Sources of Phosphates

The most available and most exploited sources of phosphorus and its compounds at the present day are the phosphatic rocks, or phosphorites, which consist of tribasic calcium phosphate associated with calcium carbonate, alumina, magnesia, etc. Phosphates of alumina are also useful. The production of these secondary rocks from the older rocks has already been mentioned. Although the apatites themselves, as pure minerals, contain a high proportion of phosphoric anhydride, they are difficult to decompose, and are admixed with other minerals of a still more refractory nature.

There are many other possible sources of phosphates in which the acid is combined with the common bases. Thus there are nearly 150 minerals which contain 1 per cent, or more of phosphoric oxide. In the following table is given a small selection of those which are of special interest:—

Minerals containing phosphorus

Name, Occurrence, etc.	Chemical Composition.	Crystal System.	Density
Apatite Igneous rocks and metamorphic limestones.	3Ca3(PO4)2.Ca(Cl or F)2	Hexagonal	3.16-3.23
Pyromorphite Upper levels of lead mines.	3Pb3(PO4)2.PbCl2	Hexagonal	6.5-7.1
Wavellite Fissures in slate.	2Al3(OH)3(PO4)2.9H2O	Rhombic	2.33-2.49
Vivianite. Some iron (copper and tin) ores and as earthy mineral.	Fe3(PO4)2.8H2O	Monoclinic	2.6-2.7
Struvite Guanos	MgNH4PO4.6H2O	Orthorhombic	1.68
Turquoise or Trachyte or breccia	Al2(OH)3.PO4.H2O or 2Al(OH)3.4AlPO4.9H2O	Amorphous with crystal granules	2.72

The next table illustrates the great variation of basicity and hydration shown by phosphoric acid in nature, and also of the bases with which it may combine. Minor constituents are omitted for the sake of brevity. When the crystalline system is not stated the mineral is amorphous or massive.

Iron ores often contain phosphates which are reduced during the smelting process, the phosphorus passing into the iron as phosphide and being again eliminated as basic calcium phosphate in basic slag.

The phosphates of lime are the most valuable natural fertilisers and raw material of the fertiliser and phosphorus industry, which uses also to a less extent phosphates of aluminium and the apatites. Over 80 per cent, of the easily decomposible phosphorites mined are used in the preparation of superphosphate or other fertilisers (1930).