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.

Editorial for Special Issue “Formation of Sulfate Minerals in Natural and Industrial Environments”

“…In this valuable contribution, the authors demonstrate the importance of small contamination particles (nm to microns), which drive the nucleation of gypsum, and whose presence is rarely, if ever, considered. This paper highlights the complexity of solution-based growth phenomena in sulfates, and clearly links to [8,9].…”

“…Importantly, these giant crystals constitute a unique "natural laboratory" for studying crystallization processes spanning many length and time scales. A detailed review on the giant crystals from Naica is presented in this volume by Carreño-Márquez and coworkers [8]. In this contribution, the authors present a description of the Naica Mine setting, where these crystals were discovered, and a state-of-the-art summary on the thermodynamic conditions favoring the crystals' growth, and the associated kinetic aspects.…”

A Challenged Evaporite Paradigm?

“…However, gypsum crystals may develop into huge dimensions, like the ones in the 'Naica.Cave of Crystals', Mexico. According to [13] such crystals can only develop in hydrothermal environments. Thus, the observation by [3] of large selenite crystals in Messinian salts might indicate hydrothermal formation as opposed to solar evaporation.…”