity. Also called a solar cell. Piezoelectricity When crystals are able to turn vibrations into an electric current and vice- versa. Polymorph When two or more minerals have the same chemical composition and yet completely different crystal structures. Precipitate When a solid is deposited out of a solution (undissolving). Pseudomorph A crystal made of one mineral but having the appearance of another.
Quartz A relatively hard mineral made of silica (SiO2) and typically occurring as colourless or white hexagonal prisms. It is often coloured by impurities. Recrystallise A metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same.
Exploring Minerals and Crystals 37 Refraction The bending of light as it travels through a gemstone. Refraction (refractive index) provides an important means of gemstone identification and is a main factor influencing brilliance in gemstones. Rock Naturally occurring solid aggregate of one or more minerals or mineraloids. For example, the common rock granite is a combination of quartz, feldspar and biotite or amphibole minerals. Saturated solution A solution in which the maximum amount of a solid has been dissolved. Sediment Naturally occurring material that is broken down by processes of weathering and erosion, and is then transported by the action of wind, water, or ice, consisting or rock or mineral particles. SHRIMP Sensitive High Resolution Ion Micro Probe. Mass spectrometer used to determine the age certain minerals in rocks. Solar cell A device which converts solar radiation into electricity. Also called a photovoltaic cell. Solute The dissolved substance in a solution. Solution A uniform mixture of two or more substances. For example, sugar dissolved in water is a solution. Solvent Able to dissolve other substances. Specific gravity Specific gravity is the ratio of the density of a substance to the density of a reference substance, usually water. Streak The colour of powered mineral, often found by scratching the mineral on an unglazed white tile. Thunder egg Alternate name for geode. Trilobite A fossil marine arthropod that occurred abundantly during the Palaeozoic era (541 to 252 million years ago). Unit cell A unit cell is the simplest repeating unit of regularly arranged atoms in a crystal. Unit cells are used to describe the crystal systems of minerals. Volcanic Igneous rocks that have formed from products of volcanic activity such as lava. Weathering This is the process in which the texture and composition of rocks, sediments and regolith change after being exposed at or near the Earth’s surface to weathering agents such as water, oxygen, organic acids and large temperature fluctuations. Weathering can be chemical or physical (mechanical) and includes changes by the effects of gravity, the atmosphere, the hydrosphere and/or the biosphere at normal temperatures and pressures.
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Resources
Books
Tompkins, D.E. (Editor). 2010. Exploring Earth and Environmental Science Stages 1, 2 and 3, Earth Science Western Australia. Challoner, J, Farndon, J and Walshaw, R. 2004. Rocks, Minerals and the Changing Earth. National Book Network. Laurie, Dr J. (Editor). 2009. Geological TimeWalk. Geoscience Australia, Canberra. Dake, H.C. and De Ment, J. 1941. Fluorescent Light and Its Applications. Chemical Publishing Co., Inc. Brooklyn, N.Y.
