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January-February 2018

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Word to the Wise: Crystal Form

The most common usage of the term form is for the shape of something and, as such, could be used synonymously with morphology or habit when describing a mineral. However, in crystallography and mineralogy form has a very specific meaning that is not equivalent to morphology. In these contexts a form is defined as a set of crystal faces that are related to one another by the symmetry of the crystal. Such faces are said to be symmetrically equivalent. Accordingly, we can define a form as a set of symmetrically equivalent crystal faces. The morphology or habit of a crystal is governed by the forms present and also by the relative development or size of the faces among those forms.


Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio.

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