Lecture 19 (12/04/2006)Lecture 19 (12/04/2006)

Systematic Description of MineralsSystematic Description of Minerals

Part 3: Part 3: Silicates I: Introduction, Silicates I: Introduction,

Nesosilicates, and SorosilicatesNesosilicates, and Sorosilicates

Predominance of Silicate Minerals Predominance of Silicate Minerals in the Earth’s Crustin the Earth’s Crust

27% of all known minerals are silicates27% of all known minerals are silicates40% of common minerals are silicates40% of common minerals are silicates>90% minerals in the earth’s crust are silicates>90% minerals in the earth’s crust are silicates

Silicon TetrahedraSilicon Tetrahedra – the basic – the basic building block of silicate mineralsbuilding block of silicate minerals

The Si-OThe Si-O bond – 50% covalent, 50% ionicbond – 50% covalent, 50% ionic

Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1Electrostatic Valence (e.v., measure of bond strength)=Z/CN=4/4 =1

Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon Each tetrahedral oxygen shares a -1 charge with the tetrahedral silicon and has an extra -1 charge to share with another cationand has an extra -1 charge to share with another cation

Polymerization of Polymerization of Silicon TetrahedraSilicon Tetrahedra

Adjacent silicon tetrahedra Adjacent silicon tetrahedra can share corners, but can share corners, but because of the high repulsive because of the high repulsive charge of Sicharge of Si+4+4 cations, they cations, they will not share edges or faces. will not share edges or faces. These shared corners are These shared corners are called called bridging oxygens.bridging oxygens.

Role of Role of AlAl in Silicate Minerals in Silicate MineralsAlAl may occur in tetrahedral (substituting for Si) or may occur in tetrahedral (substituting for Si) or

octahedral coordinationoctahedral coordination

Ionic radius of Ionic radius of AlAl+3+3 = 0.39Å (4-fold) = 0.39Å (4-fold) (Si(Si+4+4=0.26Å)=0.26Å) = 0.54Å (6-fold)= 0.54Å (6-fold)

Ionic Ionic AlAl:O Radius Ratio (4-fold) =0.39/1.36=0.286:O Radius Ratio (4-fold) =0.39/1.36=0.286(Upper limit of tetrahedral coordination RR=0.225)(Upper limit of tetrahedral coordination RR=0.225) Ionic Ionic AlAl:O Radius Ratio (6-fold) = 0.388:O Radius Ratio (6-fold) = 0.388(Upper limit of octahedral coordination RR=0.414)(Upper limit of octahedral coordination RR=0.414)

Bond strength - e.v. = 3/4 in tetrahedral coord.Bond strength - e.v. = 3/4 in tetrahedral coord.= 3/6=1/2 in octahedral coord.= 3/6=1/2 in octahedral coord.

O-coordination and Bond Strength of Other O-coordination and Bond Strength of Other Common Cations in Silicate MineralsCommon Cations in Silicate Minerals

ElectostaticElectostaticValence w/ OValence w/ O-2-2

1/8 - 1/121/8 - 1/121/6 - 1/81/6 - 1/81/3 – 1/41/3 – 1/41/3 1/3 1/31/31/31/31/21/22/32/31/21/23/43/411

WeakWeak

StrongStrong

Silicate Mineral ClassificationSilicate Mineral Classification(based on arrangement of SiO(based on arrangement of SiO44 tetrahedra) tetrahedra)

Nesosilicates (Island)Nesosilicates (Island) x(SiOx(SiO44) Unit Composition ) Unit Composition Isolated, but tightly packed (SiOIsolated, but tightly packed (SiO44))4- 4- tetrahedratetrahedra Forms silicate minerals with: Forms silicate minerals with:

High density and hardnessHigh density and hardnessEquidimensional habitsEquidimensional habitsPoor cleavagePoor cleavage

Low degree of Al substitution with SiLow degree of Al substitution with Si

OlivineOlivine GarnetGarnet

Common Nesosilicates: Common Nesosilicates: OlivineOlivine(Mg,Fe)(Mg,Fe)22SiOSiO44

High-T igneous mineral, common in mafic and ultramafic rocks; High-T igneous mineral, common in mafic and ultramafic rocks; commonly alters to serpentinecommonly alters to serpentine

