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625-102 GEOLOGY
Lecture 3
Silicate Mineralogy
Melbourne EarthEarth
Sciences
Materials
Anion groupings
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Minerals are classified according to their chemistry
– (ie not symmetry, hardness or any other physical property)
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They are grouped according to their anions
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Why?
– because minerals rarely contain more than one of these,
whereas they often contain several different cations
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Minerals are classified into 8 major groups
Earth Materials
Classification of Minerals
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Native elements
Sulphides
Halides
Oxides
Carbonates
Sulfates
Phosphates
Silicates
- no anions
- S2- anions, with variations
- halogens (Cl, F; rarely Br, I)
- O2- anions (& hydroxides)
- CO32- etc. polyanionic gps
- SO42- groups with variations
- PO43- with variations
-SiO44- with structural variations
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Native Elements
Native Sulphur, Italy
Native Silver, Germany
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Sulphides
Galena, PbS
Marcasite, FeS2
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Halides and Oxides
Fluorite, CaF2
Magnetite, Fe3O4
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Carbonates
Calcite, CaCO3
Malachite, Cu2[(OH)2/CO3])
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Sulphates, Phosphates
Gypsum, CaSO4.2H2O
Apatite, CaPO4
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Silicates
Olivine, (Fe,Mg)2SiO4
Quartz, SiO2
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Silicate Minerals
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Most abundant minerals in the Earth's crust
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Make up 95% of the Earth’s crust
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Made up of tetrahedral SiO 4 anion groups
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Polymerised into compound groups by
sharing adjacent oxygen ions
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Linked by various cations, esp.
Fe2+, Mg2+, Ca2+, K+, Na+
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Aluminium (Al3+ ) can substitute for silicon ions
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Result in complex chemical compositions
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The Basic Silicate Unit
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Unit behaves as a compound anion
Cation radius = 0.41 Å, Anion Radius = 1.40 Å
Radius ratio = 0.29, therefore 4-fold Coordination
Overall charge of - 4
one negative charge for each oxygen
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SiO4 Tetrahedra
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Arrangements of silicate groups
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Silicate groups may exist as isolated tetrahedra
joined by cations eg Olivine
or joined in more complex polymerised groups
Tetrahedra may be arranged as
– Simple groups
– Rings
– Single chains
– Double chains
– Sheets
– Frameworks
3-01
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Simple SiO4 groups
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Silicate groups are isolated from each other
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May be individual or simple polymerised groups
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Individual SiO4 tetrahedra are linked by cations
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Closely packed, high density structures
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eg Olivine (Mg,Fe)2SiO4 and Zircon ZrSiO4, Beryl
3-02
Earth Materials
Single Chain Structures
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Each silicate tetrahedron shares two oxygens
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Tetrahedra joined into a continuous chain
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Chains are linked with each other by cations
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e.g. the Pyroxene group of minerals
Diopside:
CaMg(Si2O6)
Augite:
Ca(Mg,Fe)Si2O6
Hypersthene: MgFe(Si2O6)
3-03
Earth Materials
Double Chain Structures
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Tetrahedra linked into a double chain
Each SiO4 tetrahedron shares 2 or 3 oxygens
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Chains are linked with each other by cations
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e.g. the Amphibole group of minerals
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3-04
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Sheet Silicates
3-05
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Continuous layer of linked SiO4 groups
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Each SiO4 tetrahedron shares three oxygens
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Layers are stacked and linked together by cations
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Have strong cleavage, generally soft minerals
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e.g. the Mica and Clay minerals etc.
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Stacking Silicate Sheets
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The tetrahedral layers can be stacked together in
several different ways, e.g.:
Clays
Micas
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Framework Silicates
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Interlocking framework of SiO4 tetrahedra
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Each SiO4 tetrahedron shares all four oxygens
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Basic formula unit is SiO2
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e.g.: Quartz SiO2
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Feldspars (note role of Al substitution):
Orthoclase: KAlSi3O8
Albite:
NaAlSi3O8
Anorthite: CaAl2Si2O8
3-05
Earth Materials
Feldspar Compositions
K
Potassium Feldspars
No Feldspars occur
Na
Plagioclase Feldspars
Ca
Earth Materials
REFERENCES
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Hamblin & Christiansen, Chapter 3, p.64-72
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Skinner and Porter, Chapter 3, p. 53-59
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Clark and Cook, Chapter 6b, p. 116-122
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