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


George R. Harlow1, Pete J. Dunn2, George R. Rossman3

1 Department of Mineral Sciences, American Museum of Natural History,
New York, New York 10024, U.S.A.

2 Department of Mineral Sciences, TSmithsonian Institution
Washington, DC 20560, U.S.A.

3 Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.



ABSTRACT

Gamagarite from Postmasburg district, Cape Province, Republic of South Africa is re-examined using analytical chemistry, infrared spsectra, and X-ray diffraction techniquies. The data incicate that gamagarite is isostructural with brackebuschite: space group P21/m (assumed center of symmetry) with unit cell constants a = 9.15(1), b = 6.17(1), c = 7.88(1), 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 poublikshed structure data, our interpretation prefers the brackebuschite-type structure as a partically acid vanadate (arsenate and phosphate) containing HVO42--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.