gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1986.050.355.18 Mineralogy The Descloizite-Mottramite Series of Vanadates from the Otavi Mountain Land, South West Africa: An X-Ray Study van der Westhuizen W. A. de Bruiyn H. Tordiffe E. A. W. Botha B. J. V. Department of Geology, University of the Orange Free State, PO Box 339, Bloemfontein, South Africa 3 1986 50 355 137 140 12 3 1984 31 5 1985 Copyright © 1986, The Mineralogical Society 1986 Abstract

Descloizite, cuprian descloizite, and mottramite from most of the known prospects and mines in the Otavi Mountain Land were sampled and subsequently analysed chemically and by X-ray diffraction. Unit cell parameters were refined with the aid of a computer program. It was found that, contrary to previous opinion, chemical parameters of these vanadates can be deduced from their X-ray powder data according to the equations (correlation coefficients in parenthesis):

ZnO = 75.369c−0.119b−31.177a−456.512(0.9634)

Zn2+ = 369.568c + 236.583b − 146.42a − 3726.344 (0.9642)

CuO = − 74.991c + 0.086b + 29.419a + 486.204 (0.9649)

Cu2+ = − 369.568c − 236.583b + 146.420c + 3826.344 (0.9642).

 Keywords descloizite mottramite X-ray data Otavi Mountain Land South West Africa