Giant crystals in Mexican cave face dehydration

In the Cave of Crystals in Chihuahua, Mexico, gypsum crystals in a form called selenite (CaSO₄∙2H₂O) have grown as big as trees. But researchers now report that loss of water may damage the crystal surfaces (Cryst. Growth Des. 2018, DOI: 10.1021/acs.cgd.8b00583). María Elena Montero-Cabrera of the Center for Research in Advanced Materials and colleagues put fragments of a cave crystal under 16 sets of conditions that varied the exposure to different atmospheres, different temperatures, and the presence or absence of water. The researchers expected that carbon dioxide exposure, which could come from exhaled human breath, would cause calcium carbonate to form on the crystal surface, Montero-Cabrera says. However, both infrared spectrometry and grazing incidence X-ray diffraction analysis revealed no detectable calcium carbonate. Instead, they found bassanite (CaSO₄∙½H₂O), a dehydrated form of gypsum crystal. This conversion could, over many years, cloud and scar the crystal surfaces. By learning more about the crystals’ degradation, the scientists hope to better preserve them for generations to come.