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

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

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

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

  2. Silicate Mineral Classification(based on arrangement of SiO4 tetrahedra)

  3. 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

  4. Common Cyclosilicates BerylBe3Al2(Si6O18) Common accessory mineral in granite pegmatite Gem varieties – Aquamarine, Emerald, Rose Beryl, Golden Beryl Cordierite(Mg,Fe)2Al4Si5O18·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(BO3)3 (Si6O18)(OH)4 Common accessory mineral in granite pegmatite Characteristic striated prisms with trigonal outline watermelon tourmaline

  5. 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 Pyroxene Structure Amphibole Structure

  6. Pyroxenes (XYZ2O6 )X (M2) – Na, Ca, Mn, Fe+2, Mg, Li Y (M1) – Mn, Fe+2, Mg, Fe+3, Cr+3 , Ti+4Z (Tetrahedral site) - Al+3, Si+4 Clinopyroxenes Orthopyroxenes

  7. Exsolution in Pyroxene Exsolved comp Original comp

  8. Amphiboles (A0-1X2Y5Z8O22 (OH,F))A – Na, KX (M4) – Na, Ca, Mn, Fe+2, Mg, Li Y (M1-3) – Mn, Fe+2, Mg, Fe+3, Cr+3 , Ti+4Z (Tetrahedral site) - Al+3, Si+4

  9. Common Types of Amphiboles

  10. Phyllosilicates (Sheet) • x(Si2O5) Unit Composition • Infinite sheets of silicon tetrahedra • Strong single cleavage parallel to silicon sheets Pyrophyllite Muscovite

  11. Phyllosilicate StructuresAlternating Si Tetrahedral and Octahedral layers bound by large cations or weak electrostatic bonds

  12. Common Phyllosilicates Kaolinite Chrysotile Antigorite Talc Pyrophyllite Muscovite Biotite Chlorite Lepidolite

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

  14. SiO2 Group Quartz SiO2 Opal SiO2·nH2O

  15. Feldspar Group

  16. Alkali Feldspars Perthite (albite exsolution in microcline)

  17. Plagioclase Feldspars Albite Twinning Compositional Zoning (Oscillatory)

  18. Feldspathoids (Si-poor feldspars) Common in alkaline (Si-undersaturated) igneous rocks Leucite – KAlSiO4 Nepheline – (Na,K)AlSiO4 Sodalite – Na8(AlSiO4)6Cl2

  19. Hydrous Tectosilicates • Analcime (Scapolite Gp) NaAlSi2O6·H2O • Natrolite (Zeolite Gp) Na2Al2Si3O10·2H2O • Heulandite (Zeolite Gp) CaAl2Si7O18·6H2O • Stilbite (Zeolite Gp) NaCa2Al5Si13O36·14H2O

  20. Next Lecture 12/11/06 Introduction to X-ray Diffractometry Read: Klein p. 309-321 12/13 – Final Review: Mineralogy Trivia

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