Accessible Requires Authentication Published by De Gruyter April 3, 2015

Investigating Petrologic Indicators of Magmatic Processes in Volcanic Rocks. Petalite under pressure: Elastic behavior and phase stability

Nancy L. Ross, Jing Zhao, Carla Slebodnick, Elinor C. Spencer and Bryan C. Chakoumakos
From the journal American Mineralogist

Abstract

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 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 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 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 third-order Birch-Murnaghan equation 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 those of 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.

Received: 2014-6-30
Accepted: 2014-10-8
Published Online: 2015-4-3
Published in Print: 2015-4-1

© 2015 by Walter de Gruyter Berlin/Boston