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

Abstract

AbstractHögbomite 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.