Peloidal phosphorite from the Permian of Idaho, USA. (4.6 centimeters across at its widest)

This scarce rock has economic value - it’s a phosphate ore. It's from the most famous phosphorite unit in America, the Phosphoria Formation. This unit is mined extensively in the Southeast Idaho Phosphate District. Phosphate and elemental phosphorus derived from processing of phosphorite rocks are used to make agricultural fertilizers and industrial chemicals.

Phosphorites are calcium phosphate-rich sedimentary rocks, generally considered to have >15-20% phosphate content. Texturally, phosphorites can be obviously granular, with fossil fragments or oolites or peloids or lithic fragments, or they can be composed of extremely fine-grained, phosphate-rich mud. Compositionally, the phosphate component in phosphorites is principally a mix of apatite minerals: chlorapatite (Ca5(PO4)3Cl), fluorapatite (Ca5(PO4)3F), hydroxyapatite (Ca5(PO4)3OH)), and carbonate fluorapatite (Ca10(PO4,CO3)6F2-3).

Phosphorites are generally marine sedimentary rocks. They range in age from Precambrian to Holocene. In modern oceans, they tend to occur along the eastern margins of some ocean basins where deep-water upwelling occurs under areas of high biologic productivity.

Stratigraphy: Phosphoria Formation, Roadian Stage to Wordian Stage, lower Guadalupian Series, mid-Permian