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 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 Pamb 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.
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. Additional single-crystal X-ray diffraction experiments were performed in m.e.w. up
to 2.04(5) GPa. In both cases, pressure-induced over-hydration was observed in m.e.w. at high pressure.
The penetration of a new H2O molecule leads to a stiffening effect of the whole structure. Moreover,
between 2.45(5) and 2.96(5) GPa in m.e.w., a phase transition from an orthorhombic to a triclinic phase
was observed. In s.o. pentagonite also transformed to a triclinic phase above 1.71(5) GPa. The overall
compressibility of pentagonite and cavansite in s.o. is comparable, with a volume contraction of 11.6%
and 12.2%, respectively.