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.
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)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 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.