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Two isomorphous hydrous minerals, Mg(OH)2 and Ca(OH)2, were exposed to the 2.5 MeV electron beam of the SIRIUS accelerator platform. Both compounds remain stable under the beam up to high doses, in the range of 3 -3.5 GGy. No decomposition is observed.But contrary to earlier statements, a net difference of reactivity between them is highlighted as a result of the present X-ray powder diffraction study: i) a significant dilatation is observed along the c -axis, more significant in brucite than in portlandite as already reported during thermal decomposition studies, ii) but a contraction is here revealed in the basal plane of brucite, along the aaxis, while a slight dilatation is still being observed in portlandite. Contraction in the basal plane seems a specific feature of electron irradiation only once previously observed by TEM in brucite. Moreover for brucite, the decreasing intensities of Bragg lines together with the appearance of a diffuse scattering over the whole angular range is compatible with the appearance of some static structural disorder induced by electron irradiation. Finally electron irradiation leads to a significant reduction in crystallite size with increasing dose, by a factor of 2 at the intermediate dose of 310 MGy in brucite, while for a comparable effect to occur an absorbed dose of 3.5 GGy should be attained in portlandite.

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