"Rock of the Month # 15, posted for September 2002" Cabriite, ideal formula Pd₂SnCu, is one of the many rare minerals which contain high proportions of one or more of the six platinum-group elements (PGE) *. Sample 1571.

The cabriite is the pinkish "L-shaped" grain, occurring in a matrix of three tawny copper sulphides, and flanked by large angular crystals of white sperrylite, PtAs₂, which is one of the more abundant and readily-recognized PGM, and one of the few widely available as macroscopic (mm-scale) crystals for collectors. The cabriite is partially rimmed by creamy-yellow, highly reflective native silver, which also infills a brittle fracture in the sperrylite at the upper left of the view. The bluish-grey veinlet is the lead sulphide, galena. Tiny specks in the cabriite are micron-scale inclusions of geversite, PtSb₂, a third PGM. Dark grey irregular lines are narrow fractures infilled by the epoxy mounting medium. The fracture-filling silver is almost pure, but within 5 mm of this assemblage is a mm-scale mass of aurian silver, containing up to 24 weight percent gold, plus other minerals such as the nickel sulphide pentlandite +. This photomicrograph was taken in reflected, plane-polarized light, 80x magnification, long-axis field-of-view 1.1 mm.

Cabriite is named for mineralogist and PGM specialist Louis Cabri. A similar image of this particular grain appears on the cover of the special "Cabri Issue" of Canadian Mineralogist (April 2002). The late Alan Criddle, another ore mineralogist par excellence, provided two digital images of this grain for that issue, although the monochrome reproductions scarcely hint at the remarkable clarity and colour resolution in the large, state-of-the-art original. He also collected diagnostic reflectance spectra on the same grain (see Criddle, 1998, for a technical account of modern ore mineralogy). Spectra and images, plus microprobe data and a brief review of the occurrence of cabriite can be found in Wilson et al. (2002).

There are at least 109 documented PGM species (Cabri, 2002), with a majority containing essential proportions of palladium and/or platinum. The figured specimen is from the Noril'sk region of Siberia (Komarova et al., 2002), the world's largest single source of palladium, and the second-largest supplier of PGE worldwide after the Bushveld of South Africa.

The other five books in the reference list offer non-technical accounts of the discovery, history and use of precious and other metals. The most detailed treatment is that of Street and Alexander (1998), but the anecdotal style of Venetsky makes two older, illustrated books a favourite read: the 1978 volume offers chapters on platinum and gold, and a brief review of the other PGE, while the sequel has chapters on palladium, osmium and iridium.

* Terminology: Mineralogists and metallurgists naturally need to distinguish between the platinum group elements (PGE) and their mineral hosts which contain essential quantities of PGE in their formulae (PGM). Unfortunately, the financial and promotional side of the metal markets focus on the metal aspect of the PGE, and commonly use PGM for the metals themselves. The six elements in question are Platinum (Pt), Palladium (Pd), Rhodium (Rh) and the generally rarer, less economically important Osmium (Os), Iridium (Ir) and Ruthenium (Ru).

+ Chemical diversity: A common feature of PGE ores is the large number of elements, including rare metals, found at percent-levels within a scale of a few mm or less. Here we have Cu, Fe, Ni, Pb and S in sulphides, Ag and Au in silver, Pt, Pd, As, Sb, Sn (and more Cu) in the actual PGM. Twelve elements: that's over 10 percent of the periodic table!

References

CABRI,LJ (2002) The platinum-group minerals. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 13-129.

CRIDDLE,AJ (1998) Ore microscopy and photometry. In `Modern Approaches to Ore and Environmental Mineralogy' (Cabri,LJ and Vaughan,DJ editors), MAC Short Course 27, 428pp., 1-74.

KOMAROVA,MZ, KOZYREV,SM, SIMONOV,ON and LULKO,VA (2002) The PGE mineralization of disseminated sulphide ores of the Noril'sk-Taimyr region. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 547-567.

ST. JOHN,J (1984) Noble Metals. Time-Life Books, 176pp.

STREET,A and ALEXANDER,W (1998) Metals in the Service of Man. Penguin Books, 11th edition, 300pp. plus 32 plates.

VENETSKY,S (1981) Tales About Metals. Mir Publishers, Moscow, English translation of 3rd Russian edition of 1978, 211pp.

VENETSKY,S (1983) On Rare and Scattered Metals. Mir Publishers, Moscow, English translation of Russian original of 1980, 152pp.

WILSON,GC, RUCKLIDGE,JC and CERMIGNANI,C (2002) Coarse-grained cabriite from Noril'sk, Russia. In `The Cabri Issue' (Martin,RF editor), Can.Mineral. 40 part 2, 473-479.

YOUNG,G and AMOS,JL (1983) The miracle metal - platinum. National Geographic 164 no.5, 686-706, November.