The natural borate colemanite at non-ambient conditions: behavior at low temperature and high pressure

Colemanite, CaB3O4(OH)3*H2O, is a common mineral in borate sedimentary deposits in saline lakes, related to hydrothermal volcanic activity, and it is one of the main mineral commodities for the extraction of boron. Due to its relative abundance, in particular at the mine dumps, several recent works were devoted to explore its potential technological and industrial applications (see e.g. [1] for a list of references). Despite this interest, very few was known on the behavior of the colemanite crystal structure at non-ambient conditions of temperature and pressure. This contribution reports the results obtained from in situ low-temperature (T < 293 K) and high-pressure experiments. A displacive phase transition from the centrosymmetric P21/a colemanite to a ferroelectric polymorph with P21 symmetry was long time known to occur in the T-range between 273 and and 263 K (e.g. [2]). Thermal analysis and in situ single-crystal X-ray diffraction data confirmed the transition, which was found to occur between 265 and 260 K. A thorough chemical analysis, performed by a combination of techniques, revealed the relative pureness of the natural sample of colemanite investigated, supporting the hypothesis that the absence of impurities reduces the T of transition to the ferroelectric state (i.e. it reduces the stability field of the ferroelectric polymoprh) [3]. Single crystal X-ray and neutron diffraction data (down to 104 and 20 K, respectively) showed that the transition has limited effects on the crystal structure of colemanite. On the other hand, in situ high-pressure single-crystal X-ray diffraction experiments disclosed a much more complex scenario, with a first-order reconstructive phase transition occurring between 13.95 and 14.91 GPa, toward a denser polymorph with a = 3*aCOL, b = bCOL and c = 2*cCOL. Despite reconstructive, the transition is single crystal-to-single crystal and involves an increase in the average coordination number of both the Ca and B sites. The tripling of the a-axis and the doubling of the c-axis imply the split of every independent atomic site of colemanite in six new independent positions in the high-P polymorph. In particular, three of the six new sites, generated from the parent triangularly coordinated B, increase their coordination number from three to four, gaining a bond with a H2O oxygen. The elastic behavior of colemanite and of the high-P polymorph have been described by means of III- and II-order Birch-Murnaghan equations of state, respectively, yielding the following bulk moduli: 67(4) GPa (colemanite, KV' = 5.5(7)) and 50(8) GPa (high-P colemanite). [1] P. Lotti, G.D. Gatta, D. Comboni, G. Guastella, M. Merlini, A. Guastoni, H-P. Liermann J. Am. Cer. Soc. 2017, in press, DOI: 10.1111/jace.14730. [2] F.N. Hainsworth, H.E. Petch Can. J. Phys. 1966, 44, 3083. [3] H.H. Wieder, A.R. Clawson, C.R. Perkerson J. Appl. Phys. 1962, 33, 1720.