Electrical conductivity of polycrystalline Mg(OH)2 at 2 GPa: effect of grain boundary hydration–dehydration

Abstract

The effect of intergranular water on the conductivity of polycrystalline brucite, Mg(OH)2, was investigated using impedance spectroscopy at 2 GPa, during consecutive heating-cooling cycles in the 298-980 K range. The grain boundary hydration levels tested here span water activities from around unity (wet conditions) down to 10−4 (dry conditions) depending on temperature. Four orders of magnitude in water activity result in electrical conductivity variations for about 6-7 orders of magnitude at 2 GPa and room temperature. Wet brucite samples containing, initially, about 18 wt% of evaporable water (i.e. totally removed at temperatures below 393 K in air), display electrical conductivity values above 10−2-10−3 S/m. A.C. electrical conductivity as a function of temperature follows an Arrhenius behaviour with an activation energy of 0.11 eV. The electrical conductivity of the same polycrystalline brucite material dried beforehand at 393 K (dry conditions) is lower by about 5-6 orders of magnitude at room temperature and possesses an activation energy of 0.8-0.9 eV which is close to that of protonic diffusion in (001) brucitic planes. Above ca. 873 K, a non-reversible conductivity jump is observed which is interpreted as a water transfer from mineral bulk to grain boundaries (i.e. partial dehydration). Cooling of such partially dehydrated sample shows electrical conductivities much higher than those of the initially dry sample by 4 orders of magnitude at 500 K. Furthermore, the corresponding activation energy is decreased by a factor of about four (i.e. 0.21 eV). Buffering of the sample at low water activity has been achieved by adding CaO or MgO, two hygroscopic compounds, to the starting material. Then, sample conductivities reached the lowest values encountered in this study with the activation energy of 1.1 eV. The strong dependency of the electrical conductivity with water activity highlights the importance of the latter parameter as a controlling factor of diffusion rates in natural processes where water availability and activity may vary grandly. Water exchange between mineral bulk and mineral boundary suggests that grain boundary can be treated as an independent phase in dehydroxylation reactions.

Effect of Hydrogen on Corrosion and Stress Corrosion Cracking of AZ91 Alloy in Aqueous Solutions

“…1 Microstructure of as-cast AZ91 alloy [39] generation distributed mainly around these two phases. However, once corrosion products deposited, the dominant hydrogen generation sites were on the surface of the corrosion products since it is highly possible that porous magnesium hydroxide filled with solution was electronically conductive [59], and that the involvement of Al corrosion products (identified by EDS, XPS, and XRD [35,37,39]) increased this conductivity [35,47]. This indicates that the location of the anodic and cathodic sites on the alloy surface shifted during corrosion as the corrosion products become active cathodic sites.…”

The effect of water on intergranular mass transport: new insights from diffusion-controlled reaction rims in the MgO–SiO2 system

“…There is general agreement that the presence of water, or rather hydrous fluid, drastically promotes element transport along grain boundaries. For example, the grain boundary diffusivity of aluminium in 'hydrous fluid-saturated' rocks is enhanced by seven orders of magnitude compared to 'nearly anhydrous' rocks (Carlson 2010), and Gasc et al (2011) reported the same range of variations for the electrical conductivity in polycrystalline brucite from 'dry' to 'wet' conditions. Based on a consensus from literature, Farver and Yund (1995) proposed that intergranular diffusion can be classified into three categories, depending on water activity in the intergranular medium: 'water-absent', 'water-unsaturated' and 'water-saturated' regimes (see also Rubie 1986;Carlson 2010).…”

Electrical conductivity of NaCl‐H2O fluid in the crust

“…The stability and electrical conductivity of rocks [Fuji-ta et al, 2004;Kariya and Shankland, 1983;Olhoeft, 1981], minerals [Gasc et al, 2011;Guo et al, 2011;Guo and Yoshino, 2014;Reynard et al, 2011;Yang, 2011], graphite films on grain boundaries [Frost et al, 1989;Glover, 1996;Yoshino and Noritake, 2011], partial melts [ten Grotenhuis et al, 2005;Hermance, 1979;Lebedev and Khitarov, 1964;Roberts and Tyburczy, 1999], saline fluids [Glover and Vine, 1994;Guo et al, 2015;Hyndman and Hyndman, 1968;Hyndman and Shearer, 1989;Nesbitt, 1993;Shimojuku et al, 2012Shimojuku et al, , 2014, and CO 2 -H 2 O fluid [Nesbitt, 1993] have been investigated as explanations for the highly conductive zones. However, the conductive zones likely represent a composite of these materials, in which aqueous fluids are considered to be a dominant cause of the high conductivity [e.g., Guo et al, 2015;Hyndman and Shearer, 1989;Shankland and Ander, 1983], and this would be more plausible at subduction zone, since the dehydration of hydrous minerals would occur in the subducting slab [Ichiki et al, 2009;McGary et al, 2014;Reynard et al, 2011].…”