Calcium Fluoride, CaF₂

The best-known natural form of Calcium Fluoride, CaF₂, is the mineral fluorite or fluorspar, crystallising in the cubic system. It is often richly coloured, for example, blue fluorite, or Blue John, from the famous Blue John mines near Castleton, in Derbyshire, and a red fluorite found near Ashover in the same county. The colour is not homogeneously distributed, but appears to form films at the crystal surfaces, especially at the junctions. The colour of Blue John is probably of organic origin, but that of red fluorite is due to small quantities of limonite, Fe₄O₃(OH)₆. Other coloured varieties are also known, and in fluorspar districts the carving of ornamental vases from the mineral often forms an important industry. When gently warmed, the mineral fluoresces, and this circumstance gave rise to the term fluorescence.

The fluoride also occurs associated with the phosphate as fluorapatite, and is found in small quantities in teeth and bones.

It may be formed by direct union of its elements. The heat of formation is 239.18 Cal. By neutralising calcium hydroxide or carbonate with hydrofluoric acid, it is obtained as a white precipitate, crystalline in the case of the carbonate. The heat of neutralisation ½Ca(OH)₂Aq + HF is 18.155 Cal. Addition of a calcium salt to a solution of a neutral fluoride gives a gelatinous precipitate, but boiling the latter with very dilute hydrochloric acid transforms it into the crystalline variety.

A crystalline compound may also be prepared if a mixture of ten parts of calcium chloride and one of manganese fluoride is fused in an atmosphere of carbon dioxide at 800°-1400° C. On treating the product with water, and then dilute hydrochloric acid, calcium fluoride is obtained as octahedral crystals if the fusion has taken place under

1000° C., or cubes if over 1200° C. Potassium hydrogen fluoride may be used instead of manganese fluoride.

By pouring potassium fluoride solution into an excess of calcium chloride solution a colloidal solution of calcium fluoride can be prepared. By dialysis, succeeded by concentration over sulphuric acid in vacuo, a 2 per cent, solution can be obtained. The colloid shows the charge of the ion in excess at the time of precipitation, in accordance with Lottermoser's observation.

A plastic form of calcium fluoride has also been described. The melting-point of fluorspar is 1378° C., the density 3.16, and the specific heat 0.21492. Fluorspar is very transparent to ultra-violet rays, the shortest wave-length penetrating it being probably 1230 Å.

According to Fremy, calcium fluoride is decomposed by water and by oxygen at red heat, producing hydrofluoric acid and probably fluorine respectively. Chlorine also decomposes it, an equilibrium point being reached.

The solubility of the natural fluorspar, determined by measurement of the electrical conductivity of the saturated solution, is 15.07 mgm. per litre at 18° C., and that of the precipitated fluoride, 16.3 mgm. per litre. The solubility increases with rise in temperature, and the heat of solution is -2.7 Cal. Concentrated acids increase the solubility of the salt considerably, and on heating with concentrated sulphuric acid, hydrofluoric acid is obtained, this being, in fact, the commercial process for the production of the latter. Hydrofluoric acid itself also increases the solubility, probably through the formation of a complex ion. The salt CaF₂.2HF.6H₂O has been obtained in the solid state. Fluorite is used for optical purposes. In the soil calcium has a beneficial effect on plants. Fluorspar is used as a flux in various metallurgical operations; in fact, its employment for this purpose dates from very early times and is the origin of its name.