MS15-P01 Evidence of Screw Dislocation on Gypsum Crystals as Principal Mechanism of Growth at Low Supersaturation Joaquin Criado Reyes (Laboratorio de Estudios Cristalográficos, IACT UGR-CSIC, Granada, Spain) Linda Pastero (Dipartimento di Scienze della Terra, Università degli Studi di Torino, Torino, Italy) Marco Bruno (Dipartimento di Scienze della Terra, Università degli Studi di Torino, Torino, Italy) Dino Aquilano (Dipartimento di Scienze della Terra, Università degli Studi di Torino, Torino, Italy) Fermín Otálora Muñoz (Laboratorio de Estudios Cristalográficos, IACT UGR-CSIC, Granada, Spain) Juan Manuel García Ruiz (Laboratorio de Estudios Cristalográficos, IACT UGR-CSIC, Granada, Spain)email: joaquin.criado@csic.esGypsum mineral mainly occurs on evaporitic environment around the world [1, 2]. It has also been reported as a relevant phase in Mars [3-5]. The growth conditions during the growth of gypsum crystals influences the surface growth mechanisms and the habit. Many authors suggest that the growth mechanisms of gypsum crystals at low supersaturation is due to dislocation growth, these studies are based on kinetic data fitted to theoretical equations[6, 7]. However, the observation of hillocks on the surface gypsum crystals has been challenging. A couple of studies on the cleavage face (010) of gypsum by Atomic Force Microscopy (AFM) and Differential Interface Contrast Microscopy (DICM) shown some hillock but only one of them could be clearly identified as a screw dislocation, so the authors conclude that the main growth mechanism at low supersturation on this face is by 2D nucleation[8, 9]. Equivalent studies on the (120) face are missing, mainly due to the roughness of these faces. In a preliminary study of the gypsum (120) face using crystals growing by evaporation, we observed that hillocks spread on (120) at low supersaturation. Those hillocks are made by monolayers with a height of 4.30 Å corresponding to the d-spacing. These hillocks show an asymmetric morphology (figure 1).
 
References:

1. Krüger, Y., et al., Determining gypsum growth temperatures using monophase fluid inclusions—Application to the giant gypsum crystals of Naica, Mexico. Geology, 2013. 41(2): p. 119-122.

2. Van Driessche, A.E.S., et al., Ultraslow growth rates of giant gypsum crystals. Proceedings of the National Academy of Sciences, 2011. 108(38): p. 15721-15726.

3. Elwood Madden, M.E., R.J. Bodnar, and J.D. Rimstidt, Jarosite as an indicator of water-limited chemical weathering on Mars. Nature, 2004. 431: p. 821.

4. Gendrin, A., et al., Sulfates in Martian Layered Terrains: The OMEGA/Mars Express View. Science, 2005. 307(5715): p. 1587-1591.

5. Nachon, M., et al., Calcium sulfate veins characterized by ChemCam/Curiosity at Gale crater, Mars. Journal of Geophysical Research: Planets, 2014. 119(9): p. 1991-2016.

6. Christoffersen, M.R.C.A.J., Crystal Growth Of Calcium Sulphate Dihydrate Atlow Supersaturation. Journal of Crystal Growth, 1982. 58: p. 10.

7. G.M. Van Rosmalen, P.J.D.A.W.G.J.M., An Analysis Ofgrowth Experi
Keywords: Gypsum, Growth Mechanism, Screw Dislocation