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The giant gypsum crystals of Naica cave have fascinated scientists since their discovery in 2000. Human activity has changed the microclimate inside the cave, making scientists wonder about the potential environmental impact on the crystals. Over the last 9 years, we have studied approximately 70 samples. This paper reports on the detailed chemical–structural characterization of the impurities present at the surface of these crystals and the experimental simulations of their potential deterioration patterns. Selected samples were studied by petrography, optical and electronic microscopy, and laboratory X-ray diffraction. 2D grazing incidence X-ray diffraction, X-ray μ-fluorescence, and X-ray μ-absorption near-edge structure were used to identify the impurities and their associated phases. These impurities were deposited during the latest stage of the gypsum crystal formation and have afterward evolved with the natural high humidity. The simulations of the behavior of the crystals in microclimatic chambers produced crystal dissolution by 1–4% weight fraction under high CO2 concentration and permanent fog, and gypsum phase dehydration under air and CO2 gaseous environment. Our work suggests that most surface impurities are of natural origin; the most significant anthropogenic damage on the crystals is the extraction of water from the caves.

The Cave of Giant Crystals of Naica (Chihuahua, Mexico) is a world geological treasure worth to be preserved. These crystals of up to 12 m in length are made of selenite, the macrocrystalline variety of gypsum (CaSO4·2H2O). They have grown for thousands of years until the cave was dried, which allowed the cave and the crystals to be accessible, but exposed their surfaces in contact with air. Gypsum crystals are fragile because of their trend to dehydrate, the possible replacement to CaCO3 upon reactions with atmospheric CO2 as well as their intrinsic mechanical properties. Several laboratory experiments, designed to study the deterioration of selenite crystals under different artificial atmospheric conditions, are presented. Four atmospheric compositions rich in CO2, CH4, NOx, and air were tested for 1 year at temperatures of 25 and 60 °C and in either liquid or gaseous environments. The surface evolution was monitored by optical microscopy, infrared spectrometry, and grazing incidence X-ray diffraction with two-dimensional detectors. Surface alteration and dissolution in a water environment were observed in short exposition times, as well as the formation of bassanite (CaSO4·1/2H2O). Neither anhydrite nor calcite was detected. The gaseous environment constituted the most detrimental conditions to the gypsum crystals integrity., The authors would like to acknowledge Consejo Nacional de Ciencia y Tecnologia (grant number 183706); Industrias Peñoles for the support given to the experiments; A. Reyes-Rojas, D. Burciaga-Valencia, and E. Guerrero-Lastarjette for their continuous support in the realization of this project; and B. Aldea-Ballano and the team of the Chemical and Physicochemical Testing Unit in Instituto Eduardo Torroja de Ciencias de la Construcción in Madrid. The authors especially thank J. M. García-Ruiz for the suggestions of some experiments. Part of the experiments was performed (as part of Proposal No. 3939) at the Stanford Synchrotron Radiation Lightsource (SSRL), a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy, Office of Science, by Stanford University.

Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4·2H2O), bassanite (CaSO4 ·0.5H2O), soluble anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the unique requirements in which these crystals grow, the characterization of their environment and microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the current state and the foreseeable future of these astounding crystals., The support provided by CONACYT Project No. 183706, the proposals SSRL 3939, the ESRF HG-77, Elettra Sincrotrone-Trieste 20155328, as well as by the International Centre for Theoretical Physics (ICTP) are acknowledged. Special thanks to Manuel Reyes-Cortés, who provided some of the key research samples and supported their selection and analysis. The authors thank the cooperation of the management of the Peñoles company, the Desert Museum of Ciudad Delicias, Chihuahua, the Harvard Museum of Natural History, and the Faculty of Engineering of the Autonomous University of Chihuahua for providing essential specimens for the study. The authors thank the anonymous referees for their helpful suggestions.