The importance of gypsum in both natural and industrial environments has motivated research on the fundamental aspects of gypsum crystallization over the last century. Even so, certain fundamental aspects, such as the growth mechanisms of the different crystal faces, the equilibrium and growth morphology, or the growth kinetics of gypsum at temperatures far above the gypsum-anhydrite transition curve of gypsum are poorly understood. Some of these unanswered questions are present in the giant selenite crystals in Mexico [1] Chile [2] and Spain [3,4] which put forth a challenging problem in the field of crystal growth in natural environments.

Laser confocal differential interference contrast microscopy, atomic force microscopy  and white-beam phase-shift interferometry microscopy  were used to observe in situ the step advancement and the evolution of the surface morphology of the (010) face of gypsum crystals growing from aqueous solutions prepared in the laboratory and natural solutions obtained from the Naica Mine.

When using supersaturated laboratory solutions we found that 2D nucleation is the main step generation mechanism, even at low supersaturations, and only indirect evidence of spiral hillocks was observed. Due to the elongated morphology of 2D islands along the c-axis and the frequent nucleation of multilayer 2D islands, the (010) develops a characteristic “hill and valley” topography. This type of surface topography is observed at all temperatures. The step kinetic coefficient, βst, was determined in the temperature range 20-80 ºC, and a steep increment in the kinetic coefficient is found with increasing temperature (step velocities as fast as 300 nm/s were measured at 80ºC, [5]). The second part of this work consisted in measuring the growth/dissolution rates of the (010) face of gypsum growing from current Naica waters at different temperatures (45-90ºC). The same “hill and valley” surface topography was observed and the slowest measurable growth rate was found at 55 °C, 1.4±0.2×10⁻⁵ nm∕s [6]. At higher temperatures, growth rates increase exponentially because of decreasing gypsum solubility and higher kinetic coefficient. At 50 °C neither growth nor dissolution was observed indicating that growth of giant crystals of gypsum occurred at Naica between 58 °C (gypsum/anhydrite transition temperature) and the current temperature of Naica waters, confirming formation temperatures determined from fluid inclusion studies [1,7].

Our results demonstrate the usefulness of applying advanced in situ optical techniques to gain a better understanding of crystal growth processes occurring at fast time scales (e.g. industry) but also at a geological time scale (e.g. giant gypsum deposits).

[1] Garcıa-Ruiz, J.; Villasuso, R.; Ayora, C.; Canals, A.; Otalora, F. Geology 2007, 35, 327–330.

[2] Cannell, J.; Cooke, R. D.; Walshe, J. L.; Stein, H. Econ. Geol. 2005, 979–1003.

[3] Garcıa-Guinea, J.; Morales, S.; Delgado, A.; Recio, C.; Calaforra, J. M. Geol. Soc. J. 2002, 159, 347–350.

[4] Bernardez Gomez, M. J.; Guisado di Monti, J. C. Pallas 2007, 75, 49–57.

[5] Van Driessche A.E.S., García-Ruiz J.M., Delgado-López J.M. Sazaki G.. Crystal Growth Des. 2010, 10, 3909-3916.

[6] A.E.S. Van Driessche, J.M. García-Ruiz,  K. Tsukamoto, Patiño-López L.D.,   Satoh H. PNAS 2011, 108, 15721-15726.

[7] 5.   Y. Krüger, J.M. García-Ruiz, A. Canals, D. Martí, M. Frenz  Van Driessche A.E.S. Geology (2013)

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