The University Of Melbourne - Minerals

The University of Melbourne - Minerals

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Geology (Engineering Course) Minerals

This page includes the outline text from the classroom PowerPoint presentation, together with selected graphics from that presentation. The graphics will appear when the title hotlink is activated. A copy of the classroom presentation, in .pdf format, can be downloaded from here. MG Chapter 2.1 is a primary reference; W scatters mineral information through several sections; also see basic Geology textbooks. There are several Web sites related to mineralogy, including the Mineralogy Museum at Ecole des Mines de Paris and the Clausthal Institute of Mineralogy . Minerals in the general sense are basic inputs to modern society, and the Mineral Council of Australia is concerned about the lack of science and engineering graduates with interests in this general area - and is spending money to do something about it.

Identification | Mineral Examples | Top Earth - Chemical Composition (MG Chapter 2) 

Dominated by Fe, O, Si, Mg (93%)

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but 

Crustal Chemistry differs

Earth - Crustal Composition 

Processes Creating Crust enrich it in lighter, larger cations so

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Crust chemistry dominated by O, Si (74%)

Important Elements Crust Composition 

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Na, K, Ca (Alkaline elements) are enriched  Enrichment by differentiation Fe, Mg relatively abundant in asthenosphere, and in oceanic crust O, Si are still (more) dominant, however

Minerals (MG 2.1.1) 

Elements occur in "Distinct substances with definite atomic configuration and physicochemical properties" minerals

Minerals 

minerals are "Natural solids with a definite chemical composition and ordered atomic arrangement"

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These solids are almost always crystals. Example: Fluorite Crystals and the crystal structure

Silicates

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Most common minerals are compounds of O with Si and Al Fe Mg Ca Na K

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Over 25% of ~2000 minerals are Silicates Over 90% of Earth's Crust is Silicate

Earth Materials Rocks   

Rocks are collections of one or more minerals Behaviour of rock under stress depends on mineral composition Identification of rock requires identification of minerals

See examples in the UBC Image Gallery - enter rock names you might already know. (Unfortunately, this no longer seems to be an open site - June 2001.) Rock-Forming Minerals Clay Quartz Calcite Olivine Dolomite Pyroxene Amphibole Biotite, Muscovite Micas Orthoclase, Plagioclase Feldspars Other Minerals   

Most important are Oxides, as general class Carbonates are not restricted to calcite and dolomite Sulphides are often significant in, or as, ores

Alternative Division  

Essential minerals - necessary for classification of rock Accessory minerals - don't affect rock classification, but may be important anyway. This term is more frequently used than "esential".

Definitions | Mineral Examples | Top

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Mineral Identification (MG 2.1.1) 

mainly by physical properties, such as  crystal shape  colour, lustre, streak  cleavage or fracture  hardness  crystal habit  density

Mineral Identification 

Other techniques  optical, using thin sections of rocks  x-ray diffraction, to measure the crystal structures  chemical analyses used for detailed examination

Crystallography 

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In unhindered growth, a mineral forms a crystal with a regular pattern of faces, and angles between faces This form is characteristic of the mineral In most rocks growth not free - crystals may not exhibit all faces, but  angles between faces are still characteristic

Mineral Identification Outline 

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Mineral identification based on physical observations can be direct for common minerals Decision trees published in many texts, monographs  Example: Barnes: Earth, Time and Life

Lustre (MG t2.2) 

Lustre is  quality of light reflected from surface of mineral  Division between metallic and non-metallic can help with diagnosis of mineral

Lustre Table

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Example in photographs, but looking at the real thing is vital; Example of vitreous (quartz) and metallic (galena) lustres First Step (These steps show how the observations ascend through a decision tree) 

Lustre is a simple primary observation: simplified to Metallic or submetallic, or Nonmetallic

Hardness (MG t2.1)

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Important diagnostic property Resistance of smooth surface to scratching Relative hardness established by test of one mineral on another

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Moh's Scale is an arbitrary selection, now definitive

