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.
The most famous phosphorite unit in America is the Phosphoria Formation (see 1st specimen below). On a global scale, phosphorite deposition was at its maximum, volumetrically, during the Neoproterozoic and Cambrian (see 2nd specimen below).
Peloidal phosphorite (4.6 cm across) from the Phosphoria Formation (Roadian Stage to Wordian Stage, lower Guadalupian Series, mid-Permian) at the Simplot Mine, southern Bingham County, southeastern Idaho, USA. This rock has economic value - it’s a phosphate ore. The Phosphoria Formation is extensively mined 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.
Fossiliferous peloidal phosphorite (4.7 cm across) - sample of the “2nd SSF (small shelly fossil) assemblage” from the upper Zhongyicun Member, upper Zhujiaqing Formation (lower Meishucunian Stage, lower Lower Cambrian). This comes from just south of Ercaicun, ~4 km WSW of Haikou, Kunming City Prefecture, east-central Yunnan Province, southwestern China. This rock was deposited not long after the Cambrian Explosion (the sudden, evolutionary appearance of abundant fossil life near the Precambrian-Cambrian boundary).
Most info. synthesized from:
Boggs (1992) - Petrology of Sedimentary Rocks. 707 pp.