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The symmetry of the crystal has interesting impacts to the index of refraction. Isometric and amorphous minerals have essentially the same structure or lack there of, in all directions and so have only one index of refraction and are called isotropic minerals. But hexagonal, trigonal and tetragonal minerals have a different structure along their primary axes than they do in all other directions and for this reason they have two indices of refraction; one along the primary axis and one for every other direction. These minerals are called uniaxial minerals for their one unique direction. Orthorhombic, monoclinic and triclinic minerals have two planes of equal refractive indices and are called biaxial.

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CALCITE

The varieties of calcite, CaCO3, are so numerous and so varied that an entire display case at the Smithsonian Museum of Natural History is devoted to just calcite. Calcite is the most abundant of the carbonate minerals. The sample shown above and in the closeup view below is called cobaltian calcite. The sample is about 9x12 cm. samples are part of the gem and mineral exhibit at the Smithsonian Museum of Natural History.

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