Phosphorite
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 Ca5(PO4)3F (CFA) typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane. It is also present as hydroxyapatite Ca5(PO4)3OH or Ca10(PO4)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.
