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

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Phosphorites are scarce, 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 the above rock). On a global scale, phosphorite deposition was at its maximum, volumetrically, during the Neoproterozoic and Cambrian (see the next photo in this album).

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.

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