MOST IMPORTANT MINERAL SUITE: The Silicate Minerals

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Presentation transcript:

MOST IMPORTANT MINERAL SUITE: The Silicate Minerals Si + O = 75% of Crust Silicates make up 95% + of all Rocks SiO4: -4 charge Link Corner-To-Corner by Sharing Oxygen atoms

Nesosilicates - Isolated Tetrahedra Representatives: Garnet Kyanite Olivine Zircon Topaz Staurolite Sphene SiO4 in Formula

Sorosilicates - Paired Tetrahedra Epidote is the most common example Lawsonite Vesuvianite Si2O7 in Formula

Cyclosilicates - Rings Beryl (Emerald) Cordierite Tourmaline SinO3n in Formula

What is a Tetrahedron? In many silicates, Al, Be and other ions occur in tetrahedral coordination Substitution of Al for Si maintains charge balance in Feldspars Some mineralogists count all tetrahedra in classifying structure By this criterion, Beryl and Cordierite are tectosilicates

Single Chains (Pyroxenes) SiO3 or Si2O6 in Formula Inosilicates - Chains Single Chains (Pyroxenes) SiO3 or Si2O6 in Formula

More Realistic Picture

Major Pyroxenes Ferromagnesian Calcic Augite: XYZ2O6 Enstatite (MgSiO3) – Hypersthene (Mg,Fe)SiO3 Calcic Diopside: CaMgSi2O6 Pigeonite: Ca0.25(Mg,Fe)1.75Si2O6 Hedenbergite: CaFeSi2O6 Augite: XYZ2O6

Other Pyroxenes Sodic Spodumene: LiAlSi2O6 Jadeite: NaAlSi2O6 Breakdown of Na-Feldspar High Pressure Metamorphism Aegerine (Acmite) NaFe3Si2O6 Spodumene: LiAlSi2O6 Pegmatite Mineral

Pyroxenes and Cations

Silica Chains and Octahedra

Silica Chains and Octahedra

Two Sites in Pyroxenes

Pyroxenoids Have single chains but more complex bends to accommodate cations Lack pyroxene cleavage Wollastonite (CaSiO3) Rhodonite and Pyroxmangite (MnSiO3) Mn is a good match in size and charge for Fe and Mg Pure Mn true pyroxenes exist These have complex chains because of variable compositions

Wollastonite CaSiO3

Wollastonite End-On

Rhodonite

Pyroxmangite

Biopyriboles Biotite = Silica sheets with (Mg,Fe) Sheets in between Amphiboles and Pyroxenes are strips of Biotite sheets Biopyribole = Biotite + Pyroxene + Amphibole

Double Chains (Amphiboles Si4O11 in Formula) Inosilicates - Chains Double Chains (Amphiboles Si4O11 in Formula)

Major Amphiboles Anthophyllite (Mg,Fe)7Si8O22(OH)2) Cummingtonite (Fe2Mg5Si8O22(OH)2) -Grunerite (Fe7Si8O22(OH)2) Tremolite (Ca2Mg5Si8O22(OH)2) – Actinolite (Ca2(Mg,Fe)5Si8O22(OH)2) Hornblende (X2-3Y5Z8O22(OH)2) Glaucophane (Na2Mg3Al2Si8O22(OH)2) and Riebeckite (Na2Fe++3,Fe+++2Si8O22(OH)2)

Beyond Amphiboles There are a few 3-chain and mixed chain silicates Discovered only in 1970’s Occur in asbestos

Phyllosilicates – Sheets (Si2O5 in Formula)

Phyllosilicates - Sheets Si2O5 sheets with layers of Mg(OH)2 or Al(OH)3 Micas Clay minerals Talc Serpentine (asbestos) minerals

Tectosilicates - Three-Dimensional Networks Quartz Feldspars

Tectosilicates Quartz and Polymorphs Feldspars Feldspathoids (Foids) K-Feldspars (KAlSi3O8) Plagioclase (NaAlSi3O8)- (CaAl2Si2O8) Barium Feldspars (BaAl2Si2O8) Feldspathoids (Foids) Scapolites Zeolites

K-Feldspars and Mixtures K-Feldspars (KAlSi3O8) Microcline (Slow cooling) Orthoclase (Faster cooling) Sanidine (Fastest cooling, High T) Anorthoclase: Solid solution of K-Feldspar and Albite (NaAlSi3O8) Perthite: K-Feldspar with exsolved plagioclase Antiperthite: Plagioclase with exsolved K-Feldspar

Plagioclase Solid solution of Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8) 0-10% An = Albite: Na-Metasomatism 10-30% An = Oligoclase: Granites 30-50% An = Andesine: Andesites and Diorite 50-70% An = Labradorite: Basalt and Gabbro 70-90% An = Bytownite: Rarest 90-100% An = Anorthite: Metamorphic

Feldspathoids (Foids) Fill “ecological niche” of feldspars in Si-poor rocks Never occur with quartz Leucite (KAlSi2O6) Nepheline ((Na,K)AlSiO4)

Scapolites and Relatives Sodalite (Na8(AlSiO4)6Cl2) Lazurite (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2) Scapolite Marialite = Albite + NaCl Meionite = Anorthite + CaSO4 or CaCO3