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Moh's Scale Hardness Mineral Name 10 Diamond 9 Corundum 8 Topaz 7 Quartz 6 Orthoclase 5 Apatite 4 Fluorspar 3 Calcite 2 Gypsum 1 Talc

Chemical Composition Carbon Alumina Aluminium Silicate Silica Alkali Silicate Calcium Phosphate Calcium Fluoride Calcium Carbonate Hydrated Calcium Sulphate Hydrated Magnesium Silicate

The mineral listed define the steps in the Moh's Scale. Next Step 

Test Hardness

Streak

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Streak is colour of finely-powdered minerals (use streak plate to observe)

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Can be diagnostic, especially of ore minerals

Colour  

Result of light absorption may be affected by trace amounts of elements, especially Fe, Mn, Cu, Cr, Co, Ni, V

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Poor absolute guide, but  division between "light-" and "dark-" coloured minerals is important

See example of sapphires, from Understanding Earth Near Completion 

Note Streak and Colour

The Result 

Note Streak and Colour Pyrite | Magnetite | Hematite

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Cleavage 

Minerals may cleave on specific planes related to atomic structure (often parallel to crystal faces) Presence/absence, ease/difficulty, number of cleavage directions, can all be diagnostic of mineral

Cleavage sketches I Cleavage sketches II Muscovite (1 cleavage plane) and Anhydrite (3 cleavage planes) Cleavage 

Breakage on non-cleavage direction is called fracture-may indicate glassy material like obsidian, but may also occur in cryptocrystalline materials

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There is a second meaning, for planes formed in rock mass in response to pressure.

Cleavage assists Add Colour The Decision Talc | Muscovite | Biotite | Kaolinite Habit (MG t2.3)    

Refers to particular shape of crystals More descriptive than diagnostic, but may indicate mode of formation, and so give clues for identification Description also gives clue to rock material behaviour, so crystal habit is still useful

A few habits Botroyoidal (malachite) | Foliated (mica) | Fibrous (gypsum) | Massive and Granular (dolomite)

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Asbestos A group of silicates with fibrous habit is collectively known as asbestos. Humans exposed to the variety crocidolite have a high risk of developing cancer (mesothelioma). Crocidolite is an amphibole, which forms needle-like crystals with pointed ends. There does not seem to be significant risk associated with the variety chrysotile, which has been far more commonly used. The fibrous habit arises even though chrysotile is a sheet silicate, because the silicate sheets curl up into tubes to form the fibres. It may not be necessary to incur significant expense in removing asbestos if the asbestos variety is chrysotile. (The variety mined at Wittenoom in Western Australia, with disastrous human effects, was crocidolite.) Some forms of crocidolite are sought as semi-precious gems ("Tiger-eye") but not in fibrous habit form. (Actinolite and tremolite are two other amphiboles which belong to the asbestos group.) Other Descriptors May be useful in particular cases     

Density - where mineral is large enough Taste - halite (cinnabar?) Magnetics - especially magnetite Acid Test - effective for calcite, some sulphides Texture - (See MGp9)

Definitions | Identification | Top

Minerals A survey of common examples, illustrated here by uncommon specimens.

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You can view interactive models of many mineral crystal structures here. Common Silicates (MG 2.1.2) 

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Two major groups, based on chemistry  Mafic minerals are iron and magnesium silicates, and are darkcoloured  Light-coloured Felsic minerals contain quartz and/or aluminium silicates  Aluminium silicates may contain K, Na, or Ca, and are known as Feldspars Colour aids identification!

Silicate Structure 1 

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Basic component of all silicate minerals is SiO4(4-) group, forming tetrahedron Classification reflects polymerisation.

Silicate Structure Overview 

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Structure based on polymerisation  Island Structures  Single-Chain Structures  Double-Chain Structures  Sheet Silicates  Framework Silicates Properties reflect structure

Island Silicates 

4 negative charges might be neutralized by positive cations alone  Mg2[SiO4] - Olivine is an example. Both Olivine and Garnet (another silicate in which the tetrahedron charges are satisfied by cations) form stubby or equidimensional crystals which are relatively hard.

