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Unconventional Platinum Group Minerals
You are here: MineralCollecting.org >> Posted Articles >> Article 16

by Jacques Jedwab
UNCONVENTIONAL PLATINUM GROUP MINERALS

This is an attempt to call the attention on a group of compounds, which mere existence is challenging our set ideas about the supergene destiny of the Platinum Group Elements (PGE) and the Platinum Group Minerals (PGM). Indeed, we were brought up with the concept of natural PGM (comprised of native elements, alloys, intermetallics, sulphides, selenides, tellurides and sulfosalts), which, when undergoing alteration (if anywise), give rise to hypothetical, invisible, readily adsorbable, chemical species. One did not think of solid alteration compounds, observable under the microscope, and the very scarce observations of PGE oxides were classified "unconfirmed" not long ago.

However, the first oxidic PGE-compounds have been massively discovered in Brazil in 1695, but when analytical chemistry was in its infancy and when the PGE still awaited their discovery. These undetermined black crusts on gold grains were nevertheless named (ouro preto, i. e. black gold in Portuguese) and actively mined for the included native gold. Since then, very few discoveries have occurred, but they gathered pace in the last thirty years, most of them due to the extensive use of the scanning electron microscope (SEM) and the electron microprobe (EMP). Today, about 30 localities have been recognised in 15 countries from 5 continents.

We define arbitrarily the Unconventional Platinum Group Minerals (UPGM) as the group of
PGE-containing oxides, hydroxides, oxihydroxides, silicates, sulfates, halogenides, organometallics, etc. Representatives of these compounds have all be observed to date. The grouping is a heuristic one, and is primarily intended at convincing (and reminding) the professionals and amateurs that such weird things do exist, and are possibly frequent.

Although the reality of UPGM is no more controversial, their finding and recognition is still problematic. When conventional PGM catch the eye with their high photon and electron reflectivities, UPGM are generally dull, low reflecting, and easily confused with the common and often associated iron and manganese oxides/hydroxides. In addition, they have no crystalline outline, and present a wide range of textures, chemical compositions and associations. Even worse, they are very often complex mixtures of different sub-micrometric components, and correspond more to the definition of "mineraloids" than "minerals".

The practical consequence of these difficulties is that UPGM are often impossible to locate and recognise without the extensive (and expensive) use of the SEM and the EMP: this does of course not mean that they do not exist, or that their detection is an insuperable task. Actually, UPGM have been detected in nearly all surficial PGE-deposits examined to date, but it is still unclear why they are so widespread in some deposits, and so rare in others. The climatic factor does not seem decisive, but the nature of the precursors is more so. This is a point which should be better explored when an extensive inventory will be available.

A selection of original photographs of UPGM found in PGE-ores, classical and unconventional are presented in http://www.ulb.ac.be/sciences/upgm/. The same features are displayed under the reflected light microscope (RLM) and the SEM, as far as possible. All chemical statements are of course based on EMP analyses. Due to their highly variable properties (physical and chemical), UPGM must be detected and examined with the instruments used under their different modes, in order to display or enhance the diagnostic features: RLM-dry and -oil immersion objectives, unpolarized and polarized light, and SEM-secondary mode (-SM) and SEM-back scattered mode (-BS).

Jacques Jedwab. Université Libre de Bruxelles. Brussels, Belgium. jjedwab@ulb.ac.be

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