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American Mineralogist; June 1999; v. 84; no. 5-6; p. 977-982
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The occurrence of preiswerkite in a tourmaline-biotite-scapolite rock from Blengsvatn, Norway

Diederik Visser, Timo G. Nijland, Dick J. Lieftink, and Cees Maijer

University Museum Utrecht, Utrecht, Netherlands

We report paragenesis and chemistry of a new occurrence of the rare trioctahedral Al-rich sodium mica preiswerkite. The preiswerkite occurs in a tourmaline-biotite-scapolite rock in the contact zone of a gabbroic boudin surrounded by Proterozoic metasediments near the Blengsvatn, Bamble sector, southern Norway. The preiswerkite occurs as subhedral crystals or is intergrown with biotite in a polygonal fabric together with Cl-rich scapolite+tourmaline+ or -ilmenite+ or -plagioclase+ or -corundum. Accessory minerals are hematite, hogbomite, spinel, allanite, apatite, and zircon, with relic calcite. Preiswerkite has the compositional range: (Na (sub 1.84-2.02) K (sub 0.02-0.10) Ca (sub < or =0.04) ) (Mg (sub 3.13-3.42) Fe (sub 0.63-0.77) VI Al (sub 1.87-2.07) ) ( IV Al (sub 3.58-3.96) Si (sub 4.04-4.29) S (sub < or =0.02) )O 20 [Cl (sub < or =0.03) (OH) (sub > or =3.97) ] and coexists with Na-Al-rich biotite, with the composition: (K (sub 1.38-1.61) Na (sub 0.18-0.45) Ca (sub < or =0.03) ) (Mg (sub 3.72-3.88) Fe (sub 1.38-1.43) Ti (sub 0.10-0.16) VI Al (sub 0.63-0.85) ) ( IV Al (sub 2.71-2.93) Si (sub 5.07-5.29) )O 20 [Cl (sub 0.02) (OH) (sub > or =3.98) ]. We suggest that the assemblage preiswerkite+biotite+tourmaline+scapolite + or - ilmenite + or - plagioclase + or - corundum was formed during prograde or peak metamorphism in the area, at approximately 700 degrees C and 7 kb.

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