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January-February 2018

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Connoisseur's Choice: Muzo-habit Quartz Zigras Mine, Garland County, Arkansas

Quartz, the best known of minerals, combines the two most abundant elements in Earth's crust—oxygen (46 percent) and silicon (28 percent)—but it is not the most abundant mineral in the crust. Plagioclase feldspar, the solid-solution series between albite and anorthite, constitutes 39 percent of the crust. Quartz at 12 percent is tied with potassium feldspar (microcline, orthoclase, and sanidine) for second place (; accessed March 2017). Continents are underlain by granitic rocks (plagioclase plus K-feldspar plus quartz), whereas oceans are underlain by basalts (plagioclase plus pyroxene), so quartz can be considered a signature mineral of the continents!


Dr. Carl A. Francis, an executive editor of Rocks & Minerals, is curator at the Maine Mineral & Gem Museum in Bethel, Maine.

Alexander U. Falster, who publishes extensively on granitic pegmatites and their minerals, runs the analytical laboratory at the Maine Mineral & Gem Museum.

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