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Physical Chemistry

Giant Crystals Grow Superslowly

Scientists estimate that Mexican gypsum crystals took up to 1 million years to reach their current size

by Stephen K. Ritter
September 19, 2011 | A version of this story appeared in Volume 89, Issue 38

Scientists studying the geochemical processes at play in the formation of giant gypsum crystals (CaSO4•2H2O) found in Mexico’s Naica ore mines have determined that these natural wonders—up to about 35 feet long and 3 feet thick—grow much more slowly than a snail’s pace, taking up to 1 million years to reach their current size. To study the crystals, a team led by Juan Manuel García-Ruíz of the University of Granada, in Spain, cleaned a chunk of one of the giant crystals to activate growth sites and then immersed it in mineral-rich water from a Naica cavern. The researchers then watched the crystals grow by using a white-light phase-shift interference microscope they developed—a type of high-resolution interferometer (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1105233108). The crystal growth proceeds slowly because the chemical equilibrium is barely shifted in favor of crystal growth over gypsum solubility, the researchers note. The slowest measurable growth rate of about 16 femtometers per second occurred at 55 °C, the natural temperature of the Naica water. At that rate it would take about 990,000 years for one of the giant crystals to grow, they estimate.


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