Among the crystalline silicates the following minerals are known:--_Wollastonite_ (tabular-spar), crystallises in the monoclinic system; sp. gr. 2·8; it is semi-transparent, difficultly fusible, decomposed by acids, and has the composition of a metasilicate, CaOSiO_{2}. But isomorphous mixtures of calcium and magnesium silicates occur with particular frequency in nature. The _augites_ (sp. gr. 3·3), diallages, hypersthenes, hornblendes (sp. gr. 3·1), amphiboles, common asbestos, and many similar minerals, sometimes forming the essential parts of entire rock formations, contain various relative proportions of the bisilicates of calcium and magnesium partially mixed with other metallic silicates, and generally anhydrous, or only containing a small amount of water. In the pyroxenes, as a rule, lime predominates, and in the amphiboles (also of the monoclinic system) magnesia predominates. Details upon this subject must be looked for in works upon mineralogy. The most remarkable complex siliceous compounds are the _felspars_, which enter into nearly all the primary rocks like porphyry, granite, gneiss, &c. These felspars always contain, in addition to silica and alumina, oxides presenting more marked basic properties, such as potash, soda, and lime. Thus the _orthoclase_ (adularia), or ordinary felspar (monoclinic) of the granites, contains K_{2}O,Al_{2}O_{3},6SiO_{2}; _albite_ contains the same substances, only with Na_{2}O instead of K_{2}O (it already appertains to the triclinic system); _anorthite_ contains lime, and its composition is CaO,Al_{2}O_{3},2SiO_{2}. On expressing the two last as containing equal quantities of oxygen, we have:-- Albite Na_{2} Al_{2} Si_{6} O_{16} Anorthite Ca_{2} Al_{4} Si_{4} O_{16}