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CHAPTER 16
Orthosilicates
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
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)
Figure 16.1 Olivine structure.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Olivine Structure
Yellow and green are Mg
cations
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Nesosilicates (Island)
• x(SiO4) Unit Composition
• Isolated, but tightly packed (SiO4)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)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 (+Mg, -Fe), mod. high relief (n~1.7), high 2V, 
~.05 (2nd order IF colors)
Complete solid solution between Mg and Fe
Common Nesosilicates: Garnet
(Mg,Fe,Mn,Ca)3(Fe3+,Cr,Al)2Si3O12
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 Mg3Al2Si3O12 – deep red to black
Almandine Fe3Al2Si3O12 – deep brownish red
Spessartine Mn3Al2Si3O12 – brownish red to black
Grossular Ca3Al2Si3O12 – yellow-green to brown
Andradite Ca3Fe2Si3O12 – variable-yellow, green, brown, black
Uvarovite Ca3Cr2Si3O12 – emerald green
Common Nesosilicates: The Aluminosilicates
Kyanite, Sillimanite, Andalusite
Al2SiO5
Moderate to high grade metamorphic minerals formed from Alrich 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
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.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Common Nesosilicates: Staurolite
Fe2Al9O6(SiO4)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(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
Sorosilicates (Double)
•
•
•
•
•
x(Si2O7) Unit Composition
Double silicon tetrahedra linked by one bridging oxygen
Commonly also contains island tetrahedra (SiO4)
Typically monoclinic symmetry
Most important phase of this silicate group is
Epidote/Clinozoisite
Epidote
Structure
Common Sorosilicates: Epidote Group
Zoisite/Clinozoisite – CaAl3O(SiO4)(Si2O7)(OH)
Epidote – Ca(Fe,Al)Al2O(SiO4)(Si2O7)(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;
Common Sorosilicates: Lawsonite,
Vesuvianite (Idocrase)
Lawsonite – CaAl2(Si2O7)(OH)2·H2O
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 – 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 to columns; H: 6.5
Optics – uniaxial(-), mod. high relief (n~1.67), 2V=84°, ~0.003-.006
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 16.10 Aluminum silicates in mica schist.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 16.11 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 16.12 Chloritoid structure.
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.
Figure 16.13 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 16.14 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 16.15 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 16.16 Optical properties of topaz. After Ribbe and Rosenberg (1971).
Introduction to Mineralogy,
Second edition
William D. Nesse
Copyright © 2012, by Oxford University Press, Inc.