lecture 20.06.ppt
TRANSCRIPT
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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