High-pressure deformation mechanism in scolecite: a combined computational-experimental study

The pressure-induced structural modifications in scolecite were studied by means of in-situ synchrotron X-ray powder diffraction and by density functional computations. The experimental cell parameters were refined up to 7.5 GPa, pressure at which we found a reduction of 4.0, 5.0, 2.0, 0.9 and 11 %, in a, b, c,  and V, respectively. A slight anomaly in the slope of the volume vs pressure dependence was observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. The weakening and broadening of the diffraction peaks reveal the growth of a disordered phase with increasing pressure, preventing the refinement of the lattice parameters above 7.5 GPa. Diffraction patterns collected in decompression show that the disordering is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The relatively discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, that allows us to attribute it to the re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover, we found that, with increasing pressure, the tetrahedral chains, parallel to c, rotate along their elongation axis and display an increasing twisting along an axis perpendicular to c. At the same time, we observed the progressive squashing of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase (from 4 to 5) of the coordination number of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.