Giant gypsum crystals : the why’s and how’s

Orateur : Dr. Alexander VAN DRIESSCHE

Giant gypsum crystals : the why’s and how’s The occurrence of giant selenite crystals around the world (e.g. Spain [1,2], Mexico [3], Chile [4]) put forth a challenging problem in the field of mineral formation and these formation can be looked upon as crystal growth experiments that have been going on for a long time. Hence, they can teach us a lot about mineral deposition from solution at geologic time scales. Up to now, the main focus of research has been on the giant gypsum crystals (up to 11m long) found at the Naica ore mines (Mexico), because, as we speak, gypsum crystals are still forming from the present day waters. To understand the why´s and how´s of these spectacular mineral formations a combination of techniques common to isotope geochemistry, crystallography and ore geology were applied. In a nutshell, these gypsum crystals are formed by a self-feeding mechanism driven by solution-mediated anhydrite-gypsum phase transition taking place close to equilibrium and leading to extremely slow crystallization rates [3,5]. From fluid inclusions the formation temperature was determined to be in the range of 47-55C, depending on the depth of the cavity where crystals were formed [6]. To estimate the formation time of these crystals the growth rates of the 010 face of gypsum growing from current waters from Naica at different temperatures were measured using high-resolution phase-shift interferometry and advanced confocal microscopy [5,7]. The slowest measurable growth rate was found at 55 C, being 1.4±0.2x10-5 nm/s, for the larger crystals this corresponds with a growth age of 1 myr. This study also shows that applying advanced optical techniques in laboratory experiments can be useful to gain a better understanding of geochemical processes occurring at a geological time scale. [1] Garcıa-Guinea, J. ; Morales, S. ; Delgado, A. ; Recio, C. ; Calaforra, J. M. Geol. Soc. J. 2002, 159, 347–350. [2] Bernardez Gomez, M. J. ; Guisado di Monti, J. C. Pallas 2007, 75, 49–57. [3] Garcıa-Ruiz, J. ; Villasuso, R. ; Ayora, C. ; Canals, A. ; Otalora, F. Geology 2007, 35, 327–330. [4] Cannell, J. ; Cooke, R. D. ; Walshe, J. L. ; Stein, H. Econ. Geol. 2005, 979–1003. [5] Van Driessche A.E.S., García-Ruiz J.M., Tsukamoto, K. Patiño-López L.D., Satoh H. PNAS 2011, 108, 15721-15726. [6] Y. Krüger, J.M. García-Ruiz, A. Canals, D. Martí, M. Frenz, A.E.S. Van Driessche A.E.S. Geology, 2013, 41,119-122. [7] Van Driessche A.E.S., García-Ruiz J.M., Delgado-López J.M. Sazaki G. Crystal Growth Des. 2010, 10, 3909-3916.

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