Hydrothermal Subsilicic Sodium Gedrite from the Gåsborn Area, West Bergslagen, Central Sweden

A. Damman
Institute of Earth Sciences, Free University, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Abstract: Subsilicic sodium gedrite has been found in a hydrothermal vein together with sekaninaite (Fe-cordierite), andalusite, plagioclase, topaz, sillimanite, quartz, biotite, magnetite, ilmenite, hercynite, wolframite, pyrrhotite, chalcopyrite and pyrite. This vein is part of a hydrothermal vein system consisting predominantly of quartz-feldspar veins, some of which contain andatusite, sekaninaite, biotite, muscovite, fluorite and accessory oxides and sulphides, which was formed during the later stages of crystallization of a high-level anorogenic Svecofennian granite. Petrographic observations suggest the following crystallization sequence for the quartz-feldspar veins: plagioclase-quartz-andalusite-sek aninaite-microcline-biotite-albite-oxides and sulphides-muscovite-fluorite, and for the subsilicic sodium gedrite-bearing vein: andalusite-sekaninaite-subsilicic sodium gedrite-biotite-quartz-albite-sillimanite-topaz-oxides and sulphides. Electron microprobe analysis revealed that all subsilicic sodium gedrite is relatively homogeneous with only the following compositional variation: Na(A) 0.57–0.81, Aliv 2.31–2.57, XMg 0.15–0.21. The temperature (T) for the formation of the hydrothermal vein system is estimated at 550–600°C and the pressure (P) is estimated to be less than 3 kbar.

Keywords: hydrothermal vein • andalusite • sekaninaite • subsilicic sodium gedrite • albitization • sodium

Mineralogical Magazine; April 1988 v. 52; no. 365; p. 193-200; DOI: 10.1180/minmag.1988.052.365.05
© 1988, The Mineralogical Society
Mineralogical Society (www.minersoc.org)