Phosphates, Natural

a class of minerals of the salts of orthophosphoric acid, H₃PO₄, with highly variable composition. There are about 180 phosphate minerals; middle salts, for example, xenotime, Y[PO₄], and monazite, are rare. Natural phosphates are primarily complex compounds with two or more cations [for example, Ca²⁺, Fe²⁺, Fe³⁺, Al³⁺, (UO₂)²⁺, Cu²⁺, Mn²⁺, Pb²⁺, and Zn²⁺] and additional anions (OH^–, Cl^–, F^–, O^2–, CO₃^{2 –}) or molecular water. Their composition rarely includes acid radicals, such as [SO₄]^2–, [SiO₄]^2–, and [BO₃]^3–. Certain natural phosphates are acid salts of the type resembling monetite, CaHPO₄.

The basic crystal structure of natural phosphates consists of insular [PO₄]^3– tetrahedral units attached to one another by salt-forming cations. According to the nature of the spatial distribution of the attached [PO₄]^3– tetrahedrons and cation polyhedra, natural phosphates are classified as insular (predominant type), chain, layered, or skeletal.

Natural phosphates occur in the form of massive granular aggregates, earthy masses, oolites, concretions, and incrustations; they sometimes occur in the form of faceted crystals. The coloration is varied. Natural phosphates have a Mohs’ hardness ranging from 2 (for many aqueous natural phosphates) to 5–6 and a density ranging from 1,700 to 7,100 kg/m³.

Natural phosphates are generally disseminated in rocks. The most widely distributed minerals of practical value are apatite (also a constituent of phosphorites), torbernite, autunite, vivianite (Fe²⁺, Fe³⁺)[PO₄]8H₂O, monazite, xenotime, amblygonite LiAl[PO₄](F,OH), and turquoise.

Natural phosphates are primarily formed in the upper part of the earth’s crust—on ocean and lake bottoms, in swamps, in soils, and in weathering crusts—as well as in the oxidation zone of many ore deposits. Some natural phosphates are of magmatic origin, for example, apatite associated with alkaline igneous rocks and monazite and xenotime associated with pegmatites and granites. A number of natural phosphates occur as accessory minerals in hydrothermal veins.

Natural phosphates are used as phosphate ores (apatite and phosphorite). Some natural phosphates have a more limited application; for example, vivianite is used as an inexpensive blue pigment and local phosphorus fertilizer, monazite as a source of rare earths and thorium, amblygonite as a lithium ore, and turquoise as an ornamental stone.