Determination of phase relations of the olivine–ahrensite transition in the Mg2SiO4–Fe2SiO4 system at 1740 K using modern multi-anvil techniques

The phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)2SiO4 system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures.

Thermochemistry of MgCr2O4, MgAl2O4, MgFe2O4 spinels in SO2−O2−SO3 atmosphere

The present paper investigates high-temperature sulphate corrosion of basic refractory ceramics containing magnesium spinels (MgAl2O4, MgFe2O4, MgCr2O4 and their solid solutions) widely used in metallurgy, chemical, ceramic and glass industry. This group of refractories are exposed to a number of destructive factors during a working campaign. One of such factors is gas corrosion caused by sulphur oxides. However, gas sulphate corrosion of basic refractory materials containing magnesium spinels, which has a great practical meaning for the corrosion resistance of the material main components, is not sufficiently examined. This work presents a thermodynamic analysis of (MgCr2O4, MgAl2O4, MgFe2O4)?SO2?O2?SO3 system aimed to calculate: i) the standard free enthalpy of chemical reactions, ii) the equilibrium composition of the gas mixture initially containing SO2 and O2 and iii) sulphates equilibrium dissociation pressure and equilibrium partial pressure for the reaction of SO3 with the spinels to predict the temperature range of corrosion products? stability. A thermochemical calculation provides information about equilibrium state in the analysed system. In real conditions the state of equilibrium does not have to be achieved. For this reason, the results of calculations were compared with experimental data. The experiment results were consistent with the theoretical predictions.

Organic-inorganic interactions in the system of pyrrole-hematite-water at elevated temperatures and pressures

The distribution and abundance of pyrrolic compounds in sediments and crude oils are most likely influenced by inorganic sedimentary components. In this paper, thermal simulation experiments on the system pyrrole-hematite-water were carried out at elevated temperatures and pressures in order to investigate the effect of organic-inorganic interactions on the preservation of pyrrolic compounds. Compositions of the reaction products were analyzed with GC-MS and GC-FID methods. In the closed system pyrrole-hematite-water, the nitrogen-oxygen exchange obviously occurred at temperatures above 350ºC in accordance with the thermochemical calculation. Large amounts of furan and ammonia were generated after simulation experiments, indicating that the conversion of pyrrole into furan was the dominant reaction. Thermochemical exchange effect between organic nitrogen and inorganic oxygen was obviously facilitated by elevated temperatures and found to be catalyzed by hematite, but inhibited by the increasing volume of water. Thermodynamically water spontaneously reacts with pyrrole above 300ºC. The reaction of pyrrole-hematite-water is an exothermic process in which the reaction heat positively correlates with temperature. The heat released was estimated as 9.0 KJ/(mol) pyrrole - 15.0 KJ/(mol) pyrrole in typical oil reservoirs (100ºC–150ºC) and 15.0–23.0 KJ/(mol) pyrrole in typical gas reservoirs (150ºC–200ºC). The calculated activation energy of the nitrogen-oxygen atom exchange is about 129.59 kJ/mol. According to the experimental results, a small amount of water may effectively initiate the nitrogen-oxygen exchange. The study would improve our evaluating of the preservation and fate of pyrrolic compounds in deeply buried geologic settings and further understanding of thermochemical processes behind the degradation of petroleum.

AM1 studies on the stabilities of anionic σ-complex regioisomers: thermodynamics of regioselectivity in the reaction of methide, methoxide, and hydroxide anions with electron-deficient aromatics

Heats of formation (ΔHf) for a series of aromatics that are progressively more electron deficient (benzene, 6; nitrobenzene, 7; 4-fluoronitrobenzene, 8; 1,3-dinitrobenzene, 9; 2,4,6-trinitroanisole, 2; and 1,3,5-trinitrobenzene, 1) were determined by semiempirical AM1 calculations. As a probe of the factors that govern the regioselectivity exhibited in the formation of anionic σ-adducts (Meisenheimer complexes), experimental gas-phase ΔHf values for the prototypical oxygen and carbon nucleophiles (hydroxide, methoxide, and methide anions) were used in a thermochemical calculation along with the calculated ΔHf of the electrophiles and the adducts to determine the heats of complexation (ΔHc). The present results show that for the series of nitroaryl electrophiles, 7, 9, and 1, hydroxide and methide anions exhibit the same regioselectivity based on thermodynamics of Meisenheimer complex formation. Specifically, Meisenheimer complexes derived from attack at a position para to at least one nitro group (designated MC-4) are formed with the greatest exothermicity (ΔHc). Exothermicity of complexation increases for both hydroxide and methide adduct formation as the number of nitro groups in the electrophile is increased, from 7 to 9 and to 1, but formation of the methide adducts occurs uniformly with greater exothermicity than that of hydroxide adducts. These results are considered in light of solution calorimetric data that quantify adduct stability in condensed phases. Surprisingly, it is found that regioselectivity inverts for CH3−as compared to OH−and CH3O−in complexation with 2,4,6-trinitroanisole, 2. Thus, while methoxide and hydroxide form adducts at C-1 of TNA with higher exothermicity than at C-3, methide preferentially forms an adduct at C-3 according to the same enthalpy criterion. These results arise from the degree of stereoelectronic stabilization that may be imparted to the respective Meisenheimer complexes formed from ipso attack, that is, the adducts (MC-1) that are geminally disubstituted with electronegative heteroatom groups. For the methoxide MC-1 of TNA, 2, full stereoelectronic stabilization is provided by n–σ* donation from nonbonding electron pairs of the acetal-like methoxyl moieties to suitable C—O acceptor bonds. However, the methide moiety of the comparable MC-1 of TNA cannot partake in such an interaction and, so, with methide, MC-3 formation is preferred over MC-1. Further evidence is provided by consideration of the two energy minima obtained from optimization of the geometry of the oxygen-centred adducts formed by attack of methoxide at C-1 of TNA, 2. In the presence of a point charge that simulates an ion-paired cation, an "M-shaped" conformer is favoured for MC-1, while in the absence of a counterion the "S-shaped" conformer is favoured. Without a complexing counterion M and S conformers are both local minima, while the "S" conformer constitutes the global minimum. The AM1 optimized structure for the "M" conformer compares favourably to published X-ray data. The greater exothermicity of formation of the "S" conformer in the absence of the counterion is indicative of stereoelectronic stabilization of the O-adduct. The geometry is rationalized as a result of minimizing steric repulsion and maximizing the n-σ* stabilization of the C-1 adduct.