Crikey, it’s crocoite!This striking red-orange mineral is known as crocoite. It is a lead

Crikey, it’s crocoite!This striking red-orange mineral is known as crocoite. It is a lead chromite with the formula PbCrO4. The name “crocoite” is derived from the Greek word “krokos”, meaning crocus (the flower from which saffron is harvested).In the late 18th century, this mineral had a brief stint as the chief source of the element chromium (until chromite was discovered in 1799) and was also a source of the pigment, chrome yellow. Nowadays crocoite is mined primarily for the mineral specimen collection market due to its relative rarity. This is because of the specialised conditions required for its formation: the presence of ultramafic rock (the source of chromium, found in chromite) near a zone of oxidised lead ore. It occurs here as a secondary mineral, which means that it forms due to the alteration of primary material. Ultramafic rocks are igneous in origin and derived from the Earth’s mantle. They are typically very low in light, silica- and potassium-rich minerals (e.g. quartz and feldspars) and high in and dark, heavy magnesium- and iron-rich minerals (e.g. peridotites and pyroxenes).Crocoite is a relatively soft, brittle mineral with a hardness rating of 2.5-3. It is prized by collectors for it’s acicular (needle-like) aggregates of prismatic, translucent crystals. It also occurs as reticulated (criss-crossing) clumps, singular elongated needles, and as a grainy, encrusting mass.Crocoite is only found at a handful of locations worldwide, including Brazil, Germany, the Philippines, Romania, and South Africa, but the best specimens are reputed to have come from the Australian state of Tasmania. As the photograph testifies, the specimens found there are stunning.YKImage credit:Australian Museum (http://australianmuseum.net.au/image/Crocoite-specimen/)Further reading:Mindat entry for crocoite: http://www.mindat.org/min-1157.htmlMining crocoite in Tasmania:http://www.mineral.org.au/news/crocoitemine.htmlUses of chromium: http://www.dartmouth.edu/~toxmetal/toxic-metals/more-metals/chromium-history.html -- source link
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