Websites
Geoscience Australia Education http://www.ga.gov.au/education Rock Recipes (Splash ABC) Short video (5 mins 13 sec) designed for Year 8. http://splash.abc.net.au/media/-/m/30573/rock-recipes?source=search Exploring Rocks and Minerals (Oresome Resources) Rocks vs minerals and identifying minerals, designed for students in Years 4-9. http://www.oresomeresources.com/resources_view/resource/publication_science_rocks Mike McGarry's Australian Earth and Space Sciences Curriculum Resources Year 8 Module 1 (ASTA) Earth System Science – spheres, crystals, minerals and rocks, properties, designed for Year 8. http://moodle.asta.edu.au/course/view.php?id=46 Crystallography (Australian Museum) Examines crystal systems, symmetry, twinning, structure and chemistry. http://australianmuseum.net.au/Crystallography Gemstones (Australian Museum) Various pages relating to gemstones, their collection, origin, Australian gemstones and treating, cutting and valuing. http://australianmuseum.net.au/Gemstones/ Rocks, Minerals and Resources – Teacher background (WASP) PDF document designed for Year 8 covering minerals. http://www.wasp.edu.au/course/view.php?id=11 Minerals Form Crystals – Teacher Notes and Student Activities (WASP) PDF document designed for Year 8 with accompanying teacher notes. Includes a growing crystals activity. http://www.wasp.edu.au/course/view.php?id=11 Rocks and minerals (Earth Science WA) Teacher notes and student activities on rocks, minerals and crystals, growing crystals and properties of minerals. Designed for upper primary to high school. http://www.earthsciencewa.com.au/course/view.php?id=16
Exploring Minerals and Crystals 39 Cooling rate and crystal size (Earth Science WA). Teacher notes and student activity pages on cooling rate and crystal size designed for high school students. http://www.earthsciencewa.com.au/course/view.php?id=12 The unfair ‘build your own crystal’ race (Earth Learning Idea) Crystal race showing the greater the time available and larger the crystals. Designed for students 11-16 years old. http://www.earthlearningidea.com/PDF/99_Build_crystal_race.pdf Why do igneous rocks have different sized crystals? (Earth Learning Idea) Activity demonstrating crystallisation from a melt at different rates of cooling. Designed for students 11-18 years old. http://www.earthlearningidea.com/PDF/94_Salol.pdf Movie available to complement. http://www.earthlearningidea.com/Flash/Salol.html Salt of the Earth (Earth Learning Idea) Growing salt crystals activity designed for students 8-16 years old. http://www.earthlearningidea.com/PDF/Salt_of_Earth.pdf What are Crystals? (RACI) Information for Year 3 students explaining what crystals are, crystal shapes, how and why crystals grow. http://www.raci.org.au/education/activities/chemistry-activities Crystal Growing Competition (RACI) Competition open to all ages (primary school age to adult). Includes information on how to grow a crystal (background and teacher notes aimed towards high school students and adults). http://www.canberra.edu.au/crystal/information
Table photo credits
Table 1
Talc - Source: Wikimedia Commons, photo credit: Ondrej Pelech. Pencil - Source: Wikimedia Commons, photo credit: Magnus Manske. Gypsum - Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Fingernail – Source: Geoscience Australia, photo credit: Lara Davis. Calcite – Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 . Australian coins – Source: Geoscience Australia, photo credit: Chris Fitzgerald. Fluorite – Source: Wikimedia Commons, photo credit: Daniel Schwen. Nails – Source: Wikimedia Commons, photo credit: W.J.Pilsak at the German language Wikipedia. Apatite – Source: Wikimedia Commons, photo credit: Dave Dyet. Glass jar - Source: Wikimedia Commons, photo credit: Max. Orthoclase feldspar - Source: Wikimedia Commons, photo credit: Dave Dyet. Knife – Source: Geoscience Australia, photo credit: Chris Fitzgerald. Quartz - Source: Geoscience Australia, photo credit: Chris Fitzgerald. Knife sharpener - Source: Wikimedia Commons, photo credit: Simon A. Eugster. Beryl - Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Topaz – Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Corundum – Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Diamond - Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0.
40 Exploring Minerals and Crystals
Table 2
Acicular – Natrolite on inesite. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Columnar – Apatite. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY- SA-3.0. Cubic - Pyrite. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Dodecahedral - Garnet on chalcopyrite. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Foliated - Mica. Source: Geoscience Australia, photo credit: Lara Davis. Globular - Malachite. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY- SA-3.0. Hexagonal – Apatites on quartz. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0. Octahedral – Diamond. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC- BY-SA-3.0. Prismatic – Beryl. Source: Wikimedia Commons, photo credit: Rob Lavinsky, iRocks.com – CC-BY- SA-3.0. Rosette – Desert Rose. Source: Wikimedia Commons, photo credit: Coyau – CC-BY-SA-3.0. Stellate - Pryophyllite. Source: Geoscience Australia, photo credit: Chris Fitzgerald.
Exploring Minerals and Crystals 41