The anomalous high-pressure phase transition of inderite

Inderite, ideally [MgB3O3(OH)5∙5H2O], is a hydrated borate discovered in the Inder deposit (Kazakhstan) in 1937 and structurally characterized for the first time by Boldyreva [1]. Inderite is a Na-free hydrated borate and, unlike others common Na-bearing minerals like ulexite (NaCaB5O6(OH)6·5H2O) or borax (Na2B4O5(OH)4·8H2O). Therefore, inderite would not promote any deleterious Alcali-Silica Reactions (ASR, triggered by Na-bearing phases), if used as an aggregate in Portland cements. In the last years, phase transitions occurring at different pressures were discovered in a plethora of hydrous borates, including kurnakovite and meyerhofferrite [2,3] which share the same polyion unit [B3O3(OH)5]2-. The high-pressure stability field of this kind of hydrated borates, having polyions in isolated units, appears to be directly correlated with the total H2O content of the mineral itself. In this light, the high-pressure behaviour of inderite was investigated by an in-situ single-crystal X-ray diffraction (up to 17.4 GPa) under hydrostatic conditions. Results show: 1) inderite undergoes a first order phase transition between ~6.15 and ~6.45 GPa marked by a sudden 7.0 % volume decrease; 2) the structure of the high-pressure polymorph, inderite-II, was solved (Fig. 1); 3) as response to the phase transition, the boron site in planar-triangular coordination bonds to a H2O molecule, forming a tetrahedron; 4) inderite was found to be a highly anisotropic mineral. [1] Boldyreva A.M. Mem Soc russe Min, 1937, 2, 651–671 [2] Comboni D., Poreba T., Pagliaro F., et al. Acta Crystallographica Section B, 2021, 6, 940-945. [3] Pagliaro F., Lotti P., Battiston T., et al. Construction and Building Materials, 2021, 121094.