The Mineral WITHERITE

(under longwave UV light)

Witherite is an uncommon carbonate mineral, although it is the second most common barium mineral next to the barium sulfate mineral, barite. All members of the aragonite group of minerals can form twins, but witherite is one member that always forms twins!

Twinning is the result of an error during the growth of the crystal. It occurs when the atomic layer stacking, ABCABCABCABC etc, makes a mistake and a C layer instead of a B layer is place next to an A layer. The result is an ABCABCACBACBACBA stacking. Where the mistake occurs, a mirror plane is produced. If this occurs another time, forming three twins, that are joined in a circle, then a trilling is created. The symmetry of the crystal will appear hexagonal but is still orthorhombic. These crystals can be thought of as a "triple siamese twin" where one crystal takes up one third (or 120 degrees) of the hexagon. Witherite's twins are typically capped with a six sided pyramid and often are dipyramidal.

Witherite is an interesting and valuable collection specimen that anyone, especially a collector of twinned minerals, would love to own.

PHYSICAL CHARACTERISTICS:

 

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