Lecture 19 (12/04/2006)
Systematic Description of Minerals
Part 3:
Silicates I: Introduction,
Nesosilicates, and Sorosilicates
Predominance of Silicate Minerals
in the Earth’s Crust
27% of all known minerals are silicates
40% of common minerals are silicates
>90% minerals in the earth’s crust are silicates
Silicon Tetrahedra – the basic
building block of silicate minerals
The Si-O bond – 50% covalent, 50% ionic
Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1
Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon
and has an extra -1 charge to share with another cation
Polymerization of
Silicon Tetrahedra
Adjacent silicon tetrahedra
can share corners, but
because of the high repulsive
charge of Si+4 cations, they
will not share edges or faces.
These shared corners are
called bridging oxygens.
Role of Al in Silicate Minerals
Al may occur in tetrahedral (substituting for Si)
or octahedral coordination
Ionic radius of Al+3 = 0.39Å (4-fold) (Si+4=0.26Å)
= 0.54Å (6-fold)
Ionic Al:O Radius Ratio (4-fold) =0.39/1.36=0.286
(Upper limit of tetrahedral coordination RR=0.225)
Ionic Al:O Radius Ratio (6-fold) = 0.388
(Upper limit of octahedral coordination RR=0.414)
Bond strength - e.v. = 3/4 in tetrahedral coord.
= 3/6=1/2 in octahedral coord.
O-coordination and Bond Strength of Other
Common Cations in Silicate Minerals
Electostatic
Valence w/ O-2
1/8 - 1/12 Weak
1/6 - 1/8
1/3 – 1/4
1/3
1/3
1/3
1/2
2/3
1/2
3/4
1
Strong
Silicate Mineral Classification
(based on arrangement of SiO4 tetrahedra)
Nesosilicates (Island)
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x(SiO4) Unit Composition
Isolated, but tightly packed (SiO4)4- tetrahedra
Forms silicate minerals with:
High density and hardness
Equidimensional habits
Poor cleavage
Low degree of Al substitution with Si
Olivine
Garnet
Common Nesosilicates: Olivine
(Mg,Fe)2SiO4
High-T igneous mineral, common in mafic and ultramafic
rocks; commonly alters to serpentine
Vitreous olive green (Mg) to black (Fe)
Equigranular to prismatic habit; poor cleavage
Optics: Colorless, biaxial (+Mg, -Fe), mod. high relief (n~1.7),
high 2V,  ~.05 (2nd order IF colors)
Complete solid solution between Mg and Fe
Common Nesosilicates: Garnet
(Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12
As mod-T metamorphic mineral formed from Al-rich source rocks
and ultramafic mantle rocks (eclogites)
Equigranular, euhderal to subhedral habit; poor cleavage
Optics: Colorless, isotropic, high relief (n~1.7-1.9)
Complex solid solution with the following end-member
compositions and their characteristic colors:
Pyrope Mg3Al2Si3O12 – deep red to black
Almandine Fe3Al2Si3O12 – deep brownish red
Spessartine Mn3Al2Si3O12 – brownish red to black
Grossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black
Uvarovite Ca3Cr2Si3O12 – emerald green
Common Nesosilicates: The Aluminosilicates
Kyanite, Sillimanite, Andalusite
Al2SiO5
Moderate to high grade metamorphic minerals formed from Alrich source rocks
Al in octahedral or a mix of octahedral to tetrahedral sites.
Kyanite – Vitreous bluish bladed tablets
w/ single perfect cleavage; H: 5-7
Sillimanite – Vitreous brown to green
clustered prisms w/ single cleavage dir.
Andalusite – Vitreous flesh-red, reddish brown
square prisms; H: 7.5
Common Nesosilicates: Staurolite
Fe2Al9O6(SiO4)4(O,OH)2
Moderate to high grade metamorphic mineral formed from Al-rich
source rocks
Resinous to vitreous (dull when altered) reddish-brown to
brownish black 6-sided prisms; commonly forms
interpenetrating twins
Optics: Biaxial(-), yellow pleochroic, high relief (n~1.75),
2V=82°-88°
Common Nesosilicates: Sphene (Titanite)
CaTiO(SiO4)
Common accessory mineral in felsic igneous rocks and in some
metamorphic rocks
Resinous to adamantine gray, brown, green, yellow or black lens
crystals; distinct diamond-shaped cleavage; H: 5-5.5
Optics: Biaxial(+), yellow pleochroic, very high relief (n~2.0),
2V=27°,  = 0.13
Sorosilicates (Double)
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x(Si2O7) Unit Composition
Double silicon tetrahedra linked by one bridging
oxygen
Commonly also contains island tetrahedra (SiO4)
Typically monoclinic symmetry
Most important phase of this silicate group is
Epidote/Clinozoisite
Epidote
Structure
Common Sorosilicates: Epidote Group
Zoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH)
Epidote – Ca(Fe,Al)Al2O(SiO4)(Si2O7)(OH)
Common accessory and alteration mineral in igneous rocks and is a
common phase in various grades of metamorphic rocks
Zoisite – Orthorhombic; Clinozoisite and Epidote – Monoclinic
Physical Properties: prismatic vitreous crystals to very fine resinous
massive granules; H: 6-7
Zoisite: Gray, greenish brown (pink-thulite)
Clinozoisite: Gray, pale yellow, pale green, colorless
Epidote: Pistachio green to yellow green,
Optics:
Zoisite: Biaxial(+), high relief (n~1.7), 2V=0-70°,  ~ 0.005
Clinozoisite: Biaxial(+), high relief (n~1.7), 2V=14-90°, ~0.010
Epidote: Biaxial(-), high relief (n~1.75), 2V=74-90°, ~0.015.051, green-yellow pleochroic;
Common Sorosilicates: Lawsonite,
Vesuvianite (Idiocrase)
Lawsonite – CaAl2(Si2O7)(OH)2·H2O
Common mineral in high pressure metamorphic rocks; occurs with
glaucophane, chlorite, and epidote
Vitreous to greasy, bluish tabular to prismatic crystals, commonly
twinned; H: 8
Optics – biaxial(+), mod. high relief (n~1.67), 2V=84°, ~0.02
Vesuvianite –
Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4
Common mineral found in thermally metamorphosed limestone with
garnet, wollastonite (Ca-pyroxene), and diopside (Mg-Ca-pyroxene)
Vitreous to resinous, green to brown, columnar to granular crystals,
commonly striated parallel to columns; H: 6.5
Optics – uniaxial(-), mod. high relief (n~1.67), 2V=84°, ~0.003-.006
Next Lecture 12/06/06
Systematic Description of Minerals
Part 4
Silicates II:Cyclo-, Ino-, Phyllo-, and
Tecto-silicates
Read: Klein Parts of Chaps. 11 & 12