Phosphorus is a strategic element, as it is essential for agriculture, integrated into NPK fertilizers. There is no substitute for P in agriculture. Phosphate is also widely used as an animal feed supplement and in the chemical industry.

The only sources of P are the so-called “phosphate rocks,” which can be of two types:

– igneous rocks or
– sedimentary rocks.

Igneous rocks are plutonic, formed by carbonatites that contain an economically viable content of apatite. Generally, in these occurrences, the residual mantle, over the unaltered rock, is enriched with apatite, while the apatite content of the sound rock is much lower. This type of phosphate rock is the only one found in Brazil under exploitable conditions. These are very low volumes, only 0.72% of world reserves. In international terms, this type of deposit makes up around 15% of phosphate rock deposits.

Phosphate sedimentary rocks are chemical (non-detrital) sediments deposited in shallow marine environments. They form sand with a phosphate content that justifies mining, for example, 13% P2O5. Generally, P2O5 levels vary between 4 and 20%. Usually, sandy layers with phosphate are interspersed with sandy layers with phosphate content too low for extraction and with layers of silt and clay without phosphate, as well as layers of limestone and dolomite. Once extracted, the phosphate sand is enriched until it reaches 28 or 30% P2O5.

Phosphate in phosphorites occurs in the form of fluorapatite, which forms cryptocrystalline masses with a grain size of less than one micron. The term used for these masses is collophane. Hydroxyapatite derived from bones and teeth also occurs.

Phosphorites are classified as “pristine,” “condensed,” and “allochthonous” and can be deposited as phosphate nodules, bioclastic phosphates, or by phosphating. Each of these processes generates a set of characteristic textures, which are generally addressed in Sedimentary Petrology.

Among marine phosphorites, a distinction is made between epiric sea phosphorites, continental margin phosphorites, seamount phosphorites, and island phosphorites.

The analysis of phosphorites under a microscope presents a number of difficulties because the material usually needs to be impregnated. Furthermore, the cryptocrystalline size of apatite crystals makes more detailed analyses difficult. Other analytical techniques are necessary for ore analysis.