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Properties of Common Minerals Groups Considered Framework Silicates Feldspars Feldspar Twinning K-Spars

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Groups Considered Properties of Common • Framework silicates Minerals • Sheet silicates • Pyroxenes Figures from Winter’s web page (2002) • Amphiboles • Other Silicates • Non Silicates Framework Silicates Feldspars • Simple chemistry •Feldspars • Substitution of Na for K or NaAl for CaAl • Feldspathoids • Crystal systems • Silica polymorphs – Monoclinic (orthoclase, sanidine) – Triclinic (plagioclase, microcline) • Tabular habit Feldspar Twinning K-spars • Simple twins (two parts) • Orthoclase, Sanidine, Microcline – Carlsbad • All optically negative – Common in monoclinic feldspars • All have n ~ 1.53 • Polysynthetic twins • Distinguished by 2V – Albite and others – Sanidine 2V = 0-30º – Common in triclinic feldspars – Orthoclase 2V = 30-70º – Microcline 2V = 70-90º • Microcline has Scotch plaid twins 1 Plagioclase Common Sheet Silicates • Refractive index increases with Ca content •Muscovite – Varies between 1.53 (Ab) to 1.57 (An) • Phlogopite • 2V is large and varies with composition • Biotite • Optic sign depends on composition (+/-) • Chlorite Sheet Structures Phyllosilicates Classified on the basis of Si-O polymerism SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5] Apical O’s are unpolymerized and are bonded to other constituents 2- [Si2O5] Sheets of tetrahedra micas talc clay minerals serpentine Building Blocks Common Sheet Properties • Tetrahedral layers • Crystals are platy parallel to (001) • Perfect cleavage follows (001) • Octahedral layers • 2V is small (0-40º) • Large Cation layers • Extinction is parallel to (001) • BxA perpendicular to (001) • Week bonds along (001) • Optic sign can be determine by 1st order plate 2 Brucite: Mg(OH)2 Gibbsite Al(OH)3 Octahedral layers can be understood by analogy with hydroxides a2 Layers of octahedral Mg in coordination with (OH) c Large spacing along c due a to weak van der waals a1 bonds • Layers of octahedral Al in coordination with (OH) • Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance • Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral Kaolinite: Al2 [Si2O5] (OH)4 Serpentine: Mg3 [Si2O5] (OH)4 T T Yellow = (OH) O Yellow = (OH) O - vdw - vdw T-layers and dioctahedral (Al3+) layers T T-layers and trioctahedral (Mg2+) layers T O O → → (OH) at center of T-rings and fill base of VI layer - vdw (OH) at center of T-rings and fill base of VI layer - vdw T T weak van der Waals bonds between T-O groups O weak van der Waals bonds between T-O groups O Chlorite Mica Properties •(Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6 • All micas are optically negative • T - O - T - (brucite) - T - O - T - (brucite) - T - O - T • 2V is small (0-40°) • Very hydrated (OH)8 • (001) sheets give BxA figures • Low-temperature stability • Birefringence is large (0.035-0.045) • Low-T metamorphism and alteration product of mafics • “Birds eye” effect is obvious due to bent cleavages 3 Chlorite Optics Other Sheet Silicates • 2V is small (0-20°) •Talc • Birefringence is low (0.001-0.010) – Pale green, high birefringence • Some optically positive, some • Stilpnomelane negative – Pleochroic (yellow to brown or green) • Extinction parallel to (001) • Some types have strong dispersion • Chloritoid • Some types weakly pleochroic – Hour glass inclusions, polysynthetic twins, cross fractures, inclined extinction Mineral