Abstract
The lithium aluminosilicate mineral petalite (LiAlSi4O10) has been studied using high-pressure single-crystal X-ray diffraction (HP-XRD) up to 5 GPa. Petalite undergoes two pressure-induced first-order phase transitions, never reported in the literature, at ca. 1.5 and 2.5 GPa. The first of these transforms the low-pressure α-phase of petalite (P2/c) to an intermediate β′-phase that then fully converts to the high-pressure β-phase at ca. 2.5 GPa. The α → β transition is isomorphic and is associated with a commensurate modulation that triples 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 β-phase relative to the α-phase. Measurement of the unit cell parameters of petalite as a function of pressure, and fitting of the data with 3rd order Birch-Murnaghan equations of state, has provided revised elastic constants for petalite. The bulk moduli of the α and β-phases are 49(1) and 35(3) GPa, respectively. These values indicate that the compressibility of the-α phase of petalite lies between the alkali feldpsars and alkali feldspathoids, whereas the β-phase has a compressibility more comparable with layered silicates. Structure analysis has shown that the compression of the α-phase is facilitated by the rigid body movement of the Si2O7 units from which the silicate double-layers are constructed.
