A sedimentary rock with a high enough content of phosphate minerals to be of economic interest. Most commonly it is a bedded primary or reworked secondary marine rock composed of microcrystalline carbonate fluorapatite in the form of laminae; pellets; oolites; nodules; skeletal, shell, and bone fragments; and guano. Aluminum and iron phosphate minerals (wavellite, millisite) are usually of secondary formation.