Magnesium Chromite

The compounds 2MgO.Cr₂O₃ and 3MgO.2Cr₂O₃, and finally, at a red heat, MgO.Cr₂O₃, result from the calcination of magnesium chromate. Amorphous MgO.Cr₂O₃ is also obtained by calcining either the double chromate of magnesium and potassium, or magnesium chloride with potassium dichromate.

Dark green octahedral crystals of MgO.Cr₂O₃ are formed when magnesia and green chromium trioxide, in appropriate proportions, are heated strongly with boric acid, or, alone, in the electric furnace. Their physical properties seem to vary with their mode of production: they scratch glass and not quartz, and have a density of 4.415 at 16° C. when prepared by the boric acid method; the furnace product is harder than quartz, and its density is 4-6 at 20° C. Sulphuric acid attacks them readily; they resist boiling nitric acid and are slowly attacked by hydrochloric and hydrofluoric acids. They are very resistant to oxidising agents in general.