calcite, CaCO3
Calcite is an extremely common rock forming mineral. In pegmatites it forms large masses and it makes up some limestones almost entirely. It has a large variety of crystal forms, but these do not include the rhombus-shaped pieces that are sold as “calcite crystals.” The rhombs are produced by cleaving calcite in three directions between rows of atoms.
When calcite is free of impurities and is optically clear, it exhibits double refraction. When it is placed above a line drawn on paper and rotated, the drawn line doubles and becomes single depending on the angle of rotation. These specimens are clear enough to demonstrate this. Excellent examples of cleavage rhombs and are a decent size.
The pencil is 5 1/2" long, for scale. If tiny rhombs would be useful in your classroom, let us know and we'll include some for student experimenting and so you can demonstrate the calcium carbonate response to a drop of hydrochloric acid - it fizzes and releases carbon dioxide - without wrecking this example.
Click on an image to enlarge and click again to further enlarge.
One issue with specimens from China is that the locality information gets distorted as the specimens change hands before we get them. This was being sold by a couple at the Tucson rock, mineral and fossil trade fair in two different localities. The wife in one spot said this came from Guangdong Province. The husband in another spot said it came from Guizhou. We're not sure that either one really knew where this was mined and prepared, but we like them both anyway!
Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil, or one of those plus some number of coins, or you can let us make the selection.
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