Nepheline

Nepheline (Gr. nephelē, 'a cloud;') in allusion to the mineral becoming cloudy when immersed in a strong acid), a rock-forming mineral of some importance. It is colourless, white or yellowish, and usually crystallises in hexagonal prisms with various modifications. It has a hardness of 5½ to 6, and sp. gr. of 2½ or thereabouts. It occurs in various volcanic rocks, as in certain basalts (nepheline-basalts), in which it plays the part of feldspar in ordinary basalt (plagioclase-basalt). In these fine-grained rocks the crystalline granules of nepheline are of microscopic size; in the more coarsely crystalline nepheline-basalts (nephelinites) the crystals are readily seen by the naked eye. Fine crystals often occur in drusy cavities in such rocks. Elaeolite (Gr. elaion, 'oil') is a variety of nepheline with a greasy lustre, which seldom assumes a crystalline form. It is dark greenish, gray, or brown in colour, and appears to occur only in plutonic rocks, as in the syenites of Frederiksværn and Laurvig in Norway.