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