Structures Silicates are classified on the basis of Si-O polymerism Amphiboles • Two cleavages at 120º • Crystals elongate parallel to c • Extinction Z^c small (10-20º) • Color and pleochroism generally strong • Optic sign negative 2- 4- [SiO3] single chains Inosilicates [Si4O11] Double tetrahedra pryoxenes pyroxenoids amphiboles • 2V is large (70-90º) Amphibole Chemistry Amphibole Groups See handout for more information • Non calcic amphiboles General formula: – Anthophyllite (orthorhombic) – Cummingtonite (monoclinic) W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2 W = Na K • Calcic amphiboles X = Ca Na Mg Fe2+ (Mn Li) – Tremolite-actinolite Y = Mg Fe2+ Mn Al Fe3+ Ti – Hornblende-oxyhornblende Z = Si Al Again, the great variety of sites and sizes → a great chemical range, and • Sodic amphiboles hence a broad stability range The hydrous nature implies an upper temperature stability limit – Glaucophane, Riebeckite, Arfvedsonite 4 Amphibole Chemistry Hornblende Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) • Strongly pleochroic • Z^c = 20º Tremolite Ferroactinolite Ca Mg Si O (OH) Actinolite • Birefringence = 0.020 Ca2Mg5Si8O22(OH)2 Ca2Fe5Si8O22(OH)2 •2Vx = 70º Clinoamphiboles Cummingtonite-grunerite Anthophyllite • Can be named green or brown hornblende Mg Si O (OH) Fe7Si8O22(OH)2 7 8 22 2 Orthoamphiboles • Reddish varieties are oxyhornblende and kaersutite (Ti-rich hornblende) Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite ↔ gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions Amphibole Chemistry General Pyroxene Hornblende has Al in the tetrahedral site Characteristics Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists • Two cleavages at 87º casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member. • Stumpy crystals Sodic amphiboles • Z^c large for monoclinic minerals • Colorless or weakly pleochroic Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2 2+ 3+ Riebeckite: Na2 Fe 3 Fe 2 [Si8O22] (OH)2 • Optic sign positive • 2V is moderate (40-60º) Sodic amphiboles are commonly blue, and often called “blue amphiboles” Single Chains- Pyroxenes Pyroxene Chemistry b The general pyroxene formula: W1-P (X,Y)1+P Z2O6 Where β β –W = Ca Na 2+ a sin a sin – X = Mg Fe Mn Ni Li – Y = Al Fe3+ Cr Ti –Z = Si Al Anhydrous so high-temperature or dry Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) conditions favor pyroxenes over amphiboles 5 Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Enstatite and Hypersthene Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite • Non-calcic Pyroxenes c pigeonite li n 1200oC orthopyroxenes op • Enstatite yr ox en –2V = 60-90º es z – Birefringence = 0.008 1000oC Diopside Hedenbergite clinopyroxenes Solvus •Hypersthene o 800 C –2Vx = 50-90º pigeonite (Mg,Fe) Si O Ca(Mg,Fe)Si O – Birefringence – 0.014 orthopyroxenes 2 2 6 2 6 Enstatite Ferrosilite – Weakly pleochroic pink to pale green Diopside and Augite Sodic Pyroxenes • Colorless • Aegerine (NaFeSi2O6) • Birefringence of 0.020-0.030 – Strongly pleochroic in green – X^c = 2-6º •2Vz = 60º • Z^c = 40º – High birefringence (~0.050) • Pigeonite has 2Vz = 0-30º • Jadeite (NaAlSi2O6) – Moderate birefringence (~0.020) – Z^c = 30º Independent SiO4 Tetrahedra • Olivine Occurrences: – Principally in mafic and ultramafic igneous and meta- igneous rocks – Fayalite in meta-ironstones and in some alkalic granitoids – Forsterite in some siliceous dolomitic marbles • Monticellite CaMgSiO4 –Ca → M2 (larger ion, larger site) – High grade metamorphic siliceous carbonates 6.
