High-pressure structural behaviour of heulandite

The structural evolution up to 5 GPa of a natural heulandite was studied using in situ single-crystal Xray 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 beta (a) = 1.02(1).10(-2), beta (b) = 8.1(6).10(-3), beta (c) = 7.6(2).10(-3) GPa(-1). The largest strain vector (beta (1)= 1.16 10(-2) GPa(-1)) lies approximately on the diagonal of the system of channels along [100] and [001]. V-0, K-0, and K-0' refined with a third-order Birch-Murnaghan equation are: V-0 = 2121(2) Angstrom (3), K-0= 26.4(1.0) GPa, K-0' = 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 congruent to 60 GPa), which underwent tilting of the fundamental polyhedral unit (FPU) chains. The T5-T5-T5 angles, between the FPUs, decreased from 162.4 degrees at 0.0001 GPa to 156.2 degrees 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.