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 (1s) Cr minerals exist in Earth's crust today, indicating that 28 +/- 13 (1s) 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.