Elastic behavior and pressure-induced structural modifications of the microporous Ca(VO)Si4O10 center dot 4H(2)O dimorphs cavansite and pentagonite

The behavior of natural microporous cavansite and pentagonite, orthorhombic dimorphs of Ca(VO) (Si4O10)center dot 4H(2)O, was studied at high pressure by means of in situ synchrotron X-ray powder diffraction with a diamond anvil cell using two different pressure-transmitting fluids: methanol:ethanol:water = 16:3:1 (m.e.w.) and silicone oil (s.o.). In situ diffraction-data on a cavansite sample were collected up to 8.17(5) GPa in m.e.w, and up to 7.28(5) GPa in s.o. The high-pressure structure evolution was studied on the basis of structural refinements at 1.08(5), 3.27(5) and 6.45(5) GPa. The compressional behavior is strongly anisotropic. When the sample is compressed in s.o. from P-amb to 7.28(5) GPa, the volume contraction is 12.2%, whereas a, b and c decrease by 1.6%, 10.3% and 0.3%, respectively. The main deformation mechanisms at high-pressure are basically driven by variation of the T-O-T angles.