Petalite under pressure: Elastic behavior and phase stability

The lithium aluminosilicate mineral petalite (LiAlSi4O10) has been studied with high-pressure single-crystal X-ray diffraction (HP-XRD) up to 5 GPa. Petalite undergoes two fully reversible pressure-induced first-order phase transitions, not previously reported in the literature, at ca. 1.5 and 2.5 GPa. The first of these transforms the low-pressure alpha-phase of petalite (P2/c) to an intermediate beta'-phase that then fully converts to the high-pressure beta-phase at ca. 2.5 GPa. The alpha -> beta transition is isomorphic and is associated with tripling of the unit-cell volume. Analysis of the HP-XRD data show that although the fundamental features of the petalite structure are retained through this transition, there are subtle alterations in the internal structure of the silicate double-layers in the beta-phase relative to the alpha-phase. Measurement of the unit-cell parameters of petalite as a function of pressure, and fitting of the data with third-order Birch-Murnaghan equation of state, has provided revised elastic constants for petalite. The bulk moduli of the alpha- and beta-phases are 49(1) and 35(3) GPa, respectively. These values indicate that the compressibility of the alpha-phase of petalite lies between those of the alkali feldpsars and alkali feldspathoids, whereas the beta-phase has a compressibility more comparable with layered silicates. Structure analysis has shown that the compression of the alpha-phase is facilitated by the rigid body movement of the Si2O7 units from which the silicate double-layers are constructed.