We report the crystal structure of dolomite-IV, a high-pressure polymorph of Fe-dolomite stabilized Pagina 1 6161merlini.txt at 115 GPa and 2500 K. It is orthorhombic, space group Pnma, a = 10.091(3), b = 8.090(7), c = 4.533(3) Å, V = 370.1(4) Å3 at 115.2 GPa and ambient temperature. The structure is based on the presence of threefold C3O9 carbonate rings, with carbon in tetrahedral coordination. The starting Fe-dolomite single crystal during compression up to 115 GPa transforms into dolomite-II (at 17 GPa) and dolomite-IIIb (at 36 GPa). The dolomite-IIIb, observed in this study, is rhombohedral, space group R3, a = 11.956(3), c = 13.626(5) Å, V = 1686.9(5) Å3 at 39.4 GPa. It is different from a previously determined dolomite-III structure, but topologically similar. The density increase from dolomite-IIIb and dolomite IV is ca. 3%. The structure of dolomite-IV has not been predicted, but it presents similarities with the structural models proposed for the high-pressure polymorphs of magnesite, MgCO3. A ring-carbonate structure match with spectroscopic analysis of high pressure forms of magnesite-siderite reported in the literature, and, therefore, is a likely candidate structure for a carbonate at the bottom of the Earth's mantle, at least for magnesitic and dolomitic compositions.

at temperatures up to 550 K. High-pressure raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO 3 -IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius-Clapeyron slopes of the phase boundary lines were determined as: ΔP/ΔT = −2.79 ± 0. 28 × 10 −3 gPa K −1 (I-II); +1.87 ± 0.31 × 10 −3 gPa K −1 (II/III); +4.01 ± 0.5 × 10 −3 gPa K −1 (II/IIIb); −33.9 ± 0.4 × 10 −3 gPa K −1 (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) gPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P2 1 /c unit cell of phase II and to [101] in the pseudomonoclinic C1 setting of phase IIIb. Crystal structure investigations of triclinic CaCO 3 -III at non-ambient pressure-temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T. PVT equation of state parameters was determined from experimental data points in the range of 2.20-6.50 gPa at 298-405 K providing K T 0 = 87.5(5.1) gPa, (δK T /δT) P = −0.21(0.23) gPa K −1 , α 0 = 0.8(21.4) × 10 −5 K −1 , and α 1 = 1.0(3.7) × 10 −7 K −1 using a second-order Birch-Murnaghan equation of state formalism.Abstract High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO 3 were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. all experiments at non-ambient conditions were carried out by means of raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 gPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations T. Yagi is on sabbatical leave at

The elastic behaviour and the structural evolution of microporous materials compressed hydrostatically in a pressure-transmitting fluid are drastically affected by the potential crystal-fluid interaction, with a penetration of new molecules through the zeolitic cavities in response to applied pressure. In this manuscript, the principal mechanisms that govern the P-behaviour of zeolites with and without crystal-fluid interaction are described, on the basis of previous experimental findings and computational modelling studies.When no crystal-fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of (quasi-rigid) tetrahedra around O atoms that behave as hinges. Tilting of tetrahedra is the An overview of the intrusion phenomena of monoatomic species (e.g., He, Ar, Kr), small (e.g., H 2 O, CO ) and complex molecules, along with the P-induced polymerization phenomena, (e.g., C 2 H 2 , C 2 H 4 , C 2 H 6 O, C 2 H 6 O 2 , BNH 6 , electrolytic MgCl 2 ·21H 2 O solution) is provided, with a discussion of potential technological and geological implications of these experimental findings.

Phase stability, elastic behavior, and pressure-induced structural evolution of synthetic boron-mullite Al 5 BO 9 (a = 5.6780 (7), b = 15.035(6), and c =7.698(3) A, space group Cmc2 1 , Z = 4) were investigated up to 25.6(1) GPa by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell (DAC) under hydrostatic conditions. No evidence of phase transition was observed up to 21.7(1) GPa. At 25.6(1) GPa, the refined unit-cell parameters deviated significantly from the compressional trend, and the diffraction peaks appeared broader than at lower pressure. At 26.7(1) GPa, the diffraction pattern was not indexable, suggesting amorphization of the material or a phase transition to a high-pressure polymorph. Fitting the P-V data up to 21.7(1) GPa with a second-order Birch-Murnaghan Equation-of-State, we obtained a bulk modulus K T0 = 164(1) GPa. The axial compressibilities, here described as linearized bulk moduli, are as follows: K T0(a) = 244(9), K T0(b) = 120(4), and K T0(c) = 166(11) GPa (K T0(a) :K T0(b) : K T0(c) = 2.03:1:1.38). The structure refinements allowed a description of the main deformation mechanisms in response to the applied pressure. The stiffer crystallographic direction appears to be controlled by the infinite chains of edge-sharing octahedra running along [100], making the structure less compressible along the a-axis than along the band c-axis.

Colemanite (ideally CaB3O4(OH)3·H2O, space group P21/a, unit‐cell parameters: a ~ 8.74, b ~ 11.26, c ~ 6.10 Å, β ~ 110.1°) is one of the principal mineralogical components of borate deposits and the most important mineral commodity of boron. Its high‐pressure behavior is here described, for the first time, by means of in situ single‐crystal synchrotron X‐ray diffraction with a diamond anvil cell up to 24 GPa (and 293 K). Colemanite is stable, in its ambient‐conditions polymorph, up to 13.95 GPa. Between 13.95 and 14.91 GPa, an iso‐symmetric first‐order single‐crystal to single‐crystal phase transition (reconstructive in character) toward a denser polymorph (colemanite‐II) occurs, with: aCOL‐II=3·aCOL, bCOL‐II=bCOL, and cCOL‐II=2·cCOL. Up to 13.95 GPa, the bulk compression of colemanite is accommodated by the Ca‐polyhedron compression and the tilting of the rigid three‐membered rings of boron polyhedra. The phase transition leads to an increase in the average coordination number of both the B and Ca sites. A detailed description of the crystal structure of the high‐P polymorph, compared to the ambient‐conditions colemanite, is given. The elastic behaviors of colemanite and of its high‐P polymorph are described by means of III‐ and II‐order Birch‐Murnaghan equations of state, respectively, yielding the following refined parameters: KV0=67(4) GPa and KV′=5.5(7) [βV0=0.0149(9) GPa−1] for colemanite; KV0=50(8) GPa [βV0=0.020(3) GPa−1] for its high‐P polymorph.