The anhydrous and hydrated ortho-, meta-, and pyro-phosphates of aluminium are insoluble in water. Anhydrous aluminium orthophosphate, AlPO4, formed by heating sodium aluminate and phosphoric acid in a sealed tube at 250°, forms colourless, hexagonal prisms of specific gravity 2.59. The hydrated orthophosphate is obtained as a gelatinous precipitate by mixing sodium phosphate and neutral aluminium sulphate solutions. It is soluble in ammonia, alkali hydroxides, and mineral acids, but insoluble in acetic acid.
When a solution of aluminium phosphate in an acid is precipitated with ammonia, a basic aluminium phosphate is obtained. Several basic aluminium phosphates occur in nature, e.g. the rhombic mineral wavellite, 3Al2O3.2P2O5.12H2O, and turquoise. Turquoise (or calaite) is a basic, hydrated aluminium, copper, and ferrous phosphate. It is triclinic, has a sky-blue to greenish colour, and is used as a gem-stone. It occurs in Persia, Nevada, Arizona, California, and Colorado. |