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302-pp27 - UNIVERSITY OF SOUTH ALABAMA Last Time Class VIII...

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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)
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Tektosilicate Minerals (Feldspars) Phase diagrams to the rescue! Today’s 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 http://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) http://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 (Feldspathoids) http://www.carolsevans.com/USERIMAGES/sodalite-rough.jpg Named on the basis of its sodium content
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