Crystallographic exploration at non-ambient conditions: Discovery of a novel high-P and high-T polymorph of leucite

Leucite, ideally KAlSi2O6, is a common feldspathoid/zeolite, usually a primary component of alkaline K-rich mafic and ultramafic volcanic rocks. Understanding its physical-chemical properties as a function of temperature and pressure, including its phase stability fields and relationships with other minerals, is therefore of the outmost importance for modelling these magmatic systems at geological deep conditions. The physical-chemical properties of a mineral are a direct consequence of its crystal structure and how it re-arranges at varying conditions. Leucite, in particular, shows a quite complex polymorphic behavior with temperature. It crystallizes at high temperature in a cubic form (Ia3d), but under cooling, in the T range ca. 620-670 °C, it undergoes two phase transitions to the tetragonal polymorphs I41/acd and then I41/a [1], the latter being the stable form of leucite down to ambient conditions. These phase transitions also imply a complex merohedric and pseudo-merohedric twinning, whereas it has been observed that several factors, including chemical composition and thermal history, may significantly modify this transitions pathway [1-3]. At high pressure (and ambient temperature), the I41/a polymorph was observed to undergo a first-order phase transition to a metrically triclinic polymorph (potentially P-1, structure not determined) with a significant increase in density at about 2.4 GPa [4]. In this study, we have re-investigated the high-pressure and, for the first time, the combined high-pressure and high-temperature behavior of leucite by in situ single-crystal synchrotron X-ray diffraction (at the P02.2 and ID15B beamlines of Petra-III and ESRF synchrotrons, respectively). The results have shown an unexpected phase transition path, from the tetragonal I41/a polymorph toward a trigonal form with R-3 space group at pressures above 2.2-2.5 GPa, both at ambient and high temperature. This suggests that several factors, such as the kinetics of compression, may also control polymorphic transitions at high pressure. The trigonal polymorph of leucite, never reported by previous studies, is characterized by two independent K sites with different coordination environments and four independent tetrahedral sites. These results further demonstrate the complex polymorphic behavior of leucite to temperature and pressure, and the importance of structural determination in order to model and understand its phase stability in deep geological environments. [1] D.M. Hatch, S. Ghose, H.T. Stokes Phys. Chem. Minerals 1990, 17, 220. [2] D.C. Palmer, M.T. Dove, R.M. Ibberson, B.M. Powell Am. Mineral. 1997, 82, 16. [3] R.A. Lange, I.S.E. Carmichael, J.F. Stebbins Am. Mineral. 1986, 71, 937. [4] G.D. Gatta, N. Rotiroti, T. Boffa Ballaran, A. Pavese Am. Mineral. 2008, 93, 1588.