FELSITE FELSITE

or FELSTONE, a hard, compact igneous rock of somewhat flinty appearance, composed usually of quartz and orthoclase felspar intimately mixed, but sometimes of le highly siliceous minerals. FEL/SPAR, or FELDSPAR, a very important group of mineral silicates of aluminium, with pota ium, sodium, or calcium, ranging from orthoclase, the pota ium species, with 64-7 per cent of silica, to anorthite, the calcium species, with only 43:3. Albite, the sodium felspar, has 68:8 per cent of silica, and the species between this and anorthite are regarded as mixtures of albite and anorthite molecules. These molecules probably do not exist as such within the crystals; but the various characters of the species graduate into one another in a@ ment with the chemical constitution, so that the felspars form an admirable example of the relation of chemical composition, specific gravity, and crystalline and optical features. At the same time orthoclase and microcline are both pota ium felspars; yet the former crystallizes in the monoclinic, and the latter in the tridimic system. All the sodium, sodium-calcium, and calcium species are trictinic, except the rare monoclinic sodium felspar barbie rite. The forms throughout the felspar series are closely similar, and the hardne is uniform, being just below that of quartz, and about that of a stecl file. l'clspar is one of the principal constituents of almost all igneous rocks, such as granite, diorite, and basalt. The alkali species yield kaolin by alteration, and are thus the source of china-clay.