GEOLOGY: Very Slow Growth

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Science  13 Apr 2007:
Vol. 316, Issue 5822, pp. 174a
DOI: 10.1126/science.316.5822.174a

Gypsum [Ca(SO4)·2H2O] forms some of the largest natural single crystals on Earth (aside from the speculative iron crystals in the inner core), in some cases reaching 10 m in length. The growth of such sizable crystals requires precise maintenance of specific environmental conditions.

García-Ruiz et al. have investigated the giant gypsum crystals in deep caves of the Naica mine in Mexico, which has been the source of several museum specimens. Analyses of fluid inclusions, trapped sequentially in the crystals as they grew in caverns nearly 300 m below the surface, show that the temperature in the large fluid-filled caves was maintained near 54°C for thousands of years at least—the mineralization in the mine began about 25 million years ago—and the deep water there is still close to this temperature today. This temperature is just below the maximal solubility point for gypsum in low-salinity water and also slightly below the thermodynamic stability range of anhydrite (a polymorph of gypsum), which had formed previously. Thus, the dissolution of anhydrite maintained a slight supersaturation of gypsum in the fluid, and a temperature close to the equilibrium allowed the formation of only a few crystal nuclei in the deep large cavities. Shallower, cooler cavities have produced multiple smaller crystals. — BH

Geology 35, 327 (2007).

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