Högbomite in Sapphirine-Bearing Rocks from the Bamble Sector, South Norway

D. Visser, P. H. M. Thijssen* and J. C. Schumacher**
Department of Geochemistry, Institute of Earth Sciences, University of Utrecht, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
Mineralogisches Institut, Olshausenstrasse 40, W-2300 Kiel, Germany
*Present address: Forschergruppe Hochdruck Metamorphose, Institut für Mineralogie, Ruhr-Universität Bochum, Postfach 102148, W-4630 Bochum 1, Germany.
**Present address: Mineralogisch-Petrographisches Institut, Albertstrasse 236, W-7800 Freiburg, Germany.

Abstract: Högbomite is reported from two upper-amphibolite and granulite-facies, sapphirine-bearing, Al-Mg-Fe-rich and silica-poor lens-shaped layers within the Bamble Sector, south Norway. Primary assemblages, indicating peak metamorphic conditions of 773–844°C at 7 kbar (Mg-Fe exchange thermometry),are spinel-sapphirine-biotite-gedrite, spinel-corundum-sapphirine-cordierite and orthopyroxene-biotite-cordierite-plagioclase. Högbomite formed by hydrous alteration and oxidation of primary spinel and rutile and/or ilmenite according to the generalised reaction: spinel + ilmenite/rutile ± sapphirine ± gedrite + H2O + O2 = högbomite ± corundum ± magnetite ± chlorite. Suggested conditions of högbomite formation are 550–620°C and 6–7 kbar. The högbomites contain 10.2–14.7 wt.% MgO, 0−.03 wt.% ZnO, 58.9–62.1 wt.% Al2O3 and 15.6–17.6 wt.% Fe as FeO. The two högbomites may belong to different polytypes, as suggested by their differing TiO2 (9.9–10.1 versus 5.7–5.8 wt. %) and calculated Fe3+- and H2O-contents. The partitioning of Zn between spinel and högbomite is not uniform and is considered to depend upon prevailing ƒo2 and αH2O.

Keywords: högbomite • spinel • secondary formation • oxidation • sapphirine-bearing rock • micro-probe analysis • Bamble Sector • south Norway

Mineralogical Magazine; September 1992 v. 56; no. 384; p. 343-351; DOI: 10.1180/minmag.1992.056.384.06
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)