On the occurrence of smolyaninovite in the Mount Cobalt deposit, in north-western Queensland, Australia

OVER the last several years the Department of Chemical Engineering of Monash University has been studying the general problem of mineral beneficiation of cobalt minerals. As part of the investigation, samples were obtained from the cobalt-mineralized area around Mount Cobalt, which is situated some 11o km south of Cloncurry in north-west Queensland, Australia. The geology of the area has been described by Rayner (1953), who reported the presence of two distinct cobalt-containing minerals, the primary sulpharsenide cobaltite and its weathering product erythrite. Mineral dressing studies in the laboratory showed that a third distinct cobalt-bearing mineral was present as well, with properties quite different from the two reported by Rayner. The 'new' mineral occurs as a relatively soft polycrystalline earthy yellow-brown material in veinlets and is frequently, but not always, associated with erythrite. It is a weathering product and has probably resulted from the weathering of cobaltian arsenopyrite or aUoclase (Co, Fe)AsS, both of which have been detected by Croxford (1974) in a copper orebody in near-by Mount lsa but not as yet at Mount Cobalt. Chemical analysis of the mineral showed it to be essentially a cobalt-iron-calcium arsenate. Mrssbauer spectra indicated that the iron was present in the ferric state only. Electronmicroprobe analyses and element mapping of individual grains confirmed that the constituent elements were uniformly distributed over the entire exposed grain surfaces and hence throughout the particles, and none was present as a gangue inclusion. These analyses showed further that the iron:cobalt ratio was constant from grain to grain within the accuracy of the techniques used. The only hydrated iron (III) cobalt arsenate that has been reported is smolyaninovite, which was described by Yakhontova (1956, 1973). Smolyaninovite is an earthy oxidation product found at Bou-Azzer, Morocco, with a composition listed as: (Co, Ni, Ca, Mg)4(Fe, A1)2(As O4)4.I I H20. The chemical analyses of the mineral isolated from the Mount Cobalt deposit and of one sample of smolyaninovite given by Yakhontova are shown in Table I. The new analysis was carried out by wet-chemical methods except for water, which was determined by a microgravimetric technique. The major difference between the mineral described by Yakhontova and the one obtained from the Australian source is in the ratio of the divalent to trivalent cations. The ratio for the Mount Cobalt mineral approximates to 3 : 2 while the Russian author gives a value of 4: 2,