Crocoite

Crocoite is a mineral made of lead chromate (PbCrO₄), with crystals that form in the monoclinic system. Its chemical makeup is the same as the synthetic pigment known as chrome yellow, which is used in paints.

The mineral typically appears as large, well-formed prismatic crystals with an adamantine luster, though many crystals have poorly developed terminations. They are a bright hyacinth-red, translucent, and have a luster that ranges from adamantine to vitreous. Exposure to ultraviolet light can reduce some of their translucency and brilliance. The streak is orange-yellow. Crocoite has a Mohs hardness of 2.5 to 3 and a specific gravity of 6.0.

It was first discovered in 1766 at the Berezovskoe Au Deposit (Berezovsk Mines) near Ekaterinburg in the Urals. In 1832, F. S. Beudant named it crocoise from the Greek word *krokos* (saffron), referring to its color; the name was later changed to crocoisite and then to crocoite. At the type locality, the crystals occur in gold-bearing quartz veins that cut through granite or gneiss, associated with quartz, embreyite, phoenicochroite, and vauquelinite. Phoenicochroite is a basic lead chromate (Pb₂CrO₅) that forms dark red crystals, while vauquelinite is a lead-copper phosphate-chromate (Pb₂CuCrO₄PO₄OH) that forms brown or green monoclinic crystals. Vauquelinite was named after Louis Nicolas Vauquelin, who discovered chromium in crocoite in 1797, simultaneously with and independently of M. H. Klaproth.

Abundant masses of exceptional crocoite crystals have been found at the Extended Mine, Adelaide Mine, Red Lead Mine, West Comet, Platt, and several other mines in Dundas, Tasmania. The Adelaide Mine is considered probably the world’s prime source of crocoite. The Adelaide Mining Company Pty. Ltd. has been actively mining crocoite there since 2004. Crocoite usually forms long, slender prisms, typically 10–20 mm long but rarely reaching up to 100 mm (4 inches), with brilliant luster and color. It is the official Tasmanian mineral emblem. Other notable localities include Congonhas do Campo near Ouro Preto in Brazil, Luzon in the Philippines, Mutare in Mashonaland, near Menzies in Western Australia, as well as Brazil, Germany, and South Africa.

The relative rarity of crocoite is due to the specific conditions needed for its formation: an oxidation zone of a lead ore deposit and the presence of ultramafic rocks that supply chromium (as chromite). The oxidation of Cr³⁺ to CrO₄²⁻ (from chromite) and the decomposition of galena or other primary lead minerals are both required. These conditions are unusual. Because crocoite is composed of lead(II) chromate, it is toxic, containing both lead and hexavalent chromium.

The Adelaide Mine in Dundas, Tasmania, is famous for producing the world’s best crocoite. Adam Wright and his partners have operated it for mineral specimen removal for many years, with significant finds including the 2010 Pocket and the more recent Red River Pocket. The Mineralogical Record has documented major discoveries there, notably the "Red River Find" in 2012. Among the mines in the Dundas field, the Adelaide Mine is the most prolific source of superb crocoite specimens, both historically and currently. It is located at the junction of Comet Creek and Adelaide Creek, near the base of a spur of Stichtite Hill, about 2 km southeast of the original Dundas townsite. A video titled "The Adelaide Mine" features mineralogist Ralph Bottrill from Mineral Resources Tasmania, who takes viewers underground to explore the Red River find.

The Red Lead Mine, also in the Dundas region near Zeehan on Tasmania’s west coast, is renowned for its striking crocoite specimens. Shane Dohnt purchased it in 1986 and has been actively working it for crocoite ever since. He initially used open-cut mining but later reopened a collapsed adit from the 1890s to access lodes below the surface, reducing environmental impact and reaching areas too deep for open-cut methods. The mine has produced many fine crocoite specimens, and operations continue in hopes of further exceptional finds. Its specimens are highly sought after by collectors and have been displayed at the Smithsonian Museum of Natural History in Washington, D.C.

Crocoite from Tasmania has also been mined from the Dundas Extended Mine by Mike and Eleanor Phelan since the mid-1980s, though the mine’s origins date back to 1892, when it was a prospecting tunnel for silver lead. In April 2019, the mine was listed for sale but was not sold.