Crystal chemistry and temperature behavior of colemanite

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Marco E. Ciriotti
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Crystal chemistry and temperature behavior of colemanite

Messaggio da Marco E. Ciriotti » mar 14 nov, 2017 11:43

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▪ Lotti, P., Gatta, G.D., Demitri, N., Guastella, G., Rizzato, S., Ortenzi, M.A., Magrini, F., Comboni, D., Guastoni, A., Fernandez-Diaz, M.T. (2017): Crystal chemistry and temperature behavior of the natural hydrous borate colemanite, a mineral commodity of boron. Physics and Chemistry of Minerals, 44, (in press).

Colemanite, CaB3O4(OH)3⋅H2O, is the most common hydrous Ca-borate, as well as a major mineral commodity of boron. In this study, we report a thorough chemical analysis and the low-temperature behavior of a natural sample of colemanite by means of a multi-methodological approach. From the chemical point of view, the investigated sample resulted to be relatively pure, its composition being very close to the ideal one, with only a minor substitution of Sr2+ for Ca2+. At about 270.5 K, a displacive phase transition from the centrosymmetric P21/a to the acentric P21 space group occurs. On the basis of in situ single-crystal synchrotron X-ray (down to 104 K) and neutron diffraction (at 20 K) data, the hydrogen-bonding configuration of both the polymorphs and the structural modifications at the atomic scale at varying temperatures are described. The asymmetric distribution of ionic charges along the [010] axis, allowed by the loss of the inversion center, is likely responsible for the reported ferroelectric behavior of colemanite below the phase transition temperature.
Marco E. Ciriotti

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