Armstrongite at non-ambient conditions : an in-situ high-pressure single-crystal X-ray diffraction study

The high-pressure behavior of a natural armstrongite [(Ca0.96Ce0.01Yb0.01)Zr0.99Si6O14.97·2.02H2O, a∼14.03 Å, b∼14.14 Å, c∼7.85 Å, β∼109.4°, Sp. Gr. C2/m], a microporous heterosilicate, has been studied by single-crystal X-ray diffraction with a diamond-anvil cell up to 8 GPa, using the methanol:ethanol:H2O = 16:3:1 mixture as a pressure-transmitting fluid. A first-order phase transition, characterized by a triplication of the unit-cell volume, was detected between 4.01 (5) and 5.07 (5) GPa. The isothermal bulk modulus (KV0 = -V (∂P/∂V)) of the high-pressure polymorph was found to be ∼50% higher than that obtained for the low-pressure one (i.e., KV0 = 45 (1) GPa for the high-pressure polymorph, KV0 = 31.2 (6) GPa for the low-pressure polymorph), indicating a remarkable change in the structure compressibility. The mechanisms at the atomic scale, which govern the structure deformation of the low-P polymorphs, are described based on a series of structure refinements up to 4 GPa, and a comparison with those experienced by the structure at high temperature is provided. As observed for other microporous silicates, the polyhedral tilting is the main deformation mechanism able to accommodate the effects of the applied pressure. No evidence of crystal-fluid interaction, with a selective sorption of molecules of the pressure-transmitting fluid through the cavities, was observed at high pressure.