lecture 16 silicates i mod 9 corrected
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Lecture 16
Systematic Description of Minerals
Part 3: Silicates I: Introduction to Silicates,Nesosilicates, and Sorosilicates
Pyrope
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Predominance of Silicate Mineralsin the Earth’s Crust
CRUST MOSTLY Oxygen O and SilicoN Si27% of all known minerals are silicates40% of common minerals are silicates>90% minerals in the earth’s crust are silicates
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Silicon Tetrahedra – the basicbuilding 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 siliconand has an extra -1 charge to share with another cation
Four (4) oxygens in each tetrahedron, so total charge -4
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Polymerization ofSilicon Tetrahedra
Adjacent silicon tetrahedracan share corners, butbecause of the high repulsivecharge of Si+4 cations, theywill not share edges or faces.These shared corners arecalled bridging oxygens.
Oxygens can share electrons with two silicons
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Role of Al in Silicate MineralsAl+3 may occur in tetrahedral [4] (substituting for Si+4)
or octahedral [6] 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.
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O-coordination and Bond Strength of OtherCommon Cations in Silicate Minerals
ElectostaticValence w/ O-2
1/8 - 1/121/6 - 1/81/3 – 1/42/6 = 1/32/6 = 1/32/6 = 1/3
3/6 = 1/24/6 = 2/33/6 = 1/2
3/44/4 = 1
Weak
Strong
big
medium
small
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Silicate Mineral Classification(based on arrangement of SiO4 tetrahedra)
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Nesosilicates (independent tetrahedra)• X2(SiO4) Unit Composition X often +2 valence• Isolated, but tightly packed (SiO
4
)4- tetrahedra• Forms silicate minerals with:
High density and hardnessEqui-dimensional habitsPoor cleavage
• Low degree of Al substitution
with Si
Olivine X = Mg+2 or Fe+2
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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 (positive if Mg++, negative if Fe++), mod. highrelief (n~1.7), high 2V, ~.05 (2nd order IF colors)
Complete solid solution between Mg and Fe
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Common Nesosilicates: Zircon
Zircon is ZrSiO4. Hafnium is almostalways present in quantities ranging from1 to 4%. The crystal structure of zircon istetragonal. The natural color of zirconvaries between colorless, yellow-golden,red, brown, and green.
Zircon usually contains radioactive Uraniumand Thorium, and is frequently used to date
plutonic rocks.
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Common Nesosilicates: Garnet(Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12
As mod-T metamorphic mineral formed from Al-rich source rocks andultramafic 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 andtheir characteristic colors:
Pyrope Mg3Al2Si3O12 – deep red to black
Almandine Fe3Al2Si3O12 – deep brownish red
Spessartine Mn3Al2Si3O12 – brownish red to blackGrossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black
Uvarovite Ca3Cr2Si3O12 – emerald green
Almandine
Grossular
Andradite
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Garnet A3B2Si3O12
Usually B is Aluminum, A divalent
Almandine Fe3Al2Si3O8
B-site
Aluminumoctahedral
A-site Fe++,Mg++, Ca++,
Mn++ indistortedoctahedra
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Common Nesosilicates: The Aluminosilicates
Kyanite, Sillimanite, Andalusite
Al2SiO5
Moderate to high grade metamorphic minerals formed from Al-rich sourcerocks
Al in octahedral or a mix of octahedral to tetrahedral sites.
Kyanite – Vitreous bluish bladed tabletsw/ single perfect cleavage; H: 5-7
Sillimanite – Vitreous brown to greenclustered prisms w/ single cleavage dir.
Andalusite – Vitreous flesh-red, reddish brownsquare prisms; H: 7.5
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Common Nesosilicates: StauroliteFe2Al9O6(SiO4)4(O,OH)2
Moderate to high grade metamorphic mineral formed from Al-rich sourcerocks
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°
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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
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Common Nesosilicates: Topaz
Topaz Al2SiO4(F,OH)2, Orthorhombic prismaticterminated by pyramidal and other faces, thebasal pinacoid often being present. Perfect basal{001} cleavage The fracture conchoidal touneven. Hardness 8, specific gravity 3.4 –3.6, anda vitreous luster.
Color wine or straw-yellow. They may also bewhite, gray, green, blue, pink, or reddish-yellowand transparent or translucent.
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Sorosilicates (double tetrahedra)
• Double silicon tetrahedra linked by one bridging oxygen• Commonly also contains island tetrahedra (SiO4)
• Typically monoclinic symmetry
Epidote
Si2O7
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Common Sorosilicates: Epidote GroupZoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH)
Epidote –
Ca2(Fe,Al)Al2O(SiO4)(Si2O7)(OH) Common accessory and alteration mineral in igneous rocks and is a common phase invarious grades of metamorphic rocks
Zoisite – Orthorhombic; Clinozoisite and Epidote – Monoclinic
Physical Properties: prismatic vitreous crystals to very fine resinous massive granules;
H: 6-7Zoisite: 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-yellowpleochroic;
Epidote
Zoisite
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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 tocolumns; H: 6.5
Common Sorosilicate: Vesuvianite(aka Idocrase)
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Common Sorosilicates: HemimorphiteHemimorphite, is a sorosilicate,
Zn4(Si2O7)(OH)2.H2O from the upper partsof zinc and lead ores, chiefly associatedwith Smithsonite.
Hemimorphite most frequently occurs as
the product of the oxidation of the upperparts of Sphalerite (ZnS) bearing orebodies, accompanied by other secondaryminerals which form the so-called iron cap or gossan . Hemimorphite is an importantore of zinc and contains up to 54.2% of
the metal.