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The Mohs scale is the unit that measures the hardness of minerals. Calcite is fairly sotf and measures a 3 on the Mohs scale. You can easily scratch it with a piece of copper.
Calcite can be found in many colors from clear to white, yellow, brown, green, blue and read. There are even some calcites that are almost black. As in all minerals, color in calcite is based on the presence of other minerals like iron or magnesium. Because these minerals are not actually pure calcium carbonate, they are called impurities.
When calcite is clear you can see through it almost like glass. Often clear calcite has a special characteristic called double refraction. When you look through the calcite at pictures or words on a page below, the image or words will be doubled.
If the conditions are right, calcium carbonate dissolves in water. The water carries the calcium carbonate into cracks and fissures in rocks. In caves, the driping calcium carbonate water builds up layers over time. These columns of layered calcite are called stalactites and stalagmites. Calcium carbonate is also responsible for the some of the crystals that grow inside geodes.
Limestone caves are formed with water, which can be slightly acidic, wears away the calcium carbonate, leaving empty spaces. This is common in geographic areas where thick and extensive limestone is found. Carlsbad Caverns in New Mexico and Mammoth Cave in Kentucky are both huge caves formed by water that wore away the calcium carbonate that was part of the limestone.
Calcium carbonate even helps in the formation of sedimentary rocks like limestone and shale.
Crystals form specific shapes based on their chemical composition. The tendency to form these shapes is called the crystal habit. But don’t expect to see these specific shapes in all minerals. Often the crystals are impossible to see individually or can only be seen with a high-powered microscope. Calcite can take several different crystal habit: prisms, tabular forms, and rhombohedra.
At fossilicious.com we have many calcite specimens for sale:
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