1. Lecture 19 (12/04/2006) Systematic Description of Minerals Part 3: Silicates I: Introduction, Nesosilicates, and Sorosilicates

2. 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

3. 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

4. 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.

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

6. O-coordination and Bond Strength of Other Common Cations in Silicate Minerals Electostatic Valence w/ O -2 1/8 - 1/12 1/6 - 1/8 1/3 – 1/4 1/31/31/31/22/31/2 3/41 Weak Strong

7. Silicate Mineral Classification (based on arrangement of SiO 4 tetrahedra)

8. Nesosilicates (Island)  x(SiO 4 ) Unit Composition  Isolated, but tightly packed (SiO 4 ) 4- tetrahedra  Forms silicate minerals with: High density and hardness Equidimensional habits Poor cleavage  Low degree of Al substitution with Si Olivine Garnet

9. Common Nesosilicates: Olivine (Mg,Fe) 2 SiO 4 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 (2 nd order IF colors) Complete solid solution between Mg and Fe

10. Common Nesosilicates: Garnet (Mg,Fe,Mn,Ca) 3 (Fe 3+,Cr,Al) 2 Si 3 O 12 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 Mg 3 Al 2 Si 3 O 12 – deep red to black Almandine Fe 3 Al 2 Si 3 O 12 – deep brownish red Spessartine Mn 3 Al 2 Si 3 O 12 – brownish red to black Grossular Ca 3 Al 2 Si 3 O 12 – yellow-green to brown Andradite Ca 3 Fe 2 Si 3 O 12 – variable-yellow, green, brown, black Uvarovite Ca 3 Cr 2 Si 3 O 12 – emerald green

11. Common Nesosilicates: The Aluminosilicates Kyanite, Sillimanite, Andalusite Al 2 SiO 5 Moderate to high grade metamorphic minerals formed from Al- rich 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

12. Common Nesosilicates: Staurolite Fe 2 Al 9 O 6 (SiO 4 ) 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°

13. Common Nesosilicates: Sphene (Titanite) CaTiO(SiO 4 ) 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

14. Sorosilicates (Double)  x(Si 2 O 7 ) Unit Composition  Double silicon tetrahedra linked by one bridging oxygen  Commonly also contains island tetrahedra (SiO 4 )  Typically monoclinic symmetry  Most important phase of this silicate group is Epidote/Clinozoisite EpidoteStructure

15. Common Sorosilicates: Epidote Group Zoisite/Clinozoisite – CaAl 3 O(SiO 4 )(Si 2 O 7 )(OH) Epidote – Ca(Fe,Al)Al 2 O(SiO 4 )(Si 2 O 7 )(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;

16. Lawsonite – CaAl 2 (Si 2 O 7 )(OH) 2 ·H 2 O 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 – Ca 10 (Mg,Fe) 2 Al 4 (SiO 4 ) 5 (Si 2 O 7 ) 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 Common Sorosilicates: Lawsonite, Vesuvianite (Idiocrase)

17. Next Lecture 12/06/06 Systematic Description of Minerals Part 4 Silicates II:Cyclo-, Ino-, Phyllo-, and Tecto-silicates Silicates II:Cyclo-, Ino-, Phyllo-, and Tecto-silicates Read: Klein Parts of Chaps. 11 & 12