Are the Naica giant crystals deteriorating because of human action?

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 Permian Minnekahta Limestone: a Saline Lake Gypsum Replaced by Calcite

The late Permian Minnekahta Limestone of the Black Hills of South Dakota is not a typical limestone. Sandwiched stratigraphically between continental redbed siliciclastics, it is distinct in its gray-pink appearance and carbonate mineralogy and is considered a local and regional marker bed. Based on its calcite composition and stromatolites, it has previously been interpreted as a tidal flat deposit, leading to speculation of a regional late Permian marine transgression. Here, we show observations that question the original composition, reexamine and refine the sedimentary and stratigraphic characteristics, and suggest an alternate depositional environment. Petrography and x-ray diffraction indicate that the Minnekahta Limestone is dominated by an interlocking crystal mosaic of calcite and gypsum. We have documented pseudomorphs after bottom-growth gypsum crystals, alternating laminae of mm-scale bottom growth gypsum crystal shapes and mud drapes, as well as mudcracks, raindrop imprints, and stromatolites. No carbonate grains, fossils, or tidal rhythmites were noted. These observations suggest that the carbonate phase is a replacement of gypsum. In addition to the sedimentology and petrography, close stratigraphic association with overlying and underlying continental redbeds indicates that this was likely a shallow saline-lake deposit. Other Permo-Triassic carbonate strata in continental redbed sequences throughout the midcontinent of North America, such as those in the Goose Egg, Chugwater, Nippewalla, and Spearfish strata, may have similar continental origin. This study serves as a lesson that paired petrographic observations and mineral identifications are important data necessary for the best interpretation of depositional environments and diagenetic histories.

Analysis on Crystallization Kinetics of Gypsum Prepared by Bittern under 40°C

Crystallization kinetics experiment of calcium sulfate dehydrates, which is prepared by bittern under 40oC and 200r/min conditions, is carried out. According to the results of the experimental data, the relationship of nucleus particle-number density n0 and crystal growth rate G with the residence time is summarized. Also, the average diameter of gypsum crystal is decreasing with the residence time increase. Meanwhile, the crystallization kinetics formula is derived to be: B0=3.20×105G0.47.

Relationship between Exothermic Effect and Crystal Growth in the Hydration Process of Waterproof Gypsum

The new type of gypsum compound waterproof agent was composed of organic emulsion that emulsified by polyvinyl alcohol and stearic acid and saline waterproof agent made of carboxylic acid sodium, aluminium sulfate etc. The influences on the hydration exothermic effect and crystal growth of gypsum were studied when different amount of the organic emulsion, the saline waterproof agent were added. According to the hydration exothermic effect and microstructure of crystal growth of gypsum after mixing the different amount of waterproof agent, the relationship between gypsum hydration exothermic effect and gypsum crystal growth were discussed theoretically.