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crystal class

noun
1.
(crystallog) any of 32 possible types of crystals, classified according to their rotational symmetry about axes through a point Also called point group
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Encyclopedia Article for crystal class

in crystallography, listing of the ways in which the orientation of a crystal can be changed without seeming to change the positions of its atoms. These changes of orientation must involve just the point operations of rotation about an axis, reflection in a plane, inversion about a centre, or sequential rotation and inversion. Only 32 distinct combinations of these point operations are possible, as demonstrated by a German mineralogist, Johann F.C. Hessel, in 1830. Each possible combination is called a point group, or crystal class. A crystal can be assigned to one of these point groups on the basis of its external shape, or morphology. The addition of translational changes will yield a total of 230 possible combinations; these are called space groups.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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