Leucite at high pressure: Elastic behavior, phase stability, and petrological implications

Abstract

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 (I4 1 /a) to triclinic symmetry (analysis of diffraction intensities suggests the space group P1), accompanied by a drastic increase in 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 K 0 = 41.9(6) GPa and K′ = 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives K 0 = 33.2(5) GPa. The eulerian finite strain (f e ) vs. normalized stress (F e ) curves were calculated for the low-and high-P polymorphs, providing F e (0) = 42 (1) and F e (0) = 33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearized BM-EoS, are K 0 (a) = 34.5(5) and K 0 (c) = 78(1) GPa. For the triclinic polymorph, we obtain K 0 (a) = 35.9(5), K 0 (b) = 34.9(7), and K 0 (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 and 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 four-and six-membered rings of the tetrahedral framework. The T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal → 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.

High-pressure behavior of synthetic mordenite-Na: an in situ single-crystal synchrotron X-ray diffraction study

“…Moreover, since a structure refinement of the primitive high-pressure polymorph cannot be carried out, a univocal space group assignment is not possible. However, we expect that the P-induced phase transition is displacive in character, as already observed for other openframework compounds at high pressure (e.g., [27][28][29][30]). If a centric structure model is assumed for the low-pressure polymorph (with sp.…”

Zeolites at high pressure: A review

“…The relaxation mechanisms lead to a strong distortion of the four-and six-membered rings, along with a rearrangement of the extra-framework species. Gatta et al (2008aGatta et al ( , 2009a investigated the highpressure behaviour of leucite and pollucite, respectively and Ori et al (2008b) described that of wairakite. These experiments showed that P-induced high-symmetry to low-symmetry phase transitions occur in zeolites of ANA-framework type: leucite has a first-order phase transition from tetragonal to triclinic symmetry (I4 1 /a ?…”

Flexibility windows and phase transitions of ordered and disordered ANA framework zeolites

“…These are compositional end members distinguished by the interstitial ions typically found in the structural channels. Analcime is the Na end member (ideally Na 16 ) also has ANA framework topology, but is not a zeolitic aluminosilicate framework and we do not consider it further. The aluminosilicate end members have broadly similar behaviour under pressure, transforming displacively at relatively low pressures (< 1-3 GPa) from a high-symmetry ambient form to a very low-symmetry compressed form.…”