CALCITE (24)
CALCITE, a common rock-forming mineral, consists of calcium carbonate. The mineral is white or colorless, but impurities may tint it shades of yellow or gray. Transparent calcite is more rare than the tinted varieties.
Transparent calcite possesses the property of double refraction; an image appears double when viewed through a calcite cleavage block.
Calcite has a glassy luster, its streak is white or colorless. The mineral is of medium hardness and can be scratched by a penny or a piece of window glass but not by the fingernail. It is fairly light weight and effervesces freely in cold dilute hydrochloric acid.
Calcite has a variety of crystal forms but in Illinois flattened block-shaped crystals and elongate crystals with tapering points (“dogtooth spar”) are the most common. When broken, calcite cleaves into six-sided blocks called rhombs.
Crystals of calcite are found in Illinois as linings in geodes in certain limestones and shales, especially in the Nauvoo-Hamilton-Warsaw area, and as crystalline masses in limestone and dolomite. Small amounts of clear crystalline calcite are associated with various ores in northwestern and extreme southern Illinois.
Calcite is the principal mineral in limestones and occurs as a component of many concretions.