Rocks and Their Origins

by Grenville A. J. Cole

Microcline. See [Felspars].

Muscovite. See [Micas].

Nepheline. Sodium aluminium silicate, with some potassium, the pure sodium type being NaAlSiO₄; the types with potassium contain slightly more silica.

Oligoclase. See [Felspars].

Olivine. Magnesium iron silicate, (Mg, Fe)₂SiO₄.

Opal. Uncrystallised silica, SiO₂, with some water.

Orthoclase. See [Felspars].

Pyrite. See [Iron Pyrites].

Pyroxenes. A series of silicates corresponding in composition to the Amphiboles, but crystallising in prisms which have angles of about 87° and 93°. On the whole, the pyroxenes are richer in calcium than the amphiboles. The formula of Wollastonite is CaSiO₃. Diopside consists of Ca(Mg, Fe)(SiO₃)₂. Augite, the commonest form, is aluminous, corresponding to Hornblende among the amphiboles; but the change from Augite into Hornblende, which often occurs, may imply a loss of calcium. Enstatite and Hypersthene are species crystallising in the rhombic system; the former consists of MgSiO₃, while in Hypersthene iron replaces some of the magnesium.

Quartz. Silica, SiO₂, crystallised in the trigonal system.