P21/c Postorthopyroxene γ-LiScGe2O6, a New Dense High-Pressure Polymorph and Its Direct Transformation from the Pbca Structure

Orthorhombic β-LiScGe₂O₆ single crystals were compressed hydrostatically up to 10.35 GPa using a diamond anvil cell and investigated in situ by means of X-ray diffraction and Raman spectroscopy. Crystal-structure investigations at ambient conditions and at high pressure show a structural transition from an orthopyroxene-type Pbca structure (a ≈ 18.43 Å, b ≈ 8.85 Å, and c ≈ 5.34 Å at 8.6 ± 0.1 GPa) to a postorthopyroxene type P2₁/c structure of the new dense γ-LiScGe₂O₆ (a ≈ 18.62 Å, b ≈ 8.85 Å, c ≈ 5.20 Å, and β ≈ 93.1° at 9.5 ± 0.1 GPa). The structure refinements reveal displacive shifts of O atoms associated with a rotation of every other tetrahedral-chain unit from the O- to S-type position similar to the postorthopyroxene-type MgSiO₃. As a consequence of the oxygen displacement, the coordination number of Li atoms is changing from [5 + 1] to a proper 6-fold coordination. The transition around P_c = 9.0 ± 0.1 GPa is associated with a volume discontinuity of ΔV = −1.6%. This orthopyroxene (OEn-Pbca) to postorthopyroxene (pOEn-P2₁/c) transition is the second example of this type of transformation. Precise lattice parameters have been determined during isothermal compression. The fit of the unit-cell volumes of β-LiScGe₂O₆, using a third-order Birch–Murnaghan equation of state, yields V₀ = 943.63 ± 0.11 ų, K₀ = 89.8 ± 0.6 GPa, and dK/dP = 4.75 ± 0.18 as parameters. Evaluation of the data points beyond the critical transition pressure using a second-order Birch–Murnaghan equation suggests V₀ = 940.6 ± 4.4 ų and K₀ = 82.4 ± 4.8 GPa. A series of high-pressure Raman spectra confirm the symmetry-related structural transition, with band positions shifting in a noncontinuous manner, thus confirming the proposed first-order transition.

Synthetic lithium−scandium germanate crystals (space group Pbca at ambient conditions) were the subject of in situ high-pressure solid-state investigations (single-crystal X-ray diffraction, lattice parameter, and Raman spectra) in a diamond anvil cell up to 10 GPa. One phase transition, apparently of first order, is identified at 9.0 ± 0.1 GPa. The Birch−Murnaghan equations of state were fitted to the P−V data, and the obtained parameters are given. The transition is of the type orthoenstatite (Pbca) to postorthoenstatite (P2₁/c).

Single-crystal diffraction at the high-pressure Indo-Italian beamline Xpress at Elettra, Trieste

In this study the first in situ high-pressure single-crystal X-ray diffraction experiments at Xpress, the Indo-Italian beamline of the Elettra synchrotron, Trieste (Italy), are reported. A description of the beamline experimental setup and of the procedures for single-crystal centring, data collection and processing, using diamond anvil cells, are provided. High-pressure experiments on a synthetic crystal of clinoenstatite (MgSiO₃), CaCO₃ polymorphs and a natural sample of leucophoenicite [Mn₇Si₃O₁₂(OH)₂] validated the suitability of the beamline experimental setup to: (i) locate and characterize pressure-induced phase transitions; (ii) solve ab initio the crystal structure of high-pressure polymorphs; (iii) perform fine structural analyses at the atomic scale as a function of pressure; (iv) disclose complex symmetry and structural features undetected using conventional X-ray sources.

Single-crystal metastable high-temperature C2/c clinoenstatite quenched rapidly from high temperature and high pressure

The high-temperature clinoenstatite (HT-CEn) is one of the most important MgSiO3 pyroxene polymorphs, but the details of its structure and stability have been uncertain. The single crystal of the C2/c HT-CEn end-member is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 1173-2173 K. The single-crystal X-ray diffraction analysis shows unusual bonding distances and static disorder of the atoms frozen in this metastable structure. The degree of kinking of the silicate tetrahedral chains is 175° for HT-CEn. The chain angle for HP-CEn is substantially smaller (135°), but the angle for L-CEn is in the opposite direction at -160° (= 200°). The degree of kinking increases by being curved by more than 180° for the transition from HT-CEn to L-CEn. As for the reverse change from the expansion to the stretch, a potential barrier exists at the point of the continuity. It is suggested that the reason why a structure can be quenched under ambient conditions is as follows: the present HT-CEn single crystal has been formed by the isosymmetric phase transition from the high-pressure C2/c clinoenstatite (HP-CEn). The presence of HT-CEn from HP-CEn in natural rocks is an indicator of quenching history, which leads to the possibility that it exists in shocked meteorites and impact materials.

Structural changes upon the temperature dependent C2/c → P21/c phase transition in LiMe3+Si2O6 clinopyroxenes, Me = Cr, Ga, Fe, V, Sc and In

Within the Li-clinopyroxene series LiMe3+Si2O6, a temperature induced C2/c → P21/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al3+ and the pure In3+ compound down to 80 K. Within the LiSc1– x In x Si2O6 solid solution se ries, the transition temperature rapidly decreases with increasing In3+ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi2O6, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q2 (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc3+ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi2O6, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P21/c, decrease of the coordination number of Li+ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi2O6 is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P21/c phase transition.

Clinoenstatite in alpe arami peridotite: additional evidence of very high pressure

Observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the Alpe Arami garnet lherzolite of the Swiss Alps. The simplest interpretation of the orientation, crystallography, and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high-pressure C-centered form of clinoenstatite. These results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision.

Catalysis of the Olivine to Spinel Transformation by High Clinoenstatite

Although enstatite is a major constituent of the Earth's upper mantle and subducting lithosphere, most kinetic studies of olivine phase transformations have typically involved single-phase polycrystalline aggregates. Transmission electron microscopy investigations of olivine to spinel and modified spinel (beta phase) reactions in the (Mg, Fe)(2)SiO(4)-(Mg,Fe)SiO(3) system show that transformation of olivine in the stability field of spinel plus phase begins with coherent nucleation of spinel on high-clinoenstatite grains. These observations demonstrate that high clinoenstatite can catalyze the transformation by enhancing nucleation kinetics and therefore imply that secondary phases can influence reaction kinetics during high-pressure mineral transformations.