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Abstract: A natural crystal with NaMnSi2O6 composition (space group C2/c) belonging to the family of clinopyroxene structures was investigated by in-situ high-pressure single-crystal X-ray diffraction up to 9 GPa in order to verify if the compressibility can be predicted from the unit-cell volume V 0 at 1 bar applying an established volume–compressibility relationship. Together with data for isostructural analogue phases the equation-of-state parameters of NaMnSi2O6 showed that such simplified relationship can not be longer applied. The findings demonstrate that prediction models for isostructural series are consistent not only for the aspect of structural isotopy, and valid relationships presuppose similar character of the electronic structure. In case of pyroxenes containing transition elements the established relationship is only valid for series of compounds with cations of identical oxidation state according to isovalent electronic character. [Copyright &y& Elsevier]

Abstract: An in situ high-pressure X-ray diffraction study has been carried out at room temperature up to 9.23 GPa on a crystal of Ca2Sb2O7 having a weberite-2O structure synthesized by solid-state reaction. A 2nd-order Birch–Murnaghan Equation of State (EoS) was used to refine the pressure–volume data. The refinement of the unit-cell volume and of the isothermal bulk modulus at room pressure leads to: V 0 = 555.8 (1), KT 0 = 154.6 (9) GPa. Unit-cell parameters decrease gradually as a function of pressure with a bulk modulus anisotropy scheme Ka 0 > Kb 0 > Kc 0, with c being the softest direction. The compressional behavior of Ca2Sb2O7 is compared with that shown by ingersonite, Ca3MnSb4O14, a mineral showing a weberite-3T structure. [Copyright &y& Elsevier]