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 βa = 1.02(1)·10−2, βb = 8.1(6)·10−3, βc = 7.6(2)·10−3 GPa−1. The largest strain vector (β1 = 1.16 10−2 GPa−1) lies approximately on the diagonal of the system of channels along [100] and [001].

V0, K0, and K0' refined with a third-order Birch-Murnaghan equation are: V0 = 2121(2) Å3, K0 = 26.4(1.0) GPa, K0' = 4.9(8). If fitted with second-order Birch-Murnaghan equation of state, fixing K0' = 4, K0 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.

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