FLUORSPAR derives its name from its use in metallurgical operations as a flux (cf. Lat. fluere, to flow, and its Ger. name Muss-spat). It is the native fluoride of calcium, also called fluorite or fluor, One of the most interesting characters of fluorspar is its colour. Specimens have been found of all the colours met with in the mineral kingdom. The colours may be divided roughly into the following groups : white to honey yellow ; pale leek green to emerald green ; sky blue ; green blue to dark violet; light to carmine red. The colour of a given specimen often varies with the way in which light falls on it. Thus many green-blue specimens are green by transmitted light and blue by reflected. This is the phenomenon of fluorescence to which fluorspar gives its name.
Certain coloured varieties of fluorspar when irradiated with white or ultra-violet light glow with a vivid light, which when examined spectroscopically is seen to consist in general of a number of very fine lines. The character of the light emitted depends to a large extent on the particular specimen of fluorspar and to some extent on the quality of the light which is incident on it. A very interesting series of experiments have shown that the lines occurring in this fluorescent spectrum are due to im purities in the crystal consisting chiefly of manganese and the rare-earths. Urbain was able to prepare an artificial fluorspar containing these impurities in proportions such that the fluorescent spectrum of the product was exactly the same as for one of the common natural varieties. He also found that perfectly pure artificial fluorspar did not fluoresce and it seems likely that the same is true of the natural product since many colourless speci mens give no fluorescence. Whilst it is fairly clear that the colour of fluorspar is in many cases due to the presence of impurities like the rare-earth metals, which give a well marked fluorescence spectrum, yet there are others which partly owe their colour to the action of radioactive radiations of the (3- and y-type. These radiations are able to cause a great many fluorspars to acquire a green-blue tint, though they hardly ever produce the other colours so common in this mineral. The presence of radio active material in some fluorspars may be shown directly by the action a crystal will produce on a photographic plate; in other cases the radiations have recorded their presence in the fluorspar by pleochroic haloes surrounding the minute inclusions. It seems certain that the colour of such specimens is largely due to the radioactive radiations.
A theory has been put forward by Doelter that the impurities which give rise to the fluorescence, i.e., the rare-earths, man ganese, etc., are present in a highly dispersed condition resembling that of a colloid. This would account for the disappearance of colour on heating in the case of most fluorspars and the change of colour on irradiation with and y-rays, if we may assume the same phenomena to occur in a crystal as occur in colloidal solutions. It has also been suggested that the colouring is due to an organic pigment, as hydrocarbons have been found in many specimens. Carbon monoxide and dioxide, hydrogen, nitro gen and a little oxygen are sometimes also present.
The known facts relating to the colour of fluorspar indicate that it is in all cases due to impurities which may, like the rare earths, give a colour determined by the fluorescence spectra, or produce a colour by scattering action resulting from the fine state of their division.
Fluorescent Light, emitted in the visible spectrum, ceases instantly on cutting off the exciting light but an emission of light in the ultra-violet continues for a long time. This emission has been detected by means of a photographic plate over a period of the order of a month. Associated with this phenomenon is another remarkable property of fluorspar. When heated in the dark, after being exposed to the light, it emits a bright light of colour varying with the specimen and the temperature. The crystals begin to glow at temperatures ranging between 40°-80° C. and continue for some hours if not heated too strongly. After they have ceased glowing they do not on cooling emit any more ultra-violet light. There is no adequate theory to account for these phenomena, which are probably also due to impurities since they do not occur with all specimens.
The crystal habit of fluorspar is generally cubic : in addition the forms (III), (IIO), (311), (2I1), (210), (310), (421) are often found. The habit appears to depend very much on the mode of origin : fluorspar formed hydrothermally is generally simple cubic, but when formed more rapidly in pneumatolytic processes it is frequently octahedral and rich in faces. Twin crystals are not uncommon, two cubes interpenetrating so that they are symmetrically arranged about a face of the octahedron. Fluorspar has a hardness of 4, so that it is scratched by a knife though not so readily as calcite. Its specific gravity is about 3.2. The structure consists of two interpenetrating cubic lattices one of calcium and one of fluorine. The former is face-centred with the unit cell of length 5.45 A.U. The simple-cubic lattice of fluorine atoms has a unit cell of half this length and is placed so that each fluorine atom is at the centre of a tetrahedron formed of calcium atoms.
Fluorspar is widely used in metallurgy as a flux and an electro lyte. On account of its high melting-point it is usually mixed with other fluorides and chlorides, when it is a good solvent for aluminium oxide and metallic silicates. These properties render it particularly suitable for the extraction of aluminium since the impurities in the ore consist largely of silicates. It is also used extensively in the steel industry for the production of ferro silicon, ferromanganese and ferro-vanadium, and in lead-smelt ing. Considerable quantities of the mineral are used in the pro duction of enamel and opal glass and very perfect crystals are employed in the manufacture of apochromatic lenses. It is used as a source of hydrofluoric acid, which it evolves when heated with sulphuric acid. The coloured variety known as Blue John is much worked as an ornamental stone. By impregnation with resin the otherwise brittle spar may be worked on a lathe and very beautiful and delicate work has been done in this way.
The mineral is of very wide distribution. Some of the finest crystals occur in the lead-veins of the Carboniferous limestone series in the north of England, especially at Weardale, Allendale and Alston Moor. It is also found in Aberdeenshire, Cornwall and south Devon. Fine yellow fluorspar occurs in some of the Saxon mines and beautiful rose-red octahedra are found in the Alps near Goschenen. Many localities in the United States yield fluorspar and it is worked commercially in Colorado, Illinois, Kentucky, New Mexico, British Columbia and Guipuzcoa (Spain).
BIBLIOGRAPHY.-G. Urbain, Ann. Chim. phys. (1909) ; C. Doelter, Bibliography.-G. Urbain, Ann. Chim. phys. (1909) ; C. Doelter, Kolloid Zeit. (1920) ; Paul Niggli, Lehrbuch der Mineralogie (Berlin, 1926) ; Max Bauer, Precious Stones (transl. L. J. Spencer, London, 1904)- (W. A. W.)