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    The Stability of Amphibole in Andesite and Basalt at High Pressuresr

    American Mineralogist, Volume 68, pages 307-314, 1983 The stability of amphibole in andesiteand basalt at high pressuresr J. C. Alt-BN Department of Geology and Geography Bucknell University Lewisburg, Pennsylvania 17837 exo A. L. Bonrrcsen Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences University of Califurnia, Los Angeles Los Angeles, California 90024 Abstract Some of our earlier work (Allen et al., 1975)on the stability of amphibolesin andesite and basalt at high pressuresis subject to criticism becauseofloss ofiron from the starting material to the walls of the capsules(AgsoPdso) during the runs of/6, bufered over the range of stability of magnetite.Analyses of fused run products show substantialloss of iron from runs at magnetite-wiistite and nickel-nickel oxide conditions, but none from those at magnetite-hematiteconditions. We have now redone our earlier work on the Mt. Hood andesiteand l92l Kilauea olivine tholeiite under N-NO conditions and with silver capsules.Analyses of fused run products show no iron loss, and the reversed curve representing the maximum stability of the amphibolesin the Mt. Hood andesiteshows no changein location, although we now have better control on the high-pressurepart of this curve. The revised curve for the appetrance of garnet is significantly lower in pressure, passing through 15.5 kbar/940'C and 14.5 kbar/9fi)'C. No orthopyroxene appearedin the run products, in contrast to the results of our earlier work. The high-temperature segment of the amphibole-out curve for the tholeiite is at least as high as 1040'Cat 13kbar and 1050"Cat 16kbar, and the high-pressure part of this curve is at about 27 kbars, about 6 kbar higher than in our earlier work.
  • Kimberlites H3 RICHTERITE-ARFVEDSONITE

    Kimberlites H3 RICHTERITE-ARFVEDSONITE

    Kimberlites References SVISERO, D.P.; Meyer, H.O.A. and Tsai, H.M. (1977) : FRAGOMENI, P.R.Z. (1976) : Tectonic control of Parana- Kimberlite minerals from Vargem ( Minas Gerais) tinga Kimberlitic Province, Boletim Nucleo and Redondao (Piaui) diatremes, and garnet Centro-Oeste Soc. Bras. Geol., 5,3-10. Iherzolite xenolith from Redondao diatreme. Goiania (in Portuguese). Revista Brasileira de Geociencias, 7, 1-13. SVISERO, D.P.; Haralayi, N.L.E. and Girardi, V.A.V. Sao Paulo. (1980) : Geology of Limeira 1, Limeira 2 and SVISERO, D.P.; Meyer, H.O.A, and Tsai, H.M. (1979b) : Indaia Kimberlites. Anais 31. Congresso Bra- Kimberlites in Brazil : An Initial Report. sileiro de Geologia, 3, 1789-1801. Camboriu, Proc. Second Intern. Kimberlite Conference, (in Portuguese). 1, 92-100. Amer. Geoph. Union, Washington. SVISERO, D.P.; Hasui, Y. and Drumond, D. (1979a) : Geo¬ logy of Kimberlites from Alto Paranaiba, Minas Gerais. Mineragao e Metalurgia, 42, 34-38. Rio de Janeiro ( in Portuguese). “ Research supported by Fapesp, CNPq and FINEP. H3 RICHTERITE-ARFVEDSONITE-RIEBECKITE-ACTINOLITE ASSEMBLAGE FROM MARID DIKES ASSOCIATED WITH ULTRAPOTASSIC MAGMATIC ACTIVITY IN CENTRAL WEST GREENLAND Peter THY f ^ Nordic Volcanological Institute University of Iceland, 101 Reykjavik, Iceland. Present address: Programs in Geosciences, The University of Texas at Dallas, P.O. Box 688, Richardson, Texas 75080 U.S.A. Introduction Crystal chemistry of the alkali amphiboles Dawson & Smith (1977) proposed that a mica- The amphibole chemistry is calculated according to amphibole-rutile-ilmenite-diopside (MARID) suite of the general formula AQ_jB2C5^Tg'^022(OH)2* Estimation xenoliths in kimberlites were cumulates from a highly of Fe3+ shows the main part of the amphiboles to con¬ oxidized kimberlitic magma in the upper part of the tain excess cations for charge balance.