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Lecture 24:Lecture 24:
OrthosilicatesOrthosilicates && SorosilicatesSorosilicates
ReadRead ChptChpt 1010
ORTHOSILICATES:
!Structure: isolated [SiO4] tetrahedraheld together by interstitial
cations
!Distinguishing characteristics: high specific gravity(dense
packing); high hardness; isotropic structure(no pronounced
cleavage)
!3 Important Groups: Olivines, Garnets, Al2SiO5polymorphs
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3 Important Groups:
(1) Olivine (M2VI
SiO4)
Most important:
Fayalite Forsterite
Fe2SiO4"Mg2SiO4
Complete solid solutionbetween these two endmember
minerals
Nearly ideal(linear physical properties) solid solution
(Fe"Mg) between two endmembers
Also occurs:
Monticellite Kirschteinite
CaMgSiO4"CaFeSiO4
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3 Important Groups:
(1) Olivine ((Mg,Fe)2
VISiO4)
! Occurrence: ultrabasic igneous rocks(peridotite, gabbro,
basalt, dunite); metamorphosed serpentinites
!Physical properties: glassy luster, conchoidalfracture, greencolor, granular, high melting point, H-6.5-7, easily altered
! Structure: 2 M sites for cations; M1= slightly distorted
tetrahedron, M2=slightly larger, more distorted tetrahedron; at
high T there is solid soln. between the two series
! Symmetry: Orthorhombic
! Uses: refractory sands, peridot (semiprecious gem)
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3 Important Groups:
(2) Garnet (A3
VIIIB2
VI(SiO4)
3)
Lots of solid solutionin the A and B sites
Ugrandite series has A=Ca+2
Uvarovite: Ca3Cr2(SiO4)3Grossular: Ca3Al2(SiO4)3Andradite: Ca3Fe2(SiO4)3
Pyralspite series has B=Al+3(and ! Ca+2)
Pyrope: Mg3Al2(SiO4)3Almandine: Fe3Al2(SiO4)3Spessartine: Mn3Al2(SiO4)3
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3 Important Groups:
(2) Garnet (A3VIIIB2VI(SiO4)3)
! Occurrence: accessory mineralin igneous rocks; common
index mineral(indicative of P,T conditions of formation) in
metamorphic rocks
! Physical properties: brown-red(due to Fe, Mn, Cr);
dodecahedral(12-sided) shape with diamond-shaped faces is
common
Some colors: pyrope (pink/purple), spessartine (brownish
red), almandine (deep red)
! Symmetry: Isometric
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Almandine
Salida, ColoradoPhoto by Mary Sutherland
James Madison U.
Grossular
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Uvarovite
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3 Important Groups:
(3) Al2SiO5Group
3 polymorphs: Kyanite, Andalusite, Sillimanite
KYANITE
SILLLIMANITE
ANDALUSITE
P
T
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3 Important Groups:
Al2SiO5Group: Kyanite (AlVIAlVISiO5)
!Occurrence: common in metamorphic rocks, often associatedwith garnet, corundum, staurolite, high pressure, dense phase
!Physical properties: bladed crystals, blue color,hardness
varieswith direction (H=5 parallel to crystal; H=7 perp to crystal);
good {100} cleavage; vitreous to pearly luster
!Symmetry: triclinic
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KyaniteNamibia
University of Bristol
Kyanite
Brazil
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Andalusite, variety Chiastolite
Brazil
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Andalusite
Brazil
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3 Important Groups:
Al2SiO5Group: Sillimanite (AlIVAlVISiO5)
!Occurrence: common in high Tmetamorphic rocks (rarest of the3 polymorphs)
!Physical properties: slender crystals or fibrous aggregates;
white, brown or green color; perfect {010} cleavage
!Symmetry: orthorhombic
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Sillimanite
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Sillimanite
Brazil
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Other important orthosilicate minerals:
Zircon (ZrSiO4):used for radiometric dating; source of Zr
Topaz (Al2SiO4(OH,F)2):blue gem
Staurolite:high grade metamorphic index mineral
Chloritoid:low grade metamorphic index mineral
Titanite (sphene):common accessory mineral in igneous rocks
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ZirconZircon
StauroliteStaurolite
TopazTopaz
Keivy, Kola Peninsula Russia
Photo Mary Sutherland, James Madison U.
Marambaia Mina Gerais, Brazil
Photo Mary Sutherland, James
Madison U.
Quebec, Canada
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SOROSILICATES
!from [SiO4] joined to form dimers!Example:
Epidote(Ca2(Al,Fe) Al2OSiO4Si2O7(OH)): typically occurs in
metamorphic rocks; distinguished by pistachio- or apple-green
color
Epidote
Peru
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