Silicate Polymers 

Tetrahedra may form "polymers" by sharing Oxygen ions at  2 corners, forming a single chain (pyroxenes)  5 of 8 O atoms in 2 tetrahedra, forming a double chain (amphiboles)

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3 corners, forming sheets (micas and clays) all 4 corners, forming networks (quartz and feldspars)

Pyroxenes  

Single-chain silicates SiO3(2-) unit bonds with Mg, Fe, Ca Augite is a common example, but readily alters (why?)

Pyroxenes   

Cations also bond adjacent chains (more weakly) Cleavage develops on 2 planes parallel to chains Habit typically prismatic

Augite Cleavage Amphiboles  

Double (linked) chain silicate Si4O11 "unit" bonds with many cations; detailed formula vague example: Ca2(Mg,Fe)5Si8O22(OH)2 (Actinolite)

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Cleavage on 2 planes, at ~ 120°, parallel to chains

Hornblende Cleavage Tremolite example Micas   

Sheet silicate Substitution of Al for Si common Examples Muscovite (light) - KAl2(AlSi3O10)(OH)2 Biotite (dark) - K(Mg,Fe)3(AlSi3O10)(OH)2

Micas  

Sheet silicate Excellent cleavage into sheets

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Habit typically foliated (micaceous!)

Biotite Structure  

Mg, Fe link two sheets, forming unit K bonds units (weakly)

Biotite Habit  

One perfect cleavage, parallel to the basal face Muscovite is another mica, with the same habit, but light coloured - why?

Chlorite   

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Another sheet silicate, with (Fe, Mg) cations Greenish colour, brittle, similar to other micas Rarely primary mineral in igneous rocks, but result of some kind of alteration of primary minerals Typically metamorphic mineral, but may also be found as joint-filling mineral in weathering rocks Perfect cleavage can provide slip plane to reduce friction in joints.

Clays   

Also sheet silicates Typical "unit" contains one or two sheets of SiO4 tetrahedra, as in micas. Unit also contains "octahedral" layer built of Al or Mg and six (OH)anions

Clays 

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Clay variety depends on bonding:  1 sheet of SiO4 tetrahedra - Kaolinite  2 sheets of tetrahedra - Montmorillonite Usually products of alteration from original silicates

Clay Minerals 

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Crystals typically submicroscopic, so few visible crystal samples available Layers bonded by secondary forces/atoms (not covalent bonding) Structure therefore depends on ambient conditions

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Feldspars (MG p16)   

Widely-occurring, light-coloured Framework silicate structure Plagioclase feldspars most common mineral in igneous rocks NaAlSi3O8 (albite) to CaAl2Si2O8 (anorthite)

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Hard, stubby crystals, often gray with two good cleavages, distinguished from Orthoclase (K-feldspar) by frequent observation of twinning on cleavage face.

Feldspars 2  

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K-feldspar or Orthoclase feldspar common in Si-rich rocks Habit stubby, slightly less hard than quartz, may be pink in colour, two good cleavages at right angles Feldspars weather relatively easily, directly to clay minerals

Quartz!    

Also framework silicate No cleavage, no cations, so relatively stable Only mineral found widely in igneous, sedimentary, metamorphic rocks Crystallizes at late stage, so sheets, veins, geodes also found

Non-Silicate Minerals 

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Two important carbonates  Calcite CaCO3  White, H=3, Perfect cleavage  Dolomite CaMg(CO3)2  White, H=3, Perfect cleavage Oxides, particularly of Iron  Magnetite, Hematite, Limonite

Other samples of Hematite and Magnetite are reminders that minerals do not always (usually!) exist as cabinet crystal specimens.

Alteration (MG p19) and Weathering (MG p60)   

Related, not equivalent terms Primary minerals may change due to changed ambient conditions Recognition of alteration, weathering necessary to understand changes in rocks

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Alteration and Weathering 

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Many minerals formed at high P, T  Earth interior also reducing environment May be unstable under changed (P,T,water) conditions, so will alter  Olivine > Serpentinite (MGp19)  FeMg Minerals > Chlorite > Clays If minerals change, so will rocks

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