Vitreous olive green (Mg) to black (Fe)Vitreous olive green (Mg) to black (Fe)Equigranular to prismatic habit; poor cleavageEquigranular to prismatic habit; poor cleavageOptics: Colorless, biaxial (+Mg, -Fe), mod. high relief (n~1.7), Optics: Colorless, biaxial (+Mg, -Fe), mod. high relief (n~1.7),

high 2V, high 2V, ~.05 (2 ~.05 (2ndnd order IF colors) order IF colors)Complete solid solution between Mg and FeComplete solid solution between Mg and Fe

Common Nesosilicates: Common Nesosilicates: GarnetGarnet(Mg,Fe,Mn,Ca)(Mg,Fe,Mn,Ca)33(Fe(Fe3+3+,Cr,Al),Cr,Al)22SiSi33OO1212

As mod-T metamorphic mineral formed from Al-rich source rocks and As mod-T metamorphic mineral formed from Al-rich source rocks and ultramafic mantle rocks (eclogites)ultramafic mantle rocks (eclogites)

Equigranular, euhderal to subhedral habit; poor cleavageEquigranular, euhderal to subhedral habit; poor cleavageOptics: Colorless, isotropic, high relief (n~1.7-1.9)Optics: Colorless, isotropic, high relief (n~1.7-1.9)Complex solid solution with the following end-member compositions and Complex solid solution with the following end-member compositions and

their characteristic colors:their characteristic colors:Pyrope MgPyrope Mg33AlAl22SiSi33OO1212 – deep red to black – deep red to black Almandine FeAlmandine Fe33AlAl22SiSi33OO1212 – deep brownish red – deep brownish redSpessartine MnSpessartine Mn33AlAl22SiSi33OO1212 – brownish red to black – brownish red to blackGrossular CaGrossular Ca33AlAl22SiSi33OO1212 – yellow-green to brown – yellow-green to brownAndradite CaAndradite Ca33FeFe22SiSi33OO1212 – variable-yellow, green, brown, black – variable-yellow, green, brown, blackUvarovite CaUvarovite Ca33CrCr22SiSi33OO1212 – emerald green – emerald green

Common Nesosilicates: Common Nesosilicates: The AluminosilicatesThe Aluminosilicates Kyanite, Sillimanite, AndalusiteKyanite, Sillimanite, Andalusite

AlAl22SiOSiO55Moderate to high grade metamorphic minerals formed from Al-Moderate to high grade metamorphic minerals formed from Al-

rich source rocksrich source rocksAlAl in octahedral or a mix of octahedral to tetrahedral sites. in octahedral or a mix of octahedral to tetrahedral sites. Kyanite – Vitreous bluish bladed tabletsKyanite – Vitreous bluish bladed tablets

w/ single perfect cleavage; H: 5-7w/ single perfect cleavage; H: 5-7

Sillimanite – Vitreous brown to green Sillimanite – Vitreous brown to green clustered prisms w/ single cleavage dir.clustered prisms w/ single cleavage dir.

Andalusite – Vitreous flesh-red, reddish brownAndalusite – Vitreous flesh-red, reddish brownsquare prisms; H: 7.5square prisms; H: 7.5

Common Nesosilicates: Common Nesosilicates: StauroliteStauroliteFeFe22AlAl99OO66(SiO(SiO44))44(O,OH)(O,OH)22

Moderate to high grade metamorphic mineral formed from Al-rich Moderate to high grade metamorphic mineral formed from Al-rich source rockssource rocks

Resinous to vitreous (dull when altered) reddish-brown to brownish Resinous to vitreous (dull when altered) reddish-brown to brownish black 6-sided prisms; commonly forms interpenetrating twins black 6-sided prisms; commonly forms interpenetrating twins

Optics: Biaxial(-), yellow pleochroic, high relief (n~1.75), 2V=82°-88°Optics: Biaxial(-), yellow pleochroic, high relief (n~1.75), 2V=82°-88°

Common Nesosilicates: Common Nesosilicates: Sphene (Titanite)Sphene (Titanite)

CaTiO(SiOCaTiO(SiO44))Common accessory mineral in felsic igneous rocks and in some Common accessory mineral in felsic igneous rocks and in some

metamorphic rocksmetamorphic rocksResinous to adamantine gray, brown, green, yellow or black lens Resinous to adamantine gray, brown, green, yellow or black lens

crystals; distinct diamond-shaped cleavage; H: 5-5.5crystals; distinct diamond-shaped cleavage; H: 5-5.5Optics: Biaxial(+), yellow pleochroic, very high relief (n~2.0), Optics: Biaxial(+), yellow pleochroic, very high relief (n~2.0),

