Lecture 20 (12/06/2006) Systematic Description of Minerals Part 4: Silicates II: Cyclosilicates, Inosilicates, Phyllosilicates and Tectosilicates

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Lecture 20 (12/06/2006)Systematic Description
of Minerals Part 4 Silicates II
Cyclosilicates, Inosilicates, Phyllosilicates and
Tectosilicates
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Silicate Mineral Classification(based on
arrangement of SiO4 tetrahedra)
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Cyclosilicates (Ring)

• x(SiO3) Unit Composition
• Hexagonal and Orthorhombic (pseudohexagonal)
  symmetries most common
• Forms silicate minerals with
• Moderate density(2.6-3.2) and hardness (7-8)
• Prismatic habits
• Poor cleavage

Beryl
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Common Cyclosilicates

• Beryl Be3Al2(Si6O18)
• Common accessory mineral in granite pegmatite
• Gem varieties Aquamarine, Emerald, Rose Beryl,
  Golden Beryl

Cordierite (Mg,Fe)2Al4Si5O18nH20 Common mineral
in contact metamorphosed argillaceous
rocks Resembles quartz in appearance and
hardness (7-7.5)
Tourmaline (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3 (Si
6O18)(OH)4 Common accessory mineral in granite
pegmatite Characteristic striated prisms with
trigonal outline
watermelon tourmaline
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Inosilicates (Chain)

• x(SiO3) Unit Composition
• Single and double silicon tetrahedra chains
• Typically monoclinic and orthorhombic symmetry
• Single chains (Pyroxenes) develop 90 cleavage
• Double chains (Amphiboles) develop 120 cleavage

Amphibole Structure
Pyroxene Structure
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Pyroxenes (XYZ2O6 )X (M2) Na, Ca, Mn, Fe2,
Mg, Li Y (M1) Mn, Fe2, Mg, Fe3, Cr3 ,
Ti4Z (Tetrahedral site) - Al3, Si4
Clinopyroxenes
Orthopyroxenes
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Exsolution in Pyroxene
Exsolved comp Original comp
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Amphiboles (A0-1X2Y5Z8O22 (OH,F))A Na, KX
(M4) Na, Ca, Mn, Fe2, Mg, Li Y (M1-3) Mn,
Fe2, Mg, Fe3, Cr3 , Ti4Z (Tetrahedral site)
- Al3, Si4
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Common Types of Amphiboles
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Phyllosilicates (Sheet)

• x(Si2O5) Unit Composition
• Infinite sheets of silicon tetrahedra
• Strong single cleavage parallel to silicon sheets

Pyrophyllite
Muscovite
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Phyllosilicate StructuresAlternating Si
Tetrahedral and Octahedral layers bound by large
cations or weak electrostatic bonds
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Common Phyllosilicates
Kaolinite
Chrysotile
Antigorite
Talc
Pyrophyllite
Muscovite
Biotite
Chlorite
Lepidolite
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Tectosilicates (Framework)

• x(SiO2) Unit Composition
• 3-D framework of linked silicon tetrahedra
• Variable physical properties and symmetries
  depending on linkage of framework groupings

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SiO2 Group
Quartz SiO2
Opal SiO2nH2O
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Feldspar Group
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Alkali Feldspars
Perthite (albite exsolution in microcline)
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Plagioclase Feldspars
Albite Twinning
Compositional Zoning (Oscillatory)
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Feldspathoids (Si-poor feldspars)

• Common in alkaline (Si-undersaturated) igneous
  rocks
• Leucite KAlSiO4
• Nepheline (Na,K)AlSiO4
• Sodalite Na8(AlSiO4)6Cl2

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Hydrous Tectosilicates

• Analcime (Scapolite Gp) NaAlSi2O6H2O
• Natrolite (Zeolite Gp) Na2Al2Si3O102H2O
• Heulandite (Zeolite Gp) CaAl2Si7O186H2O
• Stilbite (Zeolite Gp) NaCa2Al5Si13O3614H2O

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Next Lecture 12/11/06

• Introduction to X-ray Diffractometry
• Read Klein p. 309-321
• 12/13 Final Review Mineralogy Trivia