X-ray single crystal structure determinations of sodalite (Na8[Al6Si6O24]Cl2, cubic, P[unk]3n) were performed under high hydrostatic pressure in diamond anvil cells. Natural (from Bancroft/Canada) and synthetic specimens were investigated.
The continuous decrease of the lattice constant with increasing pressure indicates that there are no first order phase transitions in the pressure range investigated. However, the comparison of the results with data obtained from previous experiments and on related structures of tugtupite and helvite, reveals a possible change in the compression mechanism or a phase transition at about 3 GPa.
The compression mechanism operates on the intertetrahedral bonding parameters of the alumino-silicate framework structure. The volume reduction is mainly achieved by the deformation of bonding angles between the different TO4-tetrahedra. As a result, the size of the β-cages and consequently the size of the [Na4Cl]3+-clusters decreases.
A pressure induced amorphization of the crystal structure was detected at 7.3 GPa. Experiments on the synthetic samples were done to prove this low value of pressure induced amorphization. On lowering the pressure, recrystallization from the amorphous phase occured at 5.8 GPa.
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