SIDNEY MOULDS / SCIENCE PHOTO LIBRARY SIDNEY MOULDS / SCIENCE PHOTO LIBRARY
Macrophotograph of crystals of calcite. Calcite is a stable form of calcium carbonate, and is the main constituent of limestone, chalk and marble. Large crystals of calcite such as this are found in lava cavities and hydrothermal veins. These slowly-cooling conditions allow the crystals to grow from the molten state. Over 300 different crystalline forms of the mineral have been described. This specimen is from Guanajuato, Mexico. Magnification x0.45 at 35mm size.
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