Leucite at high-pressure : elastic behaviour, phase stability and petrological implications

Elastic and structural behavior of a natural tetragonal leucite from the volcanic Lazium district (Italy) were investigated at high pressure by in-situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. A first-order phase transition, never reported in the literature, was observed at P=2.40.2 GPa from tetragonal (I41/a) to triclinic symmetry (diffraction intensities analysis suggests the space group P ), accompanied by a drastic increase of density of about 4.7%. The transition pressure was bracketed by several measurements in compression and decompression. No further phase-transition has been observed up to 7 GPa. Fitting a second-order Birch-Murnaghan Equation-of-State (BM-EoS) to the pressure-volume data of the tetragonal polymorph, we obtain: K0=41.9(6) GPa and K’= 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives: K0= 33.2(5) GPa. The eulerian finite strain (fe) vs normalised stress (Fe) curves were calculated for the low- and high-P polymorphs, providing Fe(0)=42(1) and Fe(0)=33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearised BM-EoS, are: K0(a)=34.5(5) GPa and K0(c) = 78(1) GPa. For the triclinic polymorph, we obtain: K0(a)= 35.9(5), K0(b)= 34.9(7) and K0(c) = 35.5(7) GPa. The elastic behavior of the low-P polymorph appears to be more anisotropic than that of the high-P polymorph. The HP-crystal structure evolution of the tetragonal polymorph of leucite was studied on the basis of six structural refinements at different pressures between 0.0001-1.8 GPa. The main deformation mechanisms at high-pressure are due to tetrahedral tilting, giving rise to an increase of the ellipticity of the 4- and 6-membered rings of the tetrahedral framework. The tetrahedral T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal-to-triclinic phase-transition, and the low quality of the diffraction data at pressure above the phase-transition, did not allow the refinement of the crystal structure of the triclinic polymorph.