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MG Chapter 2.1 is a primary reference; W scatters mineral information through several sections; also see basic Geology textbooks.

• Dominated by Fe, O, Si, Mg (93%)

  but

• Crustal Chemistry differs

• Processes Creating Crust enrich it in lighter, larger cations

  so

• Crust chemistry dominated by O, Si (74%)

• 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

• Elements occur in

  "Distinct substances with definite atomic configuration and physico-chemical properties"

  minerals

• minerals are

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

• These solids are almost always crystals.

Example: Fluorite Crystals and the crystal structure

Silicates

• Most common minerals are compounds of

  O with Si
  and Al Fe Mg
  Ca Na K

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

• 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.)

Clay
Quartz
Calcite
Olivine
Dolomite
Pyroxene
Amphibole
Biotite, Muscovite Micas
Orthoclase, Plagioclase Feldspars

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

• 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".

Mineral Identification (MG 2.1.1)

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

• Other techniques
  • optical, using thin sections of rocks
  • x-ray diffraction, to measure the crystal structures
  • chemical analyses

  used for detailed examination

• 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 based on physical observations can be direct for common minerals
• Decision trees published in many texts, monographs
  • Example:

    Barnes: Earth, Time and Life

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

Example in photographs, but looking at the real thing is vital;

Example of vitreous (quartz) and metallic (galena) lustres

(These steps show how the observations ascend through a decision tree)

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

• Important diagnostic property
• Resistance of smooth surface to scratching
• Relative hardness established by test of one mineral on another
  
  
• Moh's Scale is an arbitrary selection, now definitive

Moh's Scale

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

• Test Hardness

Streak

• Streak is

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

• Can be diagnostic, especially of ore minerals

• Result of light absorption
• may be affected by trace amounts of elements, especially

  Fe, Mn, Cu, Cr, Co, Ni, V

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

   

  See example of sapphires, from Understanding Earth

• Note Streak and Colour

• Note Streak and Colour

Pyrite | Magnetite | Hematite

• 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

• Breakage on non-cleavage direction is called fracture-may indicate glassy material like obsidian, but may also occur in cryptocrystalline materials
  
  
• There is a second meaning, for planes formed in rock mass in response to pressure.

• 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

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.)

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)

Minerals

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

You can view interactive models of many mineral crystal structures here.

• Two major groups, based on chemistry
  • Mafic minerals are iron and magnesium silicates, and are dark-coloured
  • 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!

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

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

• 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.

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

• Single-chain silicates
• SiO3(2-) unit bonds with Mg, Fe, Ca

  Augite is a common example, but readily alters (why?)

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

• Double (linked) chain silicate
• Si4O11 "unit" bonds with many cations; detailed formula vague

  example: Ca2(Mg,Fe)5Si8O22(OH)2 (Actinolite)

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

Tremolite example

• Sheet silicate
• Substitution of Al for Si common
• Examples

  Muscovite (light) - KAl2(AlSi3O10)(OH)2
  Biotite (dark) - K(Mg,Fe)3(AlSi3O10)(OH)2

• Sheet silicate
• Excellent cleavage into sheets
• Habit typically foliated (micaceous!)

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

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

Chlorite

• 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

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

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

• Widely-occurring, light-coloured
• Framework silicate structure
• Plagioclase feldspars most common mineral in igneous rocks

  NaAlSi3O8 (albite) to CaAl2Si2O8 (anorthite)
  

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

• 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

• 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

• 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.

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

• 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

Clays and Parent Silicates

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