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March-April 2009

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Word to the Wise

Sectoral Zoning

The January/February 2009 issue of Rocks & Minerals focused on Mineral Oddities, the theme of the 2009 Tucson Gem and Mineral Society Show. One oddity that may be common but rarely manifests itself in an obvious way is sectoral zoning. Sectoral zoning is of interest to mineral collectors if it results in zoning of color or fluorescence. In either of these cases, variations can be both beautiful and intriguing. Figures 1–3 show examples of sectoral zoning revealed by color and fluorescence, respectively. To understand what sectoral zoning is exactly, and why it occurs, we first need to define a sector. As we will see, sectors are related to crystal faces, which in turn are the result of how a crystal grows. Ultimately, sectoral zoning and an associated phenomenon, intrasectoral zoning, are related to the subtleties of crystal growth and the nature of crystal surfaces.
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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