English: Calcite in concretion from Utah, USA. (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)

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.

The carbonate minerals all contain one or more carbonate (CO3-2) anions.

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 & 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:

2HCl(aq) + CaCO3(s) -->> CO2(g)↑ + H2O(l) + CaCl2(aq)

The most important & 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.

The rock shown above is a calcite-filled septarian concretion. Concretions are post-depositional structures found in some sedimentary successions, particularly in shales. They vary in mineralogy, size, and shape, but often have slightly flattened spheroidal shapes. They typically form by post-depositional mineralization around some nucleus (a rock fragment or fossil fragment). Septarian concretions have developed cracks during their formation. In this case, cracks and cavities have filled with coarsely-crystalline calcite.

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From museum signage: Concretions that grow in clays may have shrinkage cracks in their centers that result from later dehydration of the clay minerals. These are called septarian concretions and often have calcite or pyrite crystals growing in the cracks.

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Locality: unrecorded/undisclosed site in Kane County, southern Utah, USA

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Photo gallery of calcite:

<a href="http://www.mindat.org/gallery.php?min=859" rel="noreferrer nofollow">www.mindat.org/gallery.php?min=859</a>