Aluminium Arsenates

The orthoarsenate, AlAsO₄, is obtained in the form of elongated lens-shaped crystals when normal sodium orthoarsenate is heated with an excess of aluminium sulphate in a sealed tube at 220° C. The crystals are hexagonal in structure, with a₀ = 5.030 and c₀ = 5.612 A.; the form is similar to that of quartz.

A mixture of aluminium arsenate, lime and sand has been recommended as a hydraulic cement; on dehydration insoluble calcium arsenate is formed, which improves the resistance to disintegration.

The addition of sodium monohydrogen arsenate to a solution of an aluminium salt results in the precipitation of a white powder which is probably aluminium hydrogen arsenate, Al₂(HAsO₄)₃. The powder dissolves in acids and the solution in hydrochloric acid when boiled with ammonium sulphite yields a precipitate of aluminium hydroxide, the arsenic remaining in solution. When heated in a current of hydrogen, arsenic is volatilised.

Aluminium Pyroarsenate, Al₄(As₂O₇)₃, is obtained by fusing at as low a temperature as possible a mixture of alumina and 15 to 16 times its weight of sodium or potassium dihydrogen arsenate; colourless transparent crystals are formed on cooling. These dissolve only very sparingly in hot water, but are readily soluble in dilute acids.

Complex arsenates of sodium, potassium and barium with aluminium have been described; thus by dissolving alumina in a fused mixture of sodium or potassium dihydrogen arsenate with about 20 per cent, of the alkali chloride, transparent plates of composition, respectively, Na₃Al2(AsO₄)₃ and K₃Al2(AsO₄)₃, are obtained. In the presence of sodium or barium hydroxide, arsenic pentoxide combines with aluminium hydroxide to form alumini-arsenates of composition NaH₂[Al(AsO₄)₂]. 0.5H₂O and BaH₄[Al(AsO₄)₃].H₂O, analogous to the ferriphosphates.

Aluminium arsenate occurs in numerous minerals. Liskeardite, (Al, Fe)AsO₄.8H₂O, is found in Cornwall as a pale blue incrustation on quartz; coerulite, CuO.2Al₂O₃.As₂O₅.8H₂O, occurs in Chile in the form of a clay which consists of minute turquoise-blue crystals; liroconite, Cu₉Al₄(OH)₁₅(AsO₄)₅.20H₂O, found in Cornwall and Hungary, is blue or green and occurs in monoclinic crystals; a basic arsenate of composition 4CaO.5Al₂O₃.3As₂O₅.20H₂O occurs as an amorphous mineral associated with mercury ores in Utah.