Physics and Chemistry of Minerals最新文献

We synthesized six samples in the compositional field NaMgAl(SO₄)₃–KMgAl(SO₄)₃ in 20 mol% increments from pure Na to pure K compounds. We investigated them by Powder X-Ray diffraction, ²³Na, and ²⁷Al Nuclear Magnetic Resonance spectroscopy. The results confirm NaMgAl(SO₄)₃ as a unique phase identical to a presumed new mineral found in the fumaroles of Eldfell and Hekla volcanoes in Iceland. It tolerates less than 10 mol% K substitution for Na. There exists a compositional gap to approximately Na_0.65K_0.35MgAl(SO₄)₃ from where a solid solution extends to KMgAl(SO₄)₃. The mineral koryakite [NaKMg₂Al₂(SO₄)₆] is a member of the latter solid solution series. The crystal structures of all (Na,K)MgAl(SO₄)₃ phases are akin to NASICON (NA Super Ionic CONductor). NaMgAl(SO₄)₃ has (Roverline{3}c) symmetry and a disordered distribution of Mg and Al among the octahedral sites with only one unique site for the alkali atom. The members of the solid solution have (Roverline{3}) symmetry with ordered Mg–Al distribution and two unique alkali sites with different preferences for Na and K. In the crystal structure, the coordination of Na and/or K is trigonal antiprismatic, and these share bases with two octahedral Mg (Na) or Al (K) coordinations. These polyhedra are arranged in columns parallel to [001] and interconnected by SO₄ tetrahedral groups. The alkali atoms from a column lie in the same (001) layers as the octahedrally coordinated atoms from the three neighboring rows. On the same level, parallel to (001), there are gaps in the other three neighboring columns forming channels containing Na⁺ or K⁺ ions.

The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order.

Iron-rich rocks of Orosirian Period in the Chilpi Group on the northern margin of the Bastar Craton, Central India, contain an association of hematite-magnetite-greenalite-chamosite-quartz in oxide-silicate facies. Additionally chert (quartz) and siderite occur in chert and carbonate facies. Presence of these mineral assemblages was investigated to infer the redox state of the depositional basin. The results have indicated formation temperature variation of 116–255 °C (average 198 °C) and log P_(O2) between  – 37 and  – 60 (average –44). A ferruginous state of the shallow water depositional environment, having oxygen content of 10^–2 to 10^–5 times the present atmospheric level, is inferred. The variations in composition of greenalite-chamosite association indicate development of the mineral phases from the reaction involving kaolinite-illite and magnetite-siderite as end-members. Thermodynamic requirements for the formation of the rare association of magnetite-greenalite-cronstedtite indicate the precipitation of the mineral phases from seawater with enhanced activities of Fe²⁺, Al, Si, Mg and C compared to the level in the present day seawater. The results indicate a steep fall in the atmospheric oxygen content immediately after the Great Oxidation Event of 2400–2000 Ma.

Spurrite from Negra Mine, Queretaro, Mexico is characterized by a complex chemical composition. Its empirical formula derived based on electron microprobe, wet chemical analyses and gas chromatography of annealing products is H_0.18Ca_5.01Na_0.05[(SiO₄)_1.91(SO₄)_0.08)][(CO₃)_0.71(BO₃)_0.28]O₁₁. The mineral was studied by single-crystal X-ray diffraction (SCXRD) as well as infrared (IR), Raman and nuclear magnetic resonance (NMR) spectroscopy. According to spectroscopic data, boron has three-fold coordination and sulfur occurs in the mineral in the sulfate form. A significant portion of carbonate groups is substituted by BO₃^3– anions. Charge compensation is achieved due to the substitution of a part of SiO₄^4– anions by SO₄^2– groups, as well as to the admixture of sodium. SCXRD shows that sodium occurs in its own site with a low occupancy. The studied sample is isotypic with the synthetic NaCa₅(SiO₄)₂(BO₃) compound. The IR spectrum shows possible partial protonation of the SiO₄ tetrahedra whereas bands of H₂O molecules and isolated OH^– anions are not observed. Thermal behavior of B,S,Na-bearing spurrite from Negra Mine has been studied using powder high-temperature X-ray diffraction (HTXRD) together with boron poor and S-free spurrite from Fuka Area (Japan). The studied samples are stable up to ~ 1200 °C and ~ 1100 °C, respectively, whereas synthetic B,S-free spurrite decomposes at about 900 °C. The thermal expansion is significantly anisotropic and is observed mainly in the direction perpendicular to the ac plane which is coplanar with the layers of calcium polyhedra and anionic pseudo-layers formed by (C,B)O₃ triangles and (Si,S)O₄ tetrahedra. Isomorphism and a similarity of the thermal, baric and compositional (C-B substitution) deformations of spurrite-like structures are discussed.

Complex mixtures of sulfates, silicates, and ice have been observed in a variety of planetary environments on Earth, Mars and the icy satellites of the solar system. Characterizing the properties of the corresponding compositional endmembers is important for understanding the interiors of a range of planetary bodies in which these phases are observed. To measure the electronic and vibrational properties of the pure ferrous iron endmember of the kieserite group, szomolnokite, (FeSO₄⋅H₂O), we have performed synchrotron ⁵⁷Fe nuclear resonant inelastic and forward scattering experiments in the diamond-anvil cell up to 14.5 GPa. This pressure range covers depths within Earth’s interior relevant to sulfur cycling in subduction zones and the range of pressures expected within icy satellite interiors. We find evidence of crystal lattice softening, changes in elastic properties, and changes in the electric field gradients of iron atoms associated with two structural transitions occurring within the experimental pressure range. We apply these findings to icy satellite interiors, including discussion of elastic properties, modeling of ice-sulfate aggregates, and implications for tidal observations.

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.

Crystal structures of Ca₂SiO₄, Na₂SO₄–K₂SO₄ sulfates, and related minerals bubnovaite K₂Na₈Ca(SO₄)₆ and dobrovolskyite Na₄Ca(SO₄)₃ were described as consisting of microblocks for the first time. A microblock [M(TO₄)₆] that consisted of an octahedron interlinked by six vertices with six adjacent tetrahedra was considered a structural unit inherited upon cooling from a high-temperature disordered parent unit. The relationship between the parent and inherited microblocks was established. Based on this relationship, 15 possible types of microblocks maintaining a trigonal symmetry were derived. The minerals and compounds structurally related to α-Na₂SO₄-derived superstructures were formed as a result of the cooling of the high-temperature phases containing the disordered parent microblock. Here, the inheritance driving force was the tendency of the structure to become ordered upon cooling. The reasons for the formation of a microblock from the parent microblock were mainly determined by the ionic radius and type of cation occupying the octahedral site. The identification of minerals with the described structural features could be a promising tool for the synthesis of novel compounds with useful properties.

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.