There are several silicic acids, which form a very extensive series of compounds with various metallic bases. As rock-forming minerals the silicates are of the first importance.

The felspars are essentially silicates of alumina (A1203) together with potash, soda, or lime. Orthoclase and microcline are potash felspars (K20, A1203, 6 Si02); albite is a soda felspar (Na20, A1203, 6 Si02); and anorthite, a lime felspar (CaO, A1203, 2 Si02). From the combination of these two series are formed: the lime-soda series, oligodase, andesine, and labradorite, the plagioclases, and the potash-soda series, anorthoclase.

The felspars crystallize in either the monoclinic or triclinic systems, but the forms of the crystals are very much alike. With few exceptions, these minerals are of pale colours and, except when decomposing, are very hard.

1. Monoclinic Felspars

Orthoclase is a potash felspar (K20, A1203, 6 Si02=K,Al, Si308), though soda may replace part of the potash. Hardness = 6, sp. gr. = 2.54-2.57. Orthoclase crystallizes in oblique rhombic prisms and is very generally twinned; there are two sets of cleavage planes, which intersect at a right angle and have thus given its name to the mineral. Orthoclase is usually dull and turbid, which is due to the presence of various alteration products, and even thin sections under the microscope are commonly hazy. Sanidine is a glassy, transparent variety of orthoclase, which is found in lavas of late geological date. Its clearness is due to the absence of the decomposition products, which render ordinary orthoclase turbid.

2. Triclinic Felspars

The minerals of this series are grouped together under the comprehensive term of Plagioclase, because of the difficulty of distinguishing them from each other under the microscope; they are very generally characterized by polysynthetic twinning, which makes fine parallel lines on the basal cleavage planes. Chemically, they are isomorphous mixtures of albite and anorthite. The following table (from Levy and Lacroix) gives the composition of the various members of this series, representing the soda-felspar constituent, or albite, by Ab, and the lime-felspar constituent, or anorthite, by An: -

It will be observed that the specific gravity increases with the lime constituent, and the fusibility diminishes in the same proportion. Anorthite is decomposed by hydrochloric acid, labradorite is slightly attacked by it, while the other members of the series are not affected.

Anorthoclase is a triclinic potash-soda felspar (Ab2Or1), but is less common than the plagioclases as a constituent of rocks.

Microcline has the composition of orthoclase and plays a similar role in rocks, but crystallizes in the triclinic instead of the mono-clinic system.