Calcite
Calcite is the only common non-silicate rock forming mineral, being instead calcium carbonate. It has two refractive indices causing a significant double refraction effect - when a clear calcite crystal is placed on an image, a double image is observed; See the sample below.Calcite will fizz when dilute hydrochloric acid is placed on it. It may be fluorescent, phosphorescent, thermoluminescent and triboluminescent (see fluorite for definitions of these properties).
Calcite is one of the most ubiquitous minerals, being an important rock forming mineral in sedimentary environments. It is an essential component of limestones, and occurs in other sedimentary rocks. It also occurs in metamorphic and igneous rocks, and is common in hydrothermal environments. Calcite is a common vein filling mineral in many rock types.
calcite |
Chemical composition - CaCO
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Other specimens - Click the thumbnails to enlarge 
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