High-pressure behavior of natural borate colemanite. An in situ synchrotron single-crystal X-ray diffraction study

Colemanite is an inoborate compound and a common constituent in natural borate deposits. In addition, it is an economically relevant mineraI commodity, not only as a primary source for B, but a1so for its applications in the production of 1ightweight concretes and ceramics. Despite its relevance in industriaI applications, its elastic behavior, phase stability and structure evolution with pressure have never been investigated. Here we report the high-P behavior of a natural colemanite based on an in-situ synchrotron single-crystal X-ray diffraction study performed at the P02.2 beamline at PETRAIII, Hamburg, Germany. Colemanite, which crystallizes in the monoclinic P2 la space group (a =8.712 A, b = 11.247 A, c = 6.091 A, beta= 110.12°, V = 560.4 A3), undergoes a reconstructive phase transition between 13.95 and 14.91 GPa, toward a monoclinic polymorph (S.G.: P2/n, a = 11.726 A, b = 10.206 A, c = 23.45 A, beta =95.07°, V= 2796 A3, at 14.91 GPa). A III-order Birch-Murnaghan EoS fit leads to a refined bu1k modulus at ambient conditions of 76(8) GPa [i(v' =4.4(10)], for colemanite in the phase stability field: 0.0001-13.95 GPa. The structure of colemanite is made by infinite chains of corner-sharing B-polyhedra alternated by chains of cornersharing Ca-polyhedra (coordination number 8). Of the three crystallographicaIly independent B sites, one shows a triangular coordination and the others a tetrahedral coordination. In the high-P polymorph, only three over eighteen independent B-sites (116) show a triangular coordination, the other fifteen being B(O,OH)4 tetrahedra. Two independent corner-sharing bora te chains are interconnected through corner and edge-sharing chains of Ca-polyhedra (C.N. 8 or 9). X-ray diffraction patterns collected during P-release show that the plme transition is completely reversible and colemanite fully recovers its starting structura1 features.