Post on 17-Dec-2015
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Earth MaterialsEarth MaterialsEarth Materials
625-102 GEOLOGY625-102 GEOLOGY
Silicate MineralogySilicate Mineralogy
Lecture 3Lecture 3
Melbourne Earth SciencesMelbourne Earth SciencesMelbourne Earth Sciences
Earth MaterialsEarth MaterialsEarth Materials
Anion groupingsAnion groupings
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l Minerals are Minerals are classified according toclassified according to their their chemistrychemistry
((ieie not symmetry, hardness or any other physical property) not symmetry, hardness or any other physical property)
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l They are They are grouped according togrouped according to theirtheir anionsanions
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l Why?Why?
because minerals rarely contain more than one of these,because minerals rarely contain more than one of these,whereas they often contain several different whereas they often contain several different cationscations
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l Minerals are Minerals are classified into 8 major groupsclassified into 8 major groups
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Classification of MineralsClassification of Minerals
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l Native elementsNative elements - no anions- no anions
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l SulphidesSulphides - S- S2-2- anions, with variations anions, with variations
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l HalidesHalides - halogens (- halogens (ClCl, F; rarely Br, I), F; rarely Br, I)
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l OxidesOxides - O- O2-2- anions (& hydroxides) anions (& hydroxides)
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l CarbonatesCarbonates - CO- CO332-2- etc. etc. polyanionic gpspolyanionic gps
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l SulfatesSulfates - SO- SO442- 2- groups with variationsgroups with variations
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l PhosphatesPhosphates - PO- PO443-3- with variations with variations
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l SilicatesSilicates -SiO-SiO444-4- with structural variations with structural variations
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Native ElementsNative Elements
Native Sulphur, Italy Native Silver, Germany
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SulphidesSulphides
Galena, PbS Marcasite, FeS2
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Halides and OxidesHalides and Oxides
Fluorite, CaF2 Magnetite, Fe3O4
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CarbonatesCarbonates
Malachite, Cu2[(OH)2/CO3])Calcite, CaCO3
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Sulphates, PhosphatesSulphates, Phosphates
Gypsum, CaSO4.2H2O Apatite, CaPO4
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SilicatesSilicates
Olivine, (Fe,Mg)2SiO4 Quartz, SiO2
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Silicate MineralsSilicate Minerals
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l Most abundant mineralsMost abundant minerals in the Earth's crust in the Earth's crust
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l Make up Make up 95% of95% of the the EarthEarths crusts crust
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l Made up of Made up of tetrahedral SiOtetrahedral SiO44 anion groups anion groups
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l PolymerisedPolymerised into into compound groupscompound groups by by
sharing adjacent oxygensharing adjacent oxygen ions ions
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l Linked byLinked by various various cationscations, , espesp..
FeFe2+2+, Mg, Mg2+2+, Ca, Ca2+2+, K, K++, Na, Na++
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l Aluminium (AlAluminium (Al3+3+ ) ) can can substitute for silicon ionssubstitute for silicon ions
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l Result in Result in complex chemical compositionscomplex chemical compositions
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The Basic Silicate UnitThe Basic Silicate Unit
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l Unit behaves as a Unit behaves as a compound anioncompound anion
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l Cation radius = 0.41 Cation radius = 0.41 , Anion Radius = 1.40, Anion Radius = 1.40
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l Radius ratio = 0.29Radius ratio = 0.29, therefore , therefore 4-fold Coordination4-fold Coordination
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l Overall Overall charge of - 4charge of - 4
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l one negative charge for each oxygenone negative charge for each oxygen
SiOSiO44 Tetrahedra Tetrahedra
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Arrangements of silicate groupsArrangements of silicate groups
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l Silicate groups may exist as Silicate groups may exist as isolated tetrahedraisolated tetrahedrajoined by cationsjoined by cations eg Olivine eg Olivine
