High‐pressure behavior and P‐induced phase transition of CaB₃O₄(OH)₃·H₂O (colemanite)

Abstract

Colemanite (ideally CaB3O4(OH)3·H2O, space group P21/a, unit‐cell parameters: a ~ 8.74, b ~ 11.26, c ~ 6.10 Å, β ~ 110.1°) is one of the principal mineralogical components of borate deposits and the most important mineral commodity of boron. Its high‐pressure behavior is here described, for the first time, by means of in situ single‐crystal synchrotron X‐ray diffraction with a diamond anvil cell up to 24 GPa (and 293 K). Colemanite is stable, in its ambient‐conditions polymorph, up to 13.95 GPa. Between 13.95 and 14.91 GPa, an iso‐symmetric first‐order single‐crystal to single‐crystal phase transition (reconstructive in character) toward a denser polymorph (colemanite‐II) occurs, with: aCOL‐II=3·aCOL, bCOL‐II=bCOL, and cCOL‐II=2·cCOL. Up to 13.95 GPa, the bulk compression of colemanite is accommodated by the Ca‐polyhedron compression and the tilting of the rigid three‐membered rings of boron polyhedra. The phase transition leads to an increase in the average coordination number of both the B and Ca sites. A detailed description of the crystal structure of the high‐P polymorph, compared to the ambient‐conditions colemanite, is given. The elastic behaviors of colemanite and of its high‐P polymorph are described by means of III‐ and II‐order Birch‐Murnaghan equations of state, respectively, yielding the following refined parameters: KV0=67(4) GPa and KV′=5.5(7) [βV0=0.0149(9) GPa−1] for colemanite; KV0=50(8) GPa [βV0=0.020(3) GPa−1] for its high‐P polymorph.

Thermal stability and high-temperature behavior of the natural borate colemanite: An aggregate in radiation-shielding concretes

“…The T-induced evolution of the unit-cell parameters, reported in Figure 2 and Table 2, reveals that the thermal expansion of colemanite is significantly anisotropic, being substantially accommodated only along the b and c crystallographic axes, whereas the a axis, which corresponds to the direction of the borate chains (Figure 1), experiences only a minor expansion up to 175°C, where a slight contraction takes place (Figure 2 and Table 2). These results are consistent with those reported in [46] for the range 104-300 K (i.e., from -169 to 27 °C) and an analogy can also be found with the high-pressure data [45], where the a direction is reported to be the less compressible crystallographic axis.…”

“…Based on the refined thermo-elastic parameters of this study and on the high-pressure elastic parameters reported in [45] for the same sample, it is possible to define the equation of state of colemanite that, at a first approximation, is valid in the ranges -169 ≤ T (°C) ≤ 250 and 0.0001 ≤ P (GPa) ≤ 13.59:…”

“…The same natural sample of massive and transparent colemanite, already investigated and described by [45,46], has been used for this study. The sample, provided by the Museum of Mineralogy of the University of Padua (Italy), comes from the borate deposits of the Bigadiç Mine (Balikesir Province, Marmara Region, Turkey [52]).…”

“…In this light, the scope of this study is to provide, by means of in situ differential scanning calorimetry, thermogravimetric and synchrotron X-ray diffraction analyses, a clearer picture of the behavior of colemanite at high temperature by: 1) providing a thorough characterization of the dehydration process and of its influence on the structural stability and 2) quantifying the volume thermal expansion coefficient at ambient conditions and at varying temperature. This study is part of a long-term project devoted to the characterization of the behavior of Bcontaining compounds at non-ambient conditions [40][41][42][43][44][45][46], which included: 1) the study of the high-pressure behavior of colemanite, reporting a P-induced reconstructive phase transition, characterized by the increased coordination of a fraction of B atoms from triangular to tetrahedral [45], and 2) the study of the low-T behavior of colemanite, with the transition to its ferroelectric polymorph [46].…”

High‐pressure behavior and phase stability of Na2B4O6(OH)2·3H2O (kernite)

“…Borates are also being used for the production of radiation-shielding materials because of the ability of 10 B, which accounts for ca. 20% of the natural boron, to absorb thermal neutrons, due to its high cross section, for the 10 B(n,α) 7 Li reaction (~3840 barns) 2,3 . Borates could be added to concretes or epoxy resins in order to enhance the shielding efficiency towards neutron radiations [4][5][6][7] .…”