Mineralogical Studies on Some Synthetic Alloys and Minerals of the Platinoid Group

Abstract

Concentrates from five different localities were investigated in this study. These were alluvial concentrates from Alaska (U.S.A.), Choco (Columbia), Witwatersrand(South Africa), Urals (U.S.S.R), and eluvial nuggets from Ethiopia. The principal method of investigation was optical microscopy including quantitative reflectance and microhardness measurements. Analyses of whole grains or inclusions were carried out by electron-probe microanalysis. To help in understanding natural alloys, synthetic binary alloys of iron and platinum were studied. A series of synthetic alloys of iridium-osmium-ruthenium were prepared and studied to assist in understanding the mineralogy of similar natural alloys. The phase relationships found in the study of synthetic iridium-osmium-ruthenium alloys were used to interpret the mineralogy of the Witwatersrand concentrates. As a result some hypotheses on the origin of these placer deposits are presented. Ferroplatinum grains differ in size morphology and optical properties. These differences are due to elements contained in solid solution. Iron and copper are invariably dissolved in the platinum, from which it is concluded that the three elements were brought together in the original magma. Iron was found to have a large effect on the reflectivity and microhardness of ferroplatinum. For naturally occurring ferroplatinum this effect is outweighed by the presence of dissolved iridium. In synthetic binary feroplatinum alloys it was found that the reflectivity decreased as the iron content increased. The microhardness of synthetic ferroplatinum increases with the increase of iron content, which reaches a maximum at the composition of Fe Pt. Phase relations in the iridium-osmium-ruthenium system have been determined. It was found that the two phase-field determined is much narrower than reported in previous work. The microhardness of synthetic hexagonal alloys is greater than cubic alloys. Hardness changes in naturally occurring alloys are interpreted according to the various mechanisms of hardening in solid solution, Data on the microhardness and reflectivity of native iridium were also obtained.