Calcium Borates

Calcium borate is found in nature as borocalcite, CaB₄O₇.4H₂O, colemannite, Ca₂B₆O₁₁.5H₂O, and pandermite, Ca₂B₆O₁₁.3H₂O, the latter being used for the extraction of boric acid. Calcium borates, varying in composition according to the conditions of formation, may be obtained by the action of boric acid on calcium hydroxide, or by precipitation or fusion of a calcium salt with an alkali borate.

By studying the freezing-point curve of mixtures of calcium oxide and boric anhydride, Guertler confirmed the existence of the compounds, 2CaO.B₂O₃, melting-point 1225° C.; CaO.B₂O₃, melting-point 1095° C.; CaO.2B₂O₃, and possibly 3CaO.B₂O₃, by the position of the maxima on the curve. The flatness of the curves at these points, however, indicate considerable dissociation on fusing.

The heats of formation of the different anhydrous calcium borates are as follows: -

2B₂O₃ + CaO = 2B₂O₃.CaO + 39.8 Cal.
B₂O₃ + CaO = B₂O₃.CaO + 30.9 Cal.
B₂O₃ + 2CaO = B₂O₃.2CaO + 485 Cal.
B₂O₃ + 3CaO = B₂O₃.3CaO + 62.2 Cal.

By precipitation, hydrated compounds are formed. Equivalent quantities of calcium hydroxide and boric acid give calcium metaborate hexahydrate, CaO.B₂O₃.6H₂O, which is apparently the stable form at ordinary temperatures. According to Meyerhoffer and van't Hoff, an unstable α-tetrahydrate may exist above 24° C. At 40° C., in contact with a 10 per cent, solution of sodium chloride, this is transformed into the stable β-tetrahydrate. Mandelbaum obtained an amorphous compound, probably a tetrahydrate, by precipitation at 60°-70° C., and this, after a few days, passed into a crystalline hexahydrate, if left in contact with water. The solubility curves of both the tetra- and hexa- hydrates show decided maxima between 65° and 70° C., and cut at 42° C., evidently the transition point. By heating the hexahydrate to 105° C. two-thirds of the water is driven off, leaving the dihydrate from which the remaining water can only be removed at red heat; for this reason Mandelbaum suggested the formula for the hexahydrate.

A tetrahydrate has also been obtained electrolytically.

By the action of boric acid on the metaborate hexahydrate, Meyerhoffer and van't Hoff obtained the compounds CaO.3B₂O₃.12H₂O, 8H₂O, and 4H₂O, as well as 2CaO.3B₂O₃.9H₂O and 5H₂O.

A double sodium calcium borate may be prepared and also occurs naturally as the mineral boronatrocalcite.

Halogen Compounds of Calcium Borate. - Calcium chloroborates, 3CaO.CaCl₂.5B₂O₃ and 3CaO.CaCl₂.3B₂O₃, may be obtained by fusing together boric anhydride and calcium chloride in various proportions with or without lime. Two strictly analogous bromoborates have also been similarly prepared, but no iodine compounds.

Berzelius obtained a gelatinous fluoborate.