Phosphorite, phosphate rock or
rock phosphate is a non-detrital
sedimentary rock which contains high amounts of
phosphate bearing minerals. The phosphate content of phosphorite is at least 15 to 20%; if it is assumed that the phosphate minerals in phosphorite are hydroxyapatite and fluoroapatite, phosphate minerals contain roughly 18,5 % phosphorus by weight and if phosphorite contains around 20% of these minerals, phosphorite is roughly 3,7 % phosphorus by weight, which is a considerable enrichment over the typical sedimentary rock content of less than 0.2%. The phosphate is present as
fluorapatite Ca
5(PO
4)
3F (CFA) typically in
cryptocrystalline masses (grain sizes < 1 µm) referred to as
collophane. It is also present as
hydroxyapatite Ca
5(PO
4)
3OH or Ca
10(PO
4)
6(OH)
2, which is often dissolved from vertebrate bones and teeth, whereas fluorapatite can originate from
hydrothermal veins. Other sources also include chemically dissolved
phosphate minerals from
igneous and
metamorphic rocks. Phosphorite deposits often occur in extensive layers, which cumulatively cover tens of thousands of square kilometres of the
Earth's crust.