Abstract

Högbomite and zincian högbomite are reported for the first time from a Mg-Fe-AI-rich layer within the serpentinite unit of the Vumba schist belt, Botswana. The zincian högbomite in one sample is a secondary product of a Zn-bearing hercynite. The FeO content ranges from 28 to 30 wt%, making it among one of the most Fe-rich zincian högbomites reported in the literature. Apart from the usual mineral assemblage spinel + corundum + chlorite + ilmenite ± rutile, a rare Na-mica (preiswerkite) is also found interfingered with chlorite and in contact with zincian högbomite and zincian hercynite.

In a second sample, a small grain of green spinel, found enclosed in a larger brown högbomite grain, is discontinuously zoned in Zn (1.23 wt% ZnO in core and about 24 wt% ZnO at the rim). The högbomite, which makes up about 50 vol% of the rock, contains a maximum of 0.70 wt% ZnO. Representative electron-microprobe analyses of högbomites give the following structural formulae, based on 8 oxygens:

D-136: (Mg0.39Fe1.072+Mn0.01Zn0.08)Ti0.23(Fe0.223+Al3.78)O8
D-225: (Mg0.92Fe0.572+Mn0.01Zn0.02)Ti0.24(Fe0.293+Al3.71)O8.

Based on field relations, textural features, and the composition of the minerals, the högbomites are considered to have been formed during the retrograde stage of the granulite-to amphibolite-facies metamorphism.

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