Preiswerkite, an Al-rich trioctahedral sodium mica from the Geisspfad ultramafic complex (Penninic Alps)

H. R. Keusen; T. Peters

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Other| December 01, 1980

Preiswerkite, an Al-rich trioctahedral sodium mica from the Geisspfad ultramafic complex (Penninic Alps)

H. R. Keusen;

H. R. Keusen

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T. Peters

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Author and Article Information

H. R. Keusen

T. Peters

Publisher: Mineralogical Society of America

First Online: 03 Mar 2017

Online ISSN: 1945-3027

Print ISSN: 0003-004X

American Mineralogist (1980) 65 (11-12): 1134–1137.

Abstract

The properties and occurrence of an Al-rich trioctahedral sodium mica are described. From microprobe analysis the formula

({Na}_{0.98} K_{0.02} {Ca}_{0.01}) ({Mg}_{1.91} {Al}_{0.91} {Fe}_{0.08}^{2 +} {Fe}_{0.06}^{3 +}) ({Al}_{1.95} {Si}_{2.05}) O_{12.06} {(OH)}_{2.10}

is derived. The idealized composition NaMg₂Al[(OH)₂(Al₂Si₂O₁₀)] is related to phlogopite by the coupled substitutions Na ⇄ K and AlAl ⇄ MgSi. The mica was formed during Alpine metamorphism (upper greenschist to lower amphibolite facies) with Al-pargasite and zoisite in a rodingite dike within the harzburgitic ultramafic mass of the Geisspfad ultramafic complex.

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