1090th General Monthly Meeting
Colour and Cash: the Exquisite Minerals of the Oxidation Zone
Prof. P.A. Williams
University of Western Sydney
Date: Wednesday 4th April, 2001
The geochemistry and chemical mineralogy of the oxidized zones of orebodies are extraordinarily complex, much more so than the primary deposits from which they are derived. The zones themselves are of great economic importance, especially to Australia. Many of the free-milling gold ores exploited over the last two decades are of supergene origin (secondary gold) and cobalt, nickel and copper deposits of the same kind are of increasing importance. We know much of the equilibrium chemistry of such mineral settings for some elements and something of the general processes that play a part in the development of ore grades in the oxidized zone. For other elements the situation is somewhat more sketchy and of certain aspects, including kinetics of crystallization, temperatures of mineral formation and solid-solution phenomena, we know very little indeed. Oxidized zone mineralization, some of which is exquisitely beautiful, is described in terms of its origin and the associated chemistry necessary to extract metals from these sorts of deposits. Further research directions are highlighted, together with problems that remain to be solved.
Professor Williams explained that the narrow zone of planet earth containing the air, water and solutes essential to our living existence has also been chemically transforming familiar rocks and minerals for ages past. For example, gold can be transported and accumulated in economically minable concentrations and examples shown of such secondary gold fully justified the words of the title "colour and cash". The speaker then discussed and illustrated the colourful crystals of selected natural minerals of copper (especially those of secondary origin), often drawing upon early descriptions in the Society's Journal. Most are "mixed salts"; for example, gerhardtite is a copper compound containing nitrate and hydroxide, being dichroic (appearing blue and green in different orientations). Another, connellite, is perhaps most simply described a cupric hydroxide compounded to a sky blue hue with a smidgin each of copper chloride and sulfate. However, nantokite (cuprous chloride CuCl) is colourless and shatters like quartz under the hammer, but yields thixotropically under fingernail pressure. Others seem to reach their thermodynamic destiny(say, green malachite, a basic copper carbonate) via a metal form that dissolve and redeposits (blue georgeite in the malachite case). Here the speaker invoked the Ostwald Step Rule (1897) to explain the phenomenon. The economic significance of the colourful copper minerals was emphasized.
One mineral mystery, now all but solved, was the mineral "chillagite", named on Edgeworth David's suggestion and debuting in the Society's Journal. Further work published in the Journal and Proceedings decried it but recent work has shown that it is a valid species.
Following a spirited discussion, a vote of thanks was proposed by Professor Tony Baker, who wittily remarked on the speaker's aplomb when suddenly confronted by the failure of the projector during his address and being forced to adapt an overhead projector for showing his splendid slides of mineral treasures.