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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 R2Mg(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.
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