Magnesium Borates

Only two borates can be prepared by heating magnesia with boron trioxide. The orthoborate, Mg₃(BO₃)₂, is obtained by heating boric anhydride strongly with boron trioxide - with or without potassium hydrogen fluoride. The crystals are nacreous, difficultly fusible, with a density of 2.987 at 21° C., or transparent prisms. Boiling water and dilute acetic acid do not dissolve them, but they are soluble in mineral acids. Magnesium pyroborate, 2MgO.B₂O₃, results with a larger excess of boron trioxide.

Rammelsberg prepared the hydrated orthoborate, B₂O₃.3MgO.9H₂O, and, by crystallising from a filtered solution of magnesium hydroxide in excess of boric acid solution, the compound MgO.3B₂O₃.8H₂O. The heptahydrate has been obtained similarly to the octahydrate. The hydrated metaborate, MgO.B₂O₃.8H₂O, separates as a white amorphous precipitate on warming mixed solutions of magnesium chloride and borax. Small crystals of 3MgO.4B₂O₃.11H₂O were obtained from the filtrate. Fine needles of MgO.B₂O₃.3H₂O, corresponding to the mineral pinnoite, are obtained by heating the octahydrate with a warm concentrated solution of magnesium chloride. When a solution of magnesium oxide in boric acid is concentrated and kept stirred at 100° C. after the addition of a solution containing potassium hydroxide and boric acid, KMg₂B₁₁O₁₉.9H₂O finally separates. It corresponds to the mineral kaliborite.

Boracite, 2Mg₃B₈O₁₅.MgCl₂, results from heating a mixture of magnesium borate, boric acid, magnesium chloride, and sodium chloride, or from heating 1 part of sodium borate with 2 parts of magnesium chloride and a little water in sealed tubes at 275°-280° C. The cubic crystals of artificial boracite have a density of 2.89: the density of the mineral is about 2.9. Boracite, pinnoite, and kaliborite occur in the Stassfurt deposits.

Pinnoite crystallises in the tetragonal system with a hardness of 3.4 and a density of 3.3.