Gamagarite: a re-examination and comparison with brackebuschite-like minerals

Abstract

Gamagarite from Postmasburg district, Cape Province, Republic of South Africa, is reexamined using analytical chemical, infrared spectra, and X-ray diffraction techniques. The data indicate that gamagarite is isostructural with brackebuschite; space group is P2_1/m (assumed center of symmetry) with unit cell constants ɑ = 9.15(1), b = 6.17(1), c = 7.88(l)Å, β = 112.7(2)°. Infrared spectra indicate that strongly hydrogen-bonded OH^- is the dominant hydrogen-bearing species in both gamagarite and brackebuschite. Consequently, with a reevaluation of previously published structure data, our interpretation prefers the brackebuschite-type structure as a partially acid vanadate (arsenate and phosphate) containing HVO^(2-)_4 -type units rather than as a hydrated vanadate containing structural water, as previously described. However, there are uncertainties with the assignment of Mn oxidation state.