MOST IMPORTANT MINERAL SUITE: The Silicate Minerals Si + O = 75% of Crust Silicates make up 95% + of...

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

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

More Realistic Picture

Major Pyroxenes• Ferromagnesian

– Enstatite (MgSiO3) – Hypersthene (Mg,Fe)SiO3

• Calcic– Diopside: CaMgSi2O6

– Pigeonite: Ca0.25(Mg,Fe)1.75Si2O6

– Hedenbergite: CaFeSi2O6

• Augite: XYZ2O6

Other Pyroxenes• Sodic

– 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

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

– 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