gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1934.023.146.03 Lusakite, a Cobalt-Bearing Silicate from Northern Rhodesia Skerl A. C. A.R.S.M., B.Sc. Rhodesian Beit Railway Fellow at the Royal School of Mines, London Bannister F. A. M.A. Assistant-Keeper in the Mineral Department of the British Museum of Natural History Groves A. W. Ph.D., M.Sc., D.I.C., F.G.S. Geochemistry Department, Imperial College of Science and Technology, London 9 1934 23 146 598 606 Copyright © 1934, The Mineralogical Society 1934 Summary

Lusakite is a cobalt-bearing variety of staurolite from Northern Rhodesia and has been named after the new capital of that colony. The orthorhombic crystals have unit-cell dimensions a 7·86, b 16·62, c 5·65 Å. (a: b: c = 0·473: 1: 0·340), space-group Vh17, and the unit cell contains 8[RO.Al2SiO5], where R = Fe,Co,Ni,Mg,Al,H. The empirical formula is H2O.4(Fe,Co,Ni,Mg)O.9(Al,Fe)2O3.8SiO2. Hardness 7 ½ specific gravity 3·767, n about 1·74. The following striking features justify the new name:

It is the only silicate mineral containing a significant amount of cobalt (CoO 8·48 %).

An intense cobalt-blue colour best seen in thin sections under the microscope.

Strong pleochroism from light cobalt-blue to deep violet-blue.

It has been formed by the metasomatic replacement of kyanite.