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Magnesian högbomite in a sapphirine-bearing rock from the Fiskenaesset region, W. Greenland

Published online by Cambridge University Press:  05 July 2018

D. Ackermand ,
B. F. Windley  and
R. K. Herd
D. Ackermand
Affiliation:
Mineralogisches Institut der Universität, D-2300 Kiel, Federal Republic of Germany
B. F. Windley
Affiliation:
Department of Geology, The University, Leicester LE1 7RH, U.K.
R. K. Herd
Affiliation:
Geological Survey of Canada, 601 Booth Street, Ottawa Ontario, Canada K1A OE8
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Abstract

Högbomite occurs in a 20 × 80 cm gedrite-rich lens between meta-volcanic amphibolite and anorthosite at the top of the Fiskenaesset Complex in the Angnertussoq area 13 km SE of Fiskenaesset. Between coarse prismatic gedrites there are dusters with chlorite, spinel, sapphirine, rutile, högbomite and corundum. Högbomite forms 200 µm grains often associated with rutile, surrounded by spinel and sapphirine but never in contact with gedrite. Usually sapphirine forms rims on spinel and contains relict gedrite. Corundum is intergrown with spinel and sapphirine. Reactions suggested by the textures are: 1. Gedrite + spinel + rutile → sapphirine + högbomite + chlorite. 2. Sapphirine → chlorite + corundum + spinel. 3. Högbomite → rutile + corundum + spinel. In some areas without rutile or högbomite the reaction gedrite + spinel + ? → sapphirine + chlorite took place.

Mg/(Mg + Fe + Mn) ratios are in the following order: sapphirine (0.95–0.92) > chlorite (0.94) > hornblende (∼ 0.91) > anthophyllite (∼ 0.86) > gedrite (∼ 0.86) > spinel (0.77–0.74) > högbomite (0.75–0.71). Högbomite usually has an anhydrous total of c. 98 %; on the basis of 31 oxygens and with total Fe as FeO one representative analysis has Mg 4.59, Fe 1.73, Zn 0.02, Ni 0.05, Al 14.78, Cr 0.02, Ti 1.2 atoms per formula unit.

Type
Research Article
Information
Mineralogical Magazine , Volume 47 , Issue 345 , December 1983 , pp. 555 - 561
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1983

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