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January-February 2018

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The Crystal Systems: A Qualitative Visual Guide

Have you ever watched children play with wooden blocks? My undergraduate college students are asked to undertake a similar activity with their adult observations guided by the curiosity and creativity of their “inner child.” Eventually some of the kids and collegians, who tire of building towers that fall over, sort the blocks into a few groups by similarity of shape. This apparently simple task, however, requires careful observations and the development of a set of internally consistent rules for the assignment of every block to a group with “similar” shapes. The next few pages provide a guide to the essential observations that allow visual recognition of the morphological crystal systems. A more elegant and widely applicable approach based on three-dimensional symmetry operations can be found in a good mineralogy textbook (e.g., Dyar and Gunter 2008). However, the techniques explained herein will work for most undistorted crystals larger than a few centimeters.

 

Dr. Ken Bladh, emeritus professor of geology at Wittenberg University, is the editor of the Mineralogical Society of America's Handbook of Mineralogy and currently serves as president of the Midwest Chapter of Friends of Mineralogy. His most recent article for Rocks & Minerals was titled “Arizona Mineral Classics” and appeared in the January/February 2012 issue.

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