THE TRANSFORMATION OF PENTLANDITE TO VIOLARITE UNDER MILD HYDROTHERMAL CONDITIONS: A DISSOLUTION- REPRECIPITATION REACTION

The formation of violarite from pentlandite by supergene processes has long been recognized as a replacement reaction (Misra & Fleet 1974 and references therein). Misra & Fleet (1974) suggested that the reaction might be topotactic, as pentlandite also has a structure based on cubic close packing of S with Fe and Ni atoms occupy 1/2 the tetrahedral sites and 1/8 of the octahedral sites (Rajamani & Prewitt 1973). Thornber (1975) proposed that the formation of violarite from pentlandite was an anodic process with the orebody acting as a corrosion cell. He proposed that at depth the primary sulfides such as pentlandite lose iron to solution to become more S–rich and the electrons are conducted away through the ore towards the surface. The groundwater acts as an electrolyte to complete the cell. However, Putnis (2004) suggested that the porous and cracked texture exhibited by supergene violarite indicates a dissolution-reprecipitation reaction rather than a topotactic transformation. We have undertaken a detailed laboratory study of the replacement of pentlandite by violarite using a series of waterbath and hydrothermal cell experiments. We report here preliminary results of these studies into the conditions under which the transformation proceeds and on the kinetics and mechanism of the reaction. EXPERIMENTAL Both natural and synthetic pentlandite was used as the starting material for the transformation experiments. Pure pentlandite, in grains larger than 1 mm, is very difficult to obtain from nature or to prepare synthetically and one must use either natural or synthetic exsolved pentlandite in pyrrhotite or fine grained pentlandite concentrates.