The interplay between crystal–melt and grain boundary interfaces in partially melted polycrystalline aggregates controls many physical properties of mantle rocks. To understand this process at the fundamental level requires improved knowledge about the interfacial structures and energetics. Here, we report the results of first-principles molecular dynamics simulations of two grain boundaries of (0l1)/[100] type for tilt angles of 30.4° and 49.6° and the corresponding solid–liquid interfaces in Mg2SiO4 forsterite at the conditions of the upper mantle. Our analysis of the simulated position time series shows that structural distortions at the solid–liquid interfacial region are stronger than intergranular interfacial distortions. The calculated formation enthalpy of the solid–solid interfaces increases nearly linearly from 1.0 to 1.4 J/m2 for the 30.4° tilt and from 0.8 to 1.0 J/m2 for the 49.6° tilt with pressure from 0 to 16 GPa at 1500 K, being consistent with the experimental data. The solid–liquid interfacial enthalpy takes comparable values in the range 0.9 to 1.5 J/m2 over similar pressure interval. The dihedral angle of the forsterite–melt system estimated using these interfacial enthalpies takes values in the range of 67° to 146°, showing a decreasing trend with pressure. The predicted dihedral angle is found to be generally larger than the measured data for silicate systems, probably caused by compositional differences between the simulation and the measurements.

The infrared hydroxyl bands and first hydroxyl combination bands of glaucophane are characterized under pressure. In this weakly hydrogen-bonded mineral, the anharmonicity parameter, as determined from the difference between combinations and the fundamentals, is nearly constant with pressure to 15 GPa, indicating that the ambient pressure value of hydroxyl-bond anharmonicity closely reflects its value at high pressures. Given this near-constancy, the Grüneisen parameters of the hydroxyl stretching vibrations of a wide range of minerals, as derived from the pressure dependence of their O–H stretching frequencies, are correlated with the anharmonic parameter of each vibration, as determined from the ambient pressure offset of the summed frequencies of the fundamental n = 0 to 1 transitions and the frequency of the hydroxyl combination or overtone band corresponding to the n = 0 to 2 transition. This correlation is motivated by (1) the anharmonic origin of the Grüneisen parameter; and (2) the grossly similar form of the interatomic potential governing weak- and medium-strength hydrogen bonding in many minerals. This possible correlation provides a means through which the likely pressure-induced hydroxyl mode shifts of phases might be estimated from ambient pressure near-infrared measurements and emphasizes the importance of near-infrared combination/overtone band measurements. In this context, the combination/overtone bands of high-pressure hydrous phases are almost completely uncharacterized, and thus one probe of their anharmonicity has been neglected. Such information directly constrains the nature of hydrogen bonding in these phases, and hence provides possible insights into both their retention of hydrogen and its mobility. Deviations from the anharmonicity-Grüneisen parameter correlation, when observed (as may be the case in prehnite), could provide insights into anomalous effects on the hydroxyl potential well induced by bifurcated H-bonds, pressure-dependent Davydov splitting, or the influence of neighboring cations.

Natural Fe2+-rich princivalleite was thermally treated in the air at 700 °C to study crystal-chemical and color variations due to changes in oxidation states of Fe and Mn and atom ordering. Overall, the experimental data (electron microprobe, structural refinement, Mössbauer, infrared, and optical absorption spectroscopy) show that thermal treatment of princivalleite results in an almost total Fe2+ oxidation to Fe3+ and an oxidation of approximately one-third of Mn2+ to Mn3+ along with a minor degree of disorder of Al–Fe–Mn over the Y and Z sites. This process is accompanied by a significant deprotonation of the sample. The YFe and YMn oxidation from + 2 to + 3 yields in a decrease in a-parameter, whereas the increased content of ZFe3+ results in a minor increase in the c-parameter. Optical absorption spectroscopy shows that the faint blue (azure) color of untreated princivalleite is caused by the presence of Fe2+ and the absence of Ti4+. Thermal treatment in air (700 °C) changed the color to dark brown due to the progressive oxidation of Fe2+ to Fe3+ and Mn2+ to Mn3+, as demonstrated by the evolution of optical absorption bands caused by electron transitions in these 3d-cations. However, the most evident result of the thermal treatment of the Fe-rich princivalleite sample is the simultaneous presence of Fe2+, Fe3+, Mn2+, and Mn3+, with a Fe3+/ΣFe and Mn3+/ΣMn ratio of 0.92 and 0.25, respectively. This observation suggests that the oxidation process during the heating experiments was largely controlled by kinetic factors.

Muscovite (Ms) and phlogopite (Phl) series mineral is studied in the 2M1 polytype and modeled by the substitution of three Mg2+ cations in the three octahedral sites of Phl [KMg3(Si3Al)O10(OH)2] by two Al3+ and one vacancy, increasing the substitution up to reach the Ms [KAl2□(Si3Al)O10(OH)2]. The series was computationally examined at DFT using Quantum ESPRESSO, as a function of pressure from − 3 to 9 GPa. Crystal structure is calculated, and cell parameters, and geometry of atomic groups agree with experimental values. OH in the Mg2+ octahedrons are approximately perpendicular to the (001) plane, meanwhile when they are in Al3+, octahedral groups are approximately parallel to this plane. From Quantum Theory of Atoms in Molecules, the atomic basins are calculated as a function of the pressure, K+ and basal O show the largest volumes. The bulk excess volume (Vxs) and the excess atomic volumes are analyzed as a function of the composition and the pressure. K+, basal and apical O Vxs show a behavior similar to the bulk Vxs as a function of the composition, keeping qualitatively this behavior as a function of pressure; substituent atoms do not show a Vxs behavior similar to the bulk and their effect consequently is mostly translated to atoms in the interlayer space. Atomic compressibilities are also calculated. Atomic compressibilities are separated in the different sheets of the crystal cell. Atomic moduli of K and basal O are the lowest and the ones behaving as the bulk modulus of the series. The atomic bulk modulus of the H’s is different depending of their position with respect to the (001) plane.

In this work, we investigated the thermodynamic properties of synthetic schafarzikite (FeSb2O_4) and tripuhyite (FeSbO_4). Low-temperature heat capacity (C_p) was determined by relaxation calorimetry. From these data, third-law entropy was calculated as 110.7pm 1.3 J mol^{-1}K^{-1} for tripuhyite and 187.1pm 2.2 J mol^{-1} K^{-1} for schafarzikite. Using previously published Delta _fG^o values for both phases, we calculated their Delta _fH^o as -947.8pm 2.2 for tripuhyite and -1061.2pm 4.4 for schafarzikite. The accuracy of the data sets was tested by entropy estimates and calculation of Delta _fH^o from estimated lattice energies (via Kapustinskii equation). Measurements of C_p above T = 300 K were augmented by extrapolation to T = 700 K with the frequencies of acoustic and optic modes, using the Kieffer C_p model. A set of equilibrium constants (log K) for tripuhyite, schafarzikite, and several related phases was calculated and presented in a format that can be employed in commonly used geochemical codes. Calculations suggest that tripuhyite has a stability field that extends over a wide range of pH-pepsilon conditions at T = 298.15 K. Schafarzikite and hydrothermal oxides of antimony (valentinite, kermesite, and senarmontite) can form by oxidative dissolution and remobilization of pre-existing stibnite ores.