Mrrk / Wikimedia CommonsSperrylite, a common source of platinumMrrk / Wikimedia Commons
In a stream in the municipality of Morro do Pilar, geologists Alexandre Raphael Cabral of the Federal University of Minas Gerais and Antônio Seabra Gomes Jr., from a mineral exploration company, found a tiny piece of gold measuring approximately one millimeter in length, with a platinum arsenide mineral called sperrylite inside it, measuring 2 to 10 micrometers. The surprise came later. After the grain of gold was polished by geologist Stephanie Lohmeier of the Clausthal University of Technology in Germany, the researchers found that gold atoms could replace arsenic atoms inside the mineral, albeit in small proportions: in 100 grams (g) of sperrylite, gold would correspond to 3.4 g. “There was plenty of gold and little arsenic to bond with the platinum,” says Cabral. “The auriferous sperrylite proves a possibility that had already been demonstrated theoretically and experimentally.” The discovery suggests another possible source of platinum and gold: quartz veins formed at low temperatures—close to 350 °C—as a result of earthquakes that occurred around 500 million years ago, in a 230-kilometer area between Ouro Preto and Diamantina (The Canadian Journal of Mineralogy and Petrology, January 18).
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