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

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Presentation transcript:

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

Silicate Mineral Classification (based on arrangement of SiO 4 tetrahedra)

Cyclosilicates (Ring)  x(SiO 3 ) Unit Composition  Hexagonal and Orthorhombic (pseudohexagonal) symmetries most common  Forms silicate minerals with: Moderate density( ) and hardness (7-8) Prismatic habits Poor cleavage Beryl

Common Cyclosilicates Beryl Be 3 Al 2 (Si 6 O 18 ) Common accessory mineral in granite pegmatite Gem varieties – Aquamarine, Emerald, Rose Beryl, Golden Beryl Cordierite (Mg,Fe) 2 Al 4 Si 5 O 18 ·nH20 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 (BO 3 ) 3 (Si 6 O 18 )(OH) 4 Common accessory mineral in granite pegmatite Characteristic striated prisms with trigonal outline watermelontourmaline

Inosilicates (Chain)  x(SiO 3 ) 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 AmphiboleStructurePyroxeneStructure

Pyroxenes (XYZ 2 O 6 ) X (M 2 ) – Na, Ca, Mn, Fe +2, Mg, Li Y (M 1 ) – Mn, Fe +2, Mg, Fe +3, Cr +3, Ti +4 Z (Tetrahedral site) - Al +3, Si +4 Clinopyroxenes Orthopyroxenes

Exsolution in Pyroxene Exsolved comp Original comp

Amphiboles (A 0-1 X 2 Y 5 Z 8 O 22 (OH,F)) A – Na, K X (M 4 ) – Na, Ca, Mn, Fe +2, Mg, Li Y (M 1-3 ) – Mn, Fe +2, Mg, Fe +3, Cr +3, Ti +4 Z (Tetrahedral site) - Al +3, Si +4

Common Types of Amphiboles

Phyllosilicates (Sheet)  x(Si 2 O 5 ) Unit Composition  Infinite sheets of silicon tetrahedra  Strong single cleavage parallel to silicon sheets PyrophylliteMuscovite

Phyllosilicate Structures Alternating Si Tetrahedral and Octahedral layers bound by large cations or weak electrostatic bonds

Common Phyllosilicates Antigorite Chrysotile Kaolinite Talc Pyrophyllite Muscovite Lepidolite Biotite Chlorite

Tectosilicates (Framework)  x(SiO 2 ) Unit Composition  3-D framework of linked silicon tetrahedra  Variable physical properties and symmetries depending on linkage of framework groupings

SiO 2 Group Opal SiO 2 ·nH 2 O Quartz SiO 2

Feldspar Group

Alkali Feldspars Perthite (albite exsolution in microcline)

Plagioclase Feldspars Albite Twinning CompositionalZoning(Oscillatory)

Feldspathoids (Si-poor feldspars) Common in alkaline (Si-undersaturated) igneous rocks Leucite – KAlSiO 4 Nepheline – (Na,K)AlSiO 4 Sodalite – Na 8 (AlSiO 4 ) 6 Cl 2

Hydrous Tectosilicates  Analcime (Scapolite Gp) NaAlSi 2 O 6 ·H 2 O  Natrolite (Zeolite Gp) Na 2 Al 2 Si 3 O 10 ·2H 2 O  Heulandite (Zeolite Gp) CaAl 2 Si 7 O 18 ·6H 2 O  Stilbite (Zeolite Gp) NaCa 2 Al 5 Si 13 O 36 ·14H 2 O

Next Lecture 12/11/06 Introduction to X-ray Diffractometry Read: Klein p /13 – Final Review: Mineralogy Trivia