24 Orth Silicatezxs

8/13/2019 24 Orth Silicatezxs

1/17

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


2/17

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


3/17

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)


4/17


5/17

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


6/17

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


7/17

Almandine

Salida, ColoradoPhoto by Mary Sutherland

James Madison U.

Grossular

Mineral Galleries

Uvarovite

Mineral Galleries


8/17

3 Important Groups:

(3) Al2SiO5Group

3 polymorphs: Kyanite, Andalusite, Sillimanite

KYANITE

SILLLIMANITE

ANDALUSITE

P

T


9/17

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


10/17

KyaniteNamibia

University of Bristol

Kyanite

Brazil

Mineral Galleries


11/17


12/17

Andalusite, variety Chiastolite

Brazil

Mineral Galleries

Andalusite

Brazil

Mineral Galleries


13/17

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


14/17

Sillimanite

Mineral Galleries

Sillimanite

Brazil

Mineral Galleries


15/17

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


16/17

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

Mineral Galleries


17/17

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

Mineral Galleries

Documents

24 Orth Silicatezxs