red, yellow, green, blue, violet, brown, or even black when charged with impurities. It is transparent to opaque and may occasionally show phosphorescence[?] or fluorescence. Calcite is perhaps best known because of its power to produce strong double refraction of light such that objects viewed through a clear piece of calcite appear doubled in all of their parts - a phenomenon first described by Rasmus Bartholin. A beautifully transparent variety used for optical purposes comes from Iceland, called Iceland spar. Acute scalenohedral crystals are sometimes referred to as dogtooth spar.
Calcite represents the stable form of calcium carbonate; aragonite will go over to calcite at 470°C.
Calcite is a common constituent of sedimentary rocks, limestone in particular. It also occurs as a vein mineral, in deposits from hot springs and in caverns as stalactites and stalagmites. Calcite is often found in the shells of marine organisms (e.g. plankton).
See also: List of minerals, lysocline.
External link: [Calcite information and images (http://mineral.galleries.com/minerals/carbonat/calcite/calcite.htm)]
wikipedia.org dumped 2003-03-17 with terodump