Magnesium Molybdates

The normal molybdate, MgMoO₄, occurs native in belonesite. The heptahydrate, MgMoO₄.7H₂O, may be prepared by boiling an aqueous suspension of magnesium oxide with excess of molybdic acid, or by heating a solution containing sodium molybdate and magnesium chloride. In either case, by allowing the clear liquid to crystallise below 30° C., thin transparent rhombic prisms, isomorphous with the corresponding sulphate, MgSO₄.7H₂O, separate. The crystals effloresce in the air and dissolve readily in water. If crystallisation occurs above 30° C., the pentahydrate, MgMoO₄.5H₂O, separates in large lustrous prisms or tablets belonging to the triclinic system, with

a:b:c = 0.5264:1:0.5732; α = 80° 43', β = 98° 52', γ = 107° 28';

and isomorphous with magnesium chromate, MgCrO₄.5H₂O, and with copper sulphate, CuSO₄.5H₂O. The crystals have density 2.208, and are readily soluble in water. On heating, the pentahydrate loses 3H₂O at 120° C., while at red heat both the penta- and hepta-hydrates yield the anhydrous salt, MgMoO₄, without fusion or decomposition.

By evaporation of solutions containing equimolecular quantities of magnesium molybdate and an alkali molybdate, double salts of the type R^•₂Mg(MoO₄)₂.2H₂O have been prepared; whilst from a solution containing equivalent proportions of magnesium molybdate and potassium chromate, a double salt, similar in form and behaviour to the above, and of composition K₂Mg(CrO₄)(MoO₄).2H₂O, may be obtained.