Elastic behavior and pressure-induced structural modifications of the microporous Ca(VO)Si4O10·4H2O dimorphs cavansite and pentagonite

The behavior of natural microporous cavansite and pentagonite, orthorhombic dimorphs of Ca(VO) (Si4O10) ·4H2O, was studied at high pressure by in situ synchrotron x- ray powder diffraction with a diamond anvil cell using two different pressure- transmitting fluids: MeOH:EtOH:H2O = 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 from 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 Pamb to 7.28(5) GPa, the vol. contraction is 12.2 %, whereas a, b and c decrease by 1.6 %, 10.3 % and 0.3 %, resp. The main deformation mechanisms at high- pressure are basically driven by variation of the T- O- T angles. Powder diffraction data on a pentagonite sample were collected up to 8.26(5) GPa in (m.e.w) and 8.35(5) GPa in (s.o) . Addnl. single- crystal x- ray diffraction expts. were performed in (m.e.w) up to 2.04(5) GPa. In both cases, pressure- induced over- hydration was obsd. in m.e.w. at high pressure. The penetration of a new H2O mol. leads to a stiffening effect of the whole structure. Also, between 2.45(5) and 2.96(5) GPa in (m.e.w) , a phase transition from an orthorhombic to a triclinic phase was obsd. In (s.o.) pentagonite also transformed to a triclinic phase >1.71(5) GPa. The overall compressibility of pentagonite and cavansite in (s.o.) is comparable, with a vol. contraction of 11.6 % and 12.2 %, resp. Crystallog. data and at. coordinates are given.