Rocks and Their Origins

by Grenville A. J. Cole

Aragonite. Calcium carbonate, CaCO₃, crystallising in the rhombic system, with a specific gravity of 2·93. See [Calcite].

Augite. See [Pyroxenes].

Biotite. See [Micas].

Calcite. Calcium carbonate, CaCO₃, crystallising in the trigonal system, with a specific gravity of 2·72. See Aragonite.

Chalcedony. Crystalline silica, SiO₂, in fibrous and often mammillated forms. Flint or Chert is a concretionary form, in which some interstitial opal may be present.

Chert. See [Chalcedony].

Chlorites. Hydrous aluminium magnesium iron silicates, resembling green micas, but softer and with non-elastic plates.

Chromite. Iron chromium oxide, FeCr₂O₄. Magnesium may replace part of the dyad iron, and aluminium and triad iron some of the chromium.

Diallage. An altered augite with a shimmery submetallic lustre.

Diopside. See [Pyroxenes].