Magnetic Hausmannite from Hydrothermally Altered Manganese Ore in the Palaeoproterozoic Kalahari Manganese Deposit, Transvaal Supergroup, South Africa

J. Gutzmmer, N. J. Beukes, A. S. E. Kleyenstuber and A. M. Burger
Department of Geology, Rand Afrikaans University, P.O.Box 524, Auckland Park 2006, South Africa
MINTEK, Private Bag X3015, Randburg 2125, South Africa

Abstract: Hausmannite (Mn3O4), a manganese oxide with a tetragonally distorted spinel structure, is considered to be ferrimagnetic with a very low Curie temperature of 42.5 K. However, strongly magnetic hausmannite has been discovered in some of the hydrothermally altered high-grade manganese ores of the giant Kalahari manganese deposit in South Africa. EDS-electron microprobe analyses indicate magnetic hausmannite to contain on average between 3 and 11.3 wt.% Fe2O3. In contrast non-magnetic hausmannite contains on average about 1–3 wt.% Fe2O3. X-ray powder diffraction analyses reveal small changes in cell dimensions of the magnetic hausmannite related to the high iron content. Mössbauer spectroscopy suggests that all iron is in the trivalent state. Optical microscopy and scanning electron microscopy (electron back-scatter imaging) proved the magnetic hausmannite to be homogeneous in composition, containing only a few minute inclusions of hematite. Magnetic blocking temperatures of the iron-rich hausmannite, approximating the Curie temperature, are of the order of 750 K. It is suggested that the ferrimagnetic state of hausmannite is stabilized and enhanced by replacement of Mn3+ by Fe3+.

Keywords: hausmannite • Kalahari manganese field • Wessels mine • ferrimagnetism • iron content

Mineralogical Magazine; December 1995 v. 59; no. 397; p. 703-716; DOI: 10.1180/minmag.1995.059.397.12
© 1995, The Mineralogical Society
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