2V=27°, 2V=27°, = 0.13 = 0.13

Sorosilicates (Double)Sorosilicates (Double) x(Six(Si22OO77) Unit Composition ) Unit Composition Double silicon tetrahedra linked by one bridging Double silicon tetrahedra linked by one bridging

oxygenoxygen Commonly also contains island tetrahedra (SiOCommonly also contains island tetrahedra (SiO44)) Typically monoclinic symmetryTypically monoclinic symmetry Most important phase of this silicate group is Most important phase of this silicate group is

Epidote/ClinozoisiteEpidote/ClinozoisiteEpidote Epidote StructureStructure

Common Sorosilicates: Common Sorosilicates: Epidote GroupEpidote GroupZoisite/Clinozoisite – CaAlZoisite/Clinozoisite – CaAl33O(SiOO(SiO44)(Si)(Si22OO77)(OH))(OH)

Epidote – Ca(Fe,Al)AlEpidote – Ca(Fe,Al)Al22O(SiOO(SiO44)(Si)(Si22OO77)(OH))(OH) Common accessory and alteration mineral in igneous rocks and is a common Common accessory and alteration mineral in igneous rocks and is a common

phase in various grades of metamorphic rocksphase in various grades of metamorphic rocksZoisite – Orthorhombic; Clinozoisite and Epidote – MonoclinicZoisite – Orthorhombic; Clinozoisite and Epidote – MonoclinicPhysical Properties: prismatic vitreous crystals to very fine resinous massive Physical Properties: prismatic vitreous crystals to very fine resinous massive

granules; H: 6-7granules; H: 6-7Zoisite: Gray, greenish brown (pink-thulite)Zoisite: Gray, greenish brown (pink-thulite)Clinozoisite: Gray, pale yellow, pale green, colorlessClinozoisite: Gray, pale yellow, pale green, colorlessEpidote: Pistachio green to yellow green, Epidote: Pistachio green to yellow green,

Optics: Optics: Zoisite: Biaxial(+), high relief (n~1.7), 2V=0-70°, Zoisite: Biaxial(+), high relief (n~1.7), 2V=0-70°, ~ 0.005 ~ 0.005Clinozoisite: Biaxial(+), high relief (n~1.7), 2V=14-90°, Clinozoisite: Biaxial(+), high relief (n~1.7), 2V=14-90°, ~0.010~0.010Epidote: Biaxial(-), high relief (n~1.75), 2V=74-90°, Epidote: Biaxial(-), high relief (n~1.75), 2V=74-90°, ~0.015-.051, green-~0.015-.051, green-yellow pleochroic; yellow pleochroic;

Lawsonite – CaAlLawsonite – CaAl22(Si(Si22OO77)(OH))(OH)22·H·H22OOCommon mineral in high pressure metamorphic rocks; occurs with Common mineral in high pressure metamorphic rocks; occurs with

glaucophane, chlorite, and epidoteglaucophane, chlorite, and epidoteVitreous to greasy, bluish tabular to prismatic crystals, commonly twinned; Vitreous to greasy, bluish tabular to prismatic crystals, commonly twinned;

H: 8H: 8Optics – biaxial(+), mod. high relief (n~1.67), 2V=84°, Optics – biaxial(+), mod. high relief (n~1.67), 2V=84°, ~0.02~0.02

Vesuvianite – CaVesuvianite – Ca1010(Mg,Fe)(Mg,Fe)22AlAl44(SiO(SiO44))55(Si(Si22OO77))22(OH)(OH)44Common mineral found in thermally metamorphosed limestone with Common mineral found in thermally metamorphosed limestone with

garnet, wollastonite (Ca-pyroxene), and diopside (Mg-Ca-pyroxene)garnet, wollastonite (Ca-pyroxene), and diopside (Mg-Ca-pyroxene)Vitreous to resinous, green to brown, columnar to granular crystals, Vitreous to resinous, green to brown, columnar to granular crystals,

commonly striated parallel to columns; H: 6.5commonly striated parallel to columns; H: 6.5Optics – uniaxial(-), mod. high relief (n~1.67), 2V=84°, Optics – uniaxial(-), mod. high relief (n~1.67), 2V=84°, ~0.003-.006~0.003-.006

Common Sorosilicates: Common Sorosilicates: Lawsonite, Lawsonite, Vesuvianite (Idiocrase)Vesuvianite (Idiocrase)

Next Lecture 12/06/06Next Lecture 12/06/06

Systematic Description of MineralsSystematic Description of Minerals

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

Tecto-silicates Tecto-silicates

Read: Klein Parts of Chaps. 11 & 12Read: Klein Parts of Chaps. 11 & 12