1911 Encyclopædia Britannica/Nepheline

From Wikisource
Jump to: navigation, search

NEPHELINE, a rock-forming mineral consisting of sodium, potassium and aluminium silicate, Na6K2Al8Si9O34. Its crystals belong to the hexagonal system, and usually have the form of a short six-sided prism terminated by the basal plane. The unsymmetrical etched figures produced artificially on the prism faces indicate, however, that the crystals are hemimorphic and tetartohedral, the only element of symmetry being a polar hexad axis. The hardness is 5½. The specific gravity (2.6), the low index of refraction and the feeble double refraction are nearly the same as in quartz; but since in nepheline the sign of the double refraction is negative, whilst in quartz it is positive, the two minerals are readily distinguished under the microscope. An important determinative character of nepheline is the ease with which it is decomposed by hydrochloric acid, with separation of gelatinous silica (which may be readily stained by colouring matters) and cubes of salt. A clear crystal of nepheline when immersed in acid becomes for this reason cloudy; hence the name nepheline, proposed by R. J. Haüy in 1801, from Gr. νεφέλη, a cloud.

Although in naturally occurring nepheline sodium and potassium are always present in approximately the atomic ratio 3:1, artificially prepared crystals have the composition NaAlSiO4; the corresponding potassium compound, KAlSiO4, which is the mineral kaliophilite, has also been prepared artificially. It has therefore been suggested that the orthosilicate formula, (NaK)AlSiO4, represents the true composition of nepheline.

The mineral is one specially liable to alteration, and in the laboratory various substitution products of nepheline have been prepared. In nature it is frequently altered to zeolites (especially natrolite), sodalite, kaolin, or compact muscovite. Gieseckite and liebenerite are pseudomorphs.

Two varieties of nepheline are distinguished, differing in their external appearance and in their mode of occurrence, being analogous in these respects to sanidine or glassy orthoclase and common orthoclase respectively. “Glassy nepheline” has the form of small, colourless, transparent crystals and grains with a vitreous lustre. It is characteristic of the later volcanic rocks rich in alkalis, such as phonolite, nepheline-basalt, leucite-basalt, &c., and also of certain dike-rocks, such as tinguaite. The best crystals are those which occur with mica, sanidine, garnet, &c., in the crystal-lined cavities of the ejected blocks of Monte Somma, Vesuvius. The other variety, known as elaeolite, occurs as large, rough crystals, or more often as irregular masses, which have a greasy lustre and are opaque, or at most translucent, with a reddish, greenish, brownish or grey colour. It forms an essential constituent of certain alkaline plutonic rocks of the nepheline-syenite series, which are typically developed in southern Norway.

The colour and greasy lustre of elaeolite (a name given by M. H. Klaproth in 1809, from Gr. ἔλαίον, oil, and λίθος, stone; Ger. Fettstein) are due to the presence of numerous microscopic enclosures of other minerals, possibly augite or hornblende. These enclosures sometimes give rise to a chatoyant effect like that of cat's-eye and cymophane; and elaeolite when of a good green or red colour and showing a distinct band of light is sometimes cut as a gem-stone with a convex surface.

Closely allied to nepheline, and occurring with it in some nepheline-syenites, is the species cancrinite, which has the composition H6Na4Ca(NaCO3)2Al3(SiO4)9. It is frequently of a bright yellow colour, and has sometimes been cut as a gemstone. (L. J. S.)