Cobaltoan calcite from Zaire, Africa. (3.8 centimeters across at its widest)
<p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>The carbonate minerals all contain one or more carbonate (CO3-2) anions.
</p><p>Calcite is a common mineral.  It is calcium carbonate (CaCO3).  It has a nonmetallic luster, commonly clearish to whitish to yellowish to grayish in color, is moderately soft (H≡3), moderately light-weight, has hexagonal crystals, and rhombohedral cleavage (three cleavage planes at 75º &amp; 105º angles - cleavage pieces look like lopsided boxes).  The easiest way to identify calcite is to drop acid on it - it easily bubbles (effervesces) in acid.  The bubbles are carbon dioxide gas.  If the acid is dilute hydrochloric acid, the chemical reaction is:
</p><p>2HCl(aq) + CaCO3(s) --&gt;&gt; CO2(g)↑ + H2O(l) + CaCl2(aq)
</p><p>The most important &amp; voluminous calcitic rocks in the world are limestone (sedimentary), marble (metamorphic), carbonatite (igneous), and travertine (speleothem, or "cave formations", and many hotspring deposits).  Quite a few hydrothermal veins in the world are calcitic or have calcite as a principal component.
</p><p>The deep pinkish-purple coloration present in the calcite shown above is from significant cobalt impurity (Co +2), resulting in the chemical formula (Ca,Co)CO3.
</p><p>Locality: unrecorded site in Shaba Province/Katanga Province, Zaire ("D.R. Congo"), south-central Africa
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<hr><p>Photo gallery of cobaltoan calcite:
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www.mindat.org/gallery.php?min=1094
James St. John
CC BY 2.0
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