Evolution of the Astonishing Naica Giant Crystals in Chihuahua, Mexico

Abstract

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.