HIGH-PRESSURE STRUCTURAL BEHAVIOUR OF HEULANDITE

Abstract

The structural evolution up to 5 GPa of a natural heulandite was studied using in situ single-crystal X- ray diffraction data from a diamond-anvil cell (DAC) with glycerol as the pressure transmitting medium. Linear regressions yielded mean axial compressibilities for a, b and c axes of a =1 .02(1)·10 -2, b = 8.1(6)·10-3, c = 7.6(2)·10-3 GPa-1. The largest strain vector ( 1 =1 .16 10 -2 GPa-1) lies approximately on the diagonal of the system of channels along (100) and (001). V0 ,K 0 ,a nd K 0' refined with a third-order Birch-Murnaghan equation are: V0 = 2121(2) Å3 ,K 0 = 26.4(1.0) GPa, K0' = 4.9(8). If fitted with second-order Birch-Murnaghan equation of state, fixing K 0 '=4 , K 0 becomes 27.5(2) GPa. The bulk heulandite structure compression was the result of the "soft" behaviour of the channels (K=10-19 GPa) and the more rigid behaviour of the tetrahedral framework (K ~ 60 GPa), which underwent tilting of the fundamental polyhedral unit (FPU) chains. The T5-T5-T5 angles, between the FPUs, decreased from 162.4° at 0.0001 GPa to 156.2° at 3.4 GPa. The position of extra-framework cations and water molecules was almost maintained within the investigated pressure range. Up to 3.7 GPa no phase transition was observed. Amorphization was clearly observed at pressure above 4 GPa. The transition to the amorphous phase was still reversible up to 5 GPa.