Nanoscale observations of periclase (MgO) hydration 

<p>Hydration of anhydrous minerals such as periclase (MgO) is a common process during retrograde metamorphism (mainly serpentinization) and, generally, during fluid-rock interactions. Changes in mineralogy due to hydration reactions may have an impact on rock properties (Kuleci et al. 2016) and implications for the rheology of the crustal rocks (Yardley et al., 2014). Also, the hydration of periclase is an important industrial reaction, particularly in the field of cement and lime mortars. Dolomitic lime used for building purposes contains significant amounts of periclase, which hydrates at a slower rate than lime (CaO), and commonly delayed MgO hydration and swelling occurs in hardened mortar eventually resulting in fracture formation (Jug et al. 2007). It also negatively impacts the durability of MgO-based refractory ceramics (Amaral et al., 2011). Hydration of periclase involves a volume increase of ~110%, resulting in very high stresses if the process occurs in a confined space, which can lead to reaction-induced fracturing of crustal rocks (Zheng et al., 2018). Hence, understanding the mechanisms of periclase hydration is crucial for technical applications, such as avoiding dolomitic lime mortars fracturing due to swelling, and for understanding the feedback between hydration and rock properties in nature.</p><p> </p><p>The hydration of periclase to brucite was investigated experimentally. Here we show, using <em>in situ</em> atomic force microscopy (AFM) and complementary techniques, that upon the reaction of periclase cleavage surfaces with deionized water, spherical nanoparticles form initially oriented along the periclase step edges, subsequently covering the whole periclase surface. With increasing reaction time, nanoparticles develop straight facets and acquire hexagonal features consistent with the structure of brucite. Additionally, differences in adhesion between the outer part and the centre of the nanoparticles were observed, suggesting the initial formation of a precursor (possibly amorphous) that subsequently transforms into crystalline brucite. These results reveal a nonclassical particle-mediated reaction mechanism for the hydration of periclase into brucite.</p><p> </p><p>Amaral, L. F., Oliveira, I. R., Bonadia, P., Salomão, R., & Pandolfelli, V. C. (2011). Chelants to inhibit magnesia (MgO) hydration. Ceramics International, 37(5), 1537-1542.</p><p>Jug K, Heidberg B, Bredow T (2007) Cyclic cluster study on the formation of brucite from periclase and water. J Phys Chem C 111(35):13,103–13,108</p><p>Kuleci, H., Schmidt, C., Rybacki, E., Petrishcheva, E., & Abart, R. (2016). Hydration of periclase at 350 °C to 620 °C and 200 MPa: Experimental calibration of reaction rate. Mineralogy and Petrology, 110(1), 1–10.</p><p>Yardley, B.W.D., Rhede, D., Heinrich, W.,  (2014). Rates of Retrograde Metamorphism and Their Implications for the Rheology of the Crust: An Experimental Study. Journal of Petrology, 55, (3), 623-641.</p><p>Zheng, X., Cordonnier, B., Zhu, W., Renard, F., & Jamtveit, B. (2018). Effects of confinement on reaction‐induced fracturing during hydration of periclase. Geochemistry, Geophysics, Geosystems., 19, 2661–2672.</p><p> </p>