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l or or joined injoined in more complex more complex polymerised groupspolymerised groups
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l Tetrahedra may be Tetrahedra may be arranged asarranged as
Simple groupsSimple groups
RingsRings
Single chainsSingle chains
Double chainsDouble chains
SheetsSheets
FrameworksFrameworks
3-01
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Simple SiOSimple SiO44 groups groups
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l Silicate groupsSilicate groups are are isolatedisolated from each other from each other
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l May be May be individualindividual or or simple polymerised groupssimple polymerised groups
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l Individual SiOIndividual SiO44 tetrahedratetrahedra are are linked by cationslinked by cations
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l Closely packed, Closely packed, high densityhigh density structuresstructures
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l eg eg OlivineOlivine (Mg,Fe) (Mg,Fe)22SiOSiO4 4 andand ZirconZircon ZrSiO ZrSiO44, Beryl, Beryl
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Single Chain StructuresSingle Chain Structures
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l Each silicate tetrahedron Each silicate tetrahedron shares twoshares two oxygens oxygens
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l Tetrahedra Tetrahedra joined intojoined into a a continuous chaincontinuous chain
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l ChainsChains are are linkedlinked with each other with each other by cationsby cations
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l e.g. the e.g. the Pyroxene groupPyroxene group of minerals of minerals
Diopside:Diopside: CaMgCaMg(Si(Si22OO66))
Augite: Augite: Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66Hypersthene:Hypersthene: MgFeMgFe(Si(Si22OO66))
3-03
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Double Chain StructuresDouble Chain Structures
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l TetrahedraTetrahedra linkedlinked into a into a double chaindouble chain
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l Each SiOEach SiO44 tetrahedron tetrahedron shares 2 or 3 shares 2 or 3 oxygensoxygens
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l Chains are linkedChains are linked with each other with each other by cationsby cations
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l e.g. the e.g. the AmphiboleAmphibole groupgroup of minerals of minerals
3-04
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Sheet SilicatesSheet Silicates
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l Continuous Continuous layer of linked SiOlayer of linked SiO44 groups groups
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l Each SiOEach SiO44 tetrahedron tetrahedron shares threeshares three oxygens oxygens
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l LayersLayers are are stacked and linkedstacked and linked together together by cationsby cations
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l Have Have strong cleavage, generally soft mineralsstrong cleavage, generally soft minerals
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l e.g. the e.g. the Mica Mica andand Clay Clay minerals etc.minerals etc.
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Stacking Silicate SheetsStacking Silicate Sheets
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l The tetrahedral The tetrahedral layers can be stacked togetherlayers can be stacked together in inseveral several different waysdifferent ways, e.g.:, e.g.:
ClaysClays MicasMicas
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Framework SilicatesFramework Silicates
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l Interlocking frameworkInterlocking framework of SiO of SiO44 tetrahedra tetrahedra
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l Each SiOEach SiO44 tetrahedron tetrahedron shares all shares all fourfour oxygens oxygens
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l Basic formula unit is Basic formula unit is SiOSiO22
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l e.g.: e.g.: QuartzQuartz SiO SiO22
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l FeldsparsFeldspars (note role of Al substitution): (note role of Al substitution): OrthoclaseOrthoclase:: KAlSiKAlSi 33OO88 AlbiteAlbite:: NaAlSiNaAlSi33OO88 AnorthiteAnorthite:: CaAlCaAl22SiSi22OO88
3-05
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Feldspar CompositionsFeldspar Compositions
KK
NaNa CaCa
Potassium FeldsparsPotassium Feldspars
Plagioclase FeldsparsPlagioclase Feldspars
No Feldspars occurNo Feldspars occur
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REFERENCESREFERENCES
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l HamblinHamblin & Christiansen, Chapter 3, p.64-72 & Christiansen, Chapter 3, p.64-72
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l Skinner and Porter, Chapter 3, p. 53-59Skinner and Porter, Chapter 3, p. 53-59
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l Clark and Cook, Chapter 6b, p. 116-122Clark and Cook, Chapter 6b, p. 116-122