"Rock of the Month # 109, posted July 2010" ---

Crocoite, a chromate (compound oxide of lead and chromium) is a beautiful secondary mineral, formed by the weathering of metallic ores under rather unusual circumstances. Generally speaking, chromium ores do not contain economic amounts of lead, and vice versa.

This sample, obtained from Roger's Minerals of Val Caron, Ontario, is from the Adelaide mine, a specimen mine in the classic crocoite locality, the Zeehan-Dundas district of western Tasmania. The crocoite prisms are nested on a cavernous goethite matrix, presumably an intensely-oxidized sulphide ore (?). Adelaide was one of numererous small silver-lead mines that operated in the area between 1890 and 1915 (Haupt, 1988). The silver-lead ore lay adjacent to an ultramafic body that had been completely serpentinized. The mine was built on a large gossanous outcrop, and oxidation extended to a depth of 75 m. Besides crocoite, the mine has yielded specimens of dundasite, cerussite, pyromorphite and other minerals. The Tunnel Hill locality yields the mauve Mg Cr carbonate, stichtite. This district is an interesting occurrence - juxtaposing lead in the base-metal ore with a distinct geological environment, the ultramafic body, presumed source of the chromium in crocoite. Single crystals from the mine may be as long as 9 cm (Barlow et al., 1996, p.403). Crocoite is known from several local mines around Dundas and Heazlewood, located some 43 km to the NNW. Occurrences include the Adelaide, Red Lead, Kapi, Heazlewood Silver-Lead and Kosminski mines. A major find of crocoite specimens was made at Adelaide in 1993, with some of the fine crystal pockets resting on goethite and black manganese oxides (Kissling, 1996). Some spectacular specimens are now in collections (e.g., Wilson, 2009, pp.65,155).

The following references include numerous photographs of crocoite specimens from this area, and from other noted occurrences, such as the Urals of Russia, where the mineral was originally described. Crocoite occurs on quartz at Berezovsk, in Yekaterinburg Oblast (Smith and Smith, 1995).

Other occurrences are known in South Africa. In particular, the Argent lead- zinc- silver veins have some parallels with Dundas. The veins, which produced silver, lead and some zinc, are hydrothermal veins spatially associated with the mighty mafic layered intrusives of the Bushveld complex. Roughly 100 km east of Johannesburg, the mineralized veins occur in gabbroic, likely a satellite of the Bushveld complex (Cairncross and Dixon, 1995). Thus once again we have veins rich in lead juxtaposed with mafic rocks apt to be relatively enriched in chromium. The widespread occurrence of hydrothermal veins with green, chromian micas attests to the mobility of chromium in such settings (see Rock of the Month 35, fuchsite). The Argent crocoite is found on gossan, clay and quartzite matrices, in association with other minerals such as cerussite, pyromorphite and vanadinite. In contrast, see the high-temperature, magmatic chromite deposits represented by the Bushveld's own (Rock of the Month 60) UG-2 chromitite.

The following image (field of view circa 8 mm) shows a close-up of the striated, monoclinic, lustrous (vitreous) crocoite prisms.

References

Barlow,FJ, Jones,RW and LaBerge,GL (editors) (1996) The F. John Barlow Mineral Collection. Sanco Publishing, Appleton, WI, 408pp.

Bonewitz,RL (2005) Smithsonian Rock and Gem. DK Publishing Inc. / Smithsonian Institution and Dorling Kindersley Limited, 360pp.

Cairncross,B and Dixon,R (1995) Minerals of South Africa. Geol.Soc.S.Africa, 296pp.

Cairncross,B and Windisch,W (1998) Microminerals from the Bushveld complex, South Africa. Mineral.Record 29, 461-465.

Haupt,J (1988) Minerals of Western Tasmania. Mineral.Record 19 no.6 (Australia issue, pp.353-504), 381-388.

Henderson,B (1981) Microminerals. Mineral.Record 12, 105-111.

Kissling,A (1996) Crocoite find. Mineral.Record 27 no.1, 67-68.

Lancaster,KE (1980) Minerals and Gemstones of Tasmania and their Locations. Gemcraft Publications Pty Ltd, East Malvern, Vic., 56pp.

Moore,T (2007) Tucson Show 2007. Mineral.Record 38, 211-232.

Smith,B and Smith,C (1995) A guide to mineral localities in the former Soviet Union. Mineral.Record 26 no.6, 517-549 [the same sample is also figured in Mineral.Record 26 no.2, p.150].

Wade,S (2007) A labor of love. Colored Stone 20 no.1, 342-345.

Williams,SA (1974) The naturally occurring chromates of lead. Bull.B.M.(Nat.Hist.) Min. 2 no.8, 377-419.

Wilson,WE (1997) Edward R. Swoboda. Mineral.Record 28, 449-456.

Wilson,WE (editor) (2009) Private Mineral Collections in Texas. Mineral.Record 40 no.1, supplement, 180pp.

Wilson,WE, Bartsch,JA, Van Pelt,H and Van Pelt,E (1992) Minerals of the Houston Museum of Natural Science. Mineral.Record 23 no.1, supplement, 34pp.