Phosphorite
A phosphorite or phosphate rock is a non-detrital sedimentary rock which contains high amounts of phosphate bearing minerals. The phosphate content of phosphorite is at least 20% which is a large enrichment over the typical sedimentary rock content of less than 0.2%.[1] The phosphate is present as fluoroapatite typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane.[1] The dark brown to black beds range from a few centimetres to several metres in thickness. The layers contain the same textures and structures as fine grained limestones and may represent diagenetic replacements of carbonate minerals by phosphates.[1]
Phosphorites are known from Proterozoic banded iron formations in Australia, but are more common from Paleozoic and Cenozoic sediments. The Permian Phosphoria Formation of the western United States represents some 15 million years of sedimentation reaches a thickness of 420 metres and covers an area of 350 000 km2.[1] Commercially mined phosporites occur in France, Belgium, Spain, Morocco, Tunisia and Algeria. In the United States phosphorites have been mined in Florida, Tennessee, Wyoming and Idaho.[2]