calcite - banded calcite collected from a vein near Payson, Arizona - hand specimen

Calcite, CaCO_3, 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.

This calcite was precipitated from groundwater in a linear cavity, with the banding representing slight changes in the chemical composition of the groundwater as additional calcium carbonate precipitated. These look good enough to eat! A good example of massive calcite.

The pencil is 5.5" long, for scale.

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