Crystal chemistry and temperature behavior of the natural hydrous borate colemanite, a mineral commodity of boron
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  axis, allowed by the loss of the inversion center, is likely responsible for the reported ferroelectric behavior of colemanite below the phase transition temperature.
KeywordsColemanite Borates Low temperature Phase transition Neutron diffraction Synchrotron Ferroelectric behavior
Emanuela Schingaro, Mario Tribaudino and the Editor, Milan Rieder, are gratefully thanked for the valuable comments and suggestions, which improved the manuscript quality. ELETTRA (Trieste, Italy) and ILL (Grenoble, France) are acknowledged for the allocation of beamtime.
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