HIGH-PRESSURE STRUCTURAL BEHAVIOUR OF SCOLECITE

Abstract

The RP structural evolution of a natural scolecite from Iceland (space group Cc) was studied up to 5 GPa using in situ single-crystal X-ray diffraction data from a diamond-anvil cell (DAC) with silicon oil as non penetrating pressure transmitting medium. Linear regressions yielded mean axial compressibilities for a, b and c axes of beta(a) = 4.4(2)(.)10(-3), beta(b) = 6.1(2)(.)10(-3), beta(c) = 6.0(1)(.)10(-3) GPa(-1). K-0, refined with a second-order Birch-Murnaghan equation, fixing K-0' at 4, is 54.6(7) GPa. The bulk scolecite structure compression was the result of the "soft" behaviour of the channels (K congruent to 17 GPa for [100]-channels; K congruent to 50 GPa for [001]-channels) and the more rigid behaviour of the tetrahedral framework (K = 96 GPa), which underwent kinking of the Secondary Building Unit (SBU) along [100]-chains. The angle between the SBUs (phi), increased from 20.80(2)degrees at 0.0001 GPa, to 22.00(6)degrees at 3.38 GPa. Within the investigated pressure range, the position of the extra-framework cations and water molecules remained almost unchanged. Up to 4.2 GPa no phase transition was observed.