Colemanite, CaB3O4(OH)3 x H2O, is a common constituent of natural borate deposits and a relevant mineral commodity, for its applications in the B-extraction processes and in the production of lightweight cements and ceramics [1,2]. Its crystal structure is monoclinic (P21/a, a = 8.712 Å, b = 11.247 Å, c = 6.091 Å, β = 110.12°, V = 560.4 Å3), characterized by infinite chains of Bpolyhedra based on a fundamental ring-unit made by one triangular BO3 and two B(O,OH)4 tetrahedra. These chains are alternated with chains of Ca-polyhedra (coordination number, CN =8), giving rise to sheets parallel to (010). Adjacent sheets are mainly interconnected through an H-bonding system involving hydroxyl groups and H2O molecules. The links among structural polyhedra, both B and Ca, are all corner sharing in type [3]. Despite its relevance in industrial applications, the behavior of colemanite at high pressure has never been investigated. In this work, we have characterized the elastic behavior, the phase stability, the phase transition and the structural evolution with pressure of a natural colemanite single crystal based on an in-situ synchrotron X-ray diffraction study, performed at the P02.2 beamline of PETRA-III, Hamburg, Germany. Colemanite, in its ambient conditions polymorph, is found to be stable up to 13.95 GPa. A IIIorder Birch-Murnaghan (III-BM) EoS fit leads to a refined bulk modulus (at ambient conditions) (KV0) of 76(8) GPa [K = 4.4(10)]. Comparative structure analysis, based on the refinements of the structure model at different pressures, shows that the B-polyhedra act as quasi-rigid units and the bulk compression is mainly accommodated by the compression of Ca-polyhedra and by polyhedral tilting. Between 13.95 and 14.91 GPa, colemanite undergoes a reconstructive phase transition toward a monoclinic polymorph (P21/n, a = 11.726(11) Å, b = 10.206(1) Å, c = 23.45(3) Å, β = 95.07(9)°, V = 2796(4) Å3, at 14.91 GPa). The average coordination number of boron is found to increase: of the 18 independent B sites, 3 are triangularly coordinated and 15 are tetrahedra. Two independent infinite chains of B-polyhedra are alternated with two independent chains of Capolyhedra (CN = 8 or 9), by means of corner- and edge-sharing links. A II-BM EoS fit leads to a refined KV0 of 60(6) GPa for the high-P polymorph of colemanite. X-ray diffraction patterns collected during P-release show that the phase transition is completely reversible and colemanite fully recovers its starting structural features. [1] K. Okuno Radiat. Prot. Dosim. 2005, 115, 258-261. [2] A. Christogerou, T. Kavas, Y. Pontikes, S. Koyas, Y. Tabak, G.N. Angelopoulos Ceram. Int. 2009, 35, 447-452. [3] P.C. Burns, F.C. Hawthorne Can. Mineral. 1993, 31, 297-304
