302-pp27 - GY 302: Crystallography & Mineralogy GY 302:...

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Unformatted text preview: GY 302: Crystallography & Mineralogy GY 302: Crystallography & Mineralogy UNIVERSITY OF SOUTH ALABAMA Lecture 27: Class VIII-Silicates Tektosilicates 3: Feldspathoids & Zeolites Lecture 27: Class VIII-Silicates Tektosilicates 3: Feldspathoids & Zeolites Last Time Class VIII Minerals (Tektosilicates) 1. Feldspar Group Tektosilicate Minerals (Feldspars) Mineral Formula System Potassium Feldspar Group *Sanidine KAlSi 3 O 8 Monoclinic Orthoclase KAlSi 3 O 8 Monoclinic Anorthoclase (Na, K)AlSi 3 O 8 Triclinic Microcline KAlSi 3 O 8 Triclinic v. Amazonite KAlSi 3 O 8 Triclinic Plagioclase Feldspars Albite (Ab) NaAlSi 3 O 8 Triclinic Oligoclase An 10-30 Triclinic Andesine An 30-50 Triclinic Labradorite An 50-70 Triclinic *Bytownite An 70-90 Triclinic Anorthite (An) CaAl 2 Si 2 O 8 Triclinic The Feldspars General Formula: XAlSi 3 O 8 or XAl 2 Si 2 O 8 X= Ca 2+ , Na + , K + Two Varieties: 1) Alkali Feldspars (incl Orthoclase Group) 2) Plagioclase Feldspars The Feldspars Plagioclase Feldspars (NaAlSi 3 O 8-CaAl 2 Si 2 O 8 ) 1) Albite (Ab) = NaAlSi 3 O 8 (An 0-10 ) 2) Oligoclase = An 10-30 3) Andesine = An 30-50 4) Labradorite = An 50-70 5) Bytownite = An 70-90 6) Anorthite (An) = CaAl 2 Si 2 O 8 solid solution Tektosilicate Minerals (Feldspars) Tektosilicate Minerals (Feldspars) Phase diagrams to the rescue! Todays Agenda Class VIII Minerals (Tektosilicates) 1. Feldspathoids 2. Zeolites Tektosilicate Minerals (Feldspathoids) Mineral Formula System Feldspathoids (38) Leucite* KAlSi 2 O 6 Tetragonal Nepheline* NaAlSiO 4 Hexagonal Sodalite* Na 8 (AlSiO 4 ) 6 Cl 2 Isometric Lazurite (Na,Ca) 8 (AlSiO 4 ) 6 (SO 4 ,S,Cl) 2 Isometric * groups Feldspathoids contain 2/3 the silica that feldspars do (hence the name). They form in alkali-rich igneous rocks in place of feldspars when there is an Si 4+ deficiency You never find feldspathoids and quartz together in the same rock NaAlSiO 4 (nepheline) + 2SiO 2 NaAlSi 3 O 8 (albite) Leucite [ KAlSi 2 O 6 ] Crystal: Tetragonal Pt. Group: 4/m Habit: equant crystals SG: 2.47; H: 6 L: vitreous; Str: white Col: colourless, grey, yellow Clev: Indistinct [110] Optics: isotropic; n=1.508-1.511 Occurrence: Felsic igneous rocks Tektosilicate Minerals (Feldspathoid) From the Greek leukos for white ht p:/ webmineral.com Nepheline [(Na,K)AlSiO 4 ] Crystal: Hexagonal Pt. Group: 6 Habit: prismatic, slender prisms SG: 2.59; H: 6 L: vitreous-greasy; Str: white Col: white, grey, brown, reddish Clev: poor [1010] Optics: uniaxial (-); bir=0.003-0.005 n e =1.528; n w =1.531 Occurrence: Si-poor igneous rocks Tektosilicate Minerals (Feldspathoids) ht p:/ webmineral.com From the Greek nephele , "cloud," because it becomes clouded when put in strong acid Sodalite [Na 8 Al 6 Si 6 O 24 C l2 ] Crystal: Isometric Pt. Group: 4 3m Habit: massive, vein-fill SG: 2.29; H: 6 L: vitreous-greasy; Str: white Col: azure blue, white, pink, grey Clev: poor [110] Optics: isotropic; n=1.483-1.484 Occurrence: Nepheline syenites Tektosilicate Minerals...
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This note was uploaded on 02/05/2012 for the course GLY 302 taught by Professor Staff during the Spring '11 term at S. Alabama.

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302-pp27 - GY 302: Crystallography & Mineralogy GY 302:...

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