Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. CHAPTER 16 Orthosilicates

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.1 Olivine structure. Characterized by Z:O = 1:4 (e.g., SiO4) Group I: Olivine, Garnet, Zircon All oxygen are part of SiO4 tetrahedron Group II: Al-silicates, Staurolite, Chloritoid, Topaz, Titanite Al3+ or Ti4+ joins with oxygen that are not part of SiO4 tetrahedra Group I Structure: isolated SiO4 tetrahedra bonded laterally through various cations(Ca, Mg, Fe, Al etc) held in octahedral or higher co-ordination. The difference in charge and size accounts for differences in structure. Group II: Chains or sheets of Al3+ or Ti4+ octahedra linked laterally by SiO4 tetrahedra or other cations All except Titanite are Al rich and are found only in Al rich rocks – meta-pelites or felsic pegmatites (Topaz)

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Olivine Structure Yellow and green are Mg cations

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

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

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~ ) 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

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

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Sillimanite consists of chains of aluminum octahedra (green) joined by alternating silica and aluminum tetrahedra (yellow and blue). This view, slightly oblique to the c axis, shows four layers of the structure. Note how the tetrahedra bridge the gaps between the octahedra and alternate.

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°

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: Optics: Biaxial(+), yellow pleochroic, very high relief (n~2.0), 2V=27°,  = 0.13

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

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°,  ~ Clinozoisite: Biaxial(+), high relief (n~1.7), 2V=14-90°, ~0.010 Epidote: Biaxial(-), high relief (n~1.75), 2V=74-90°, ~ , green-yellow pleochroic;

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°,  ~ Common Sorosilicates: Lawsonite, Vesuvianite (Idocrase)

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.3 Olivine phenocryst (O) in basaltic dike. Note the alteration to iddingsite (I) a long grain boundaries and cracks. The phenocryst at the upper right is augite (A). Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.2 Specific gravity, d (130), and optical properties of olivine.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.5 Garnet.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.4 Garnet structure. Isolated silicon tetrahedra share single oxygen anions with Y cations in 6-fold coordination. The spaces between are distorted 8-fold sites occupied by the larger X cations.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.6 Optical properties of common garnet.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.7 Photomicrograph of a thin section of granite showing a small zircon crystal (Z) in quartz (Q). The dark mineral is biotite (B). Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.8 Structure of the aluminum silicates.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure 16.9 Aluminum silicate stability fields, From Hemingway and others (1991).

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Aluminum silicates in mica schist.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Sieved staurolite porphyroblast (St) with porphyroblasts of garnet (G) and biotite (B) in thin section of mica schist. Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Chloritoid structure.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Photomicrograph of chloritoid in a thin section of schist. Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Photomicrograph of subhedral to anhedral titanite (T) grains with hornblende (H), plagioclase (P), and Ca-clinopyroxene (Cpx) in a thin section of amphibolite. Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Topaz. (a) Topaz crystal. (b) Photomicrograph of basal section of topaz in thin section. Plane light.

Introduction to Mineralogy, Second edition William D. Nesse Copyright © 2012, by Oxford University Press, Inc. Figure Optical properties of topaz. After Ribbe and Rosenberg (1971).