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Lecture 20 (12/06/2006)

Systematic Description of Minerals

Part 4:Silicates II: Cyclosilicates,

Inosilicates, Phyllosilicates andTectosilicates

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Silicate Mineral Classification(based on arrangement of SiO4tetrahedra)

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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 habitsPoor cleavage

Beryl

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Common Cyclosilicates

BerylBe3Al2(Si6O18)Common accessory mineral in granite pegmatite

Gem varieties Aquamarine, Emerald, RoseBeryl, Golden Beryl

Cordierite(Mg,Fe)2Al4Si5O18nH20Common 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 (Si6O18)(OH)4Common accessory mineral in granite pegmatite

Characteristic striated prisms with trigonal outlinewatermelontourmaline

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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, Fe

+2, Mg, Li

Y (M1) Mn, Fe+2, Mg, Fe+3, Cr+3 , Ti+4

Z (Tetrahedral site) - Al+3, Si+4

Clinopyroxenes

Orthopyroxenes

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Exsolution in Pyroxene

Exsolved comp

Original comp

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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 layersbound by large cations

or weak electrostatic

bonds

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Common Phyllosilicates

Antigorite

ChrysotileKaolinite

Talc

Pyrophyllite

Muscovite

Lepidolite

BiotiteChlorite

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Tectosilicates (Framework)

x(SiO2) Unit Composition

3-D framework of linkedsilicon tetrahedra

Variable physicalproperties andsymmetries dependingon linkage of frameworkgroupings

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SiO2Group

Opal SiO2nH2O

Quartz SiO2

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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

CompositionalZoning(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

12/13Final Review: Mineralogy Trivia