Chromium mineral ecology

Minerals containing chromium (Cr) as an essential element display systematic trends in their diversity and distribution. We employ data for 72 approved terrestrial Cr mineral species (http://rruff.info/ima, as of 15 April 2016), representing 4089 mineral species-locality pairs (http://mindat.org and other sources, as of 15 April 2016). We find that Cr-containing mineral species, for which 30% are known at only one locality and more than half are known from three or fewer localities, conform to a Large Number of Rare Events (LNRE) distribution. Our model predicts that at least 100 ± 13 (1σ) Cr minerals exist in Earth's crust today, indicating that 28 ± 13 (1σ) species have yet to be discovered - a minimum estimate because our model assumes that new minerals will be found only using the same methods as in the past. Numerous additional Cr minerals likely await discovery using micro-analytical methods. We propose 117 compounds as plausible Cr minerals to be discovered, including 7 oxides, 11 sulfides, 7 silicates, 7 sulfates, and 82 chromates. Depending on their compositions and crystal structures, new Cr minerals are likely to be discovered in various environments, including meteorites, basalt, evaporites, and oxidized Pb ore deposits.