Lecture 16 Silicates I Mod 9 Corrected

8/4/2019 Lecture 16 Silicates I Mod 9 Corrected

http://slidepdf.com/reader/full/lecture-16-silicates-i-mod-9-corrected 1/22

Lecture 16

Systematic Description of Minerals

Part 3: Silicates I: Introduction to Silicates,Nesosilicates, and Sorosilicates

Pyrope



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



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





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



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.



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



Silicate Mineral Classification(based on arrangement of SiO4 tetrahedra)



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



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



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.



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



Garnet A3B2Si3O12

Usually B is Aluminum, A divalent

Almandine Fe3Al2Si3O8

B-site

Aluminumoctahedral

A-site Fe++,Mg++, Ca++,

Mn++ indistortedoctahedra



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



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°



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



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.



Sorosilicates (double tetrahedra)

• Double silicon tetrahedra linked by one bridging oxygen• Commonly also contains island tetrahedra (SiO4)

• Typically monoclinic symmetry

Epidote

Si2O7



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

http://www.gc.maricopa.edu/earthsci/imagearchive/epidote.jpg

http://www.gc.maricopa.edu/earthsci/imagearchive/epidote.jpg



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)



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.



Next Lecture

Systematic Description of Minerals

Part 4Silicates II:Cyclo-, Ino-, Phyllo-, and

Tecto-silicates

Lecture 16 Silicates I Mod 9 Corrected

Documents