Atomistic computer modeling of the local structure and mixing properties of a grossular-uvarovite solid solution

An original methodology for the atomistic computer modeling of solid solutions was applied for the study of the mixing properties and local structure of the grossular-uvarovite, i.e., Ca₃Al₂[SiO₄]₃ Ca₃Cr₂][SiO₄]₃, garnet series. The parameters of the interatomic potentials for end members of this series were optimized using experimental data on their structural, elastic, and thermodynamic characteristics. The optimized model of the potentials allowed us to describe the elastic, structural, and thermodynamic characteristics of grossular and uvarovite and estimate the energy of point defects in these crystal structures. Calculations of the mixing properties and local structure for seven different compositions of the solid solution were carried out on a “Chebyshev” supercomputer (Moscow State University) in a 2 × 2 × 4 supercell of the garnet-type structure containing 2560 atoms. Mixing properties, such as the enthalpy of mixing, parameters of interaction, excess mixing volume, deviation of bulk modulus from additivity, and the vibrational and configuration contribution to the entropy of mixing, were determined. This allowed us to estimate the stability field for the grossular-uvarovite solid solution. Histograms of the interatomic distances M-O (M = Ca, Al, Cr, Si) and O-O in supercells were plotted and the parameters of relaxation and changes of the CrO₆ and AlO₆ octahedron volumes were estimated. The data of the simulation are quite consistent with the experimental data on this system and supplement it significantly.     

Volcanic-associated Besshi-type copper sulfide deposits

Besshi-type volcanic-associated massive sulfide deposits (VMSD) are associated with undifferentiated basaltic formations. They form within the mid-ocean ridges near the continental margins, in back-arc spreading zones, and rarely in intracontinental rift basins. They are characterized by a wide spread of turbidites in ore-bearing strata, Co-rich copper-zinc ores, the predominance of subvolcanic sills, sheet-like ore bodies, an absence of clear structural control, relatively low Cu, Zn, Ag, and Au grades, enrichment in Pb, and relatively large ore and metal reserves.     

Volcanogenic massive sulfide deposits of ophiolite associations

Volcanogenic massive sulfide deposits in ophiolite complexes are usually attributed to the Cyprus type. They associate with basaltic volcanics that are formed in mid-ocean or back-arc spreading centers and much less frequently in intra-plate settings. The deposits are characterized by copper or copper-zinc ores that are enriched in Ni, Co, and in places Mn and As, but are very poor in Pb and demonstrate a low to moderate content of Ag and Au. Typically, the deposits are low to very low in ore and metal reserves. Cyprus-type deposits were irregularly distributed during geological history. The most ancient of them were formed in the Neoproterozoic, while the bulk of the deposits are Ordovician or Cretaceous in age. Their possible Paleoproterozoic analogues can be found in the Svecofennian belt (Outokumpu ore district), while modern ones are confined to the Explorer and Endeavour Ridges and southern segment of the Juan de Fuca Ridge.     

Main Stages of Evolution of Pacific Ferromanganese Stromatolites and Oncolites

As exemplified by cobalt–bearing Fe–Mn crusts (ferromanganese stromatolite) of the Magellan Seamounts (Clarion–Clipperton Province), the main stages of the structural evolution of bacterial communities have been recognized. It has been demonstrated that the change of the stages is caused by the effect of major tectonic, volcanic, and other geological events.     

Possibilities of electric field measurements in the audio frequency range using ungrounded electric sensors

Possibilities of electric field measurements in the audio frequency range using underground (capacitive) electric sensors are considered. Features of parameters of electric sensors and input circuits of the measurement equipment, robust methods, and software tools for processing obtained data are analyzed. Examples of experimental studies by the audiomagnetotelluric sounding (AMTS) method, demonstrating possibilities of implementing ungrounded electric field measurements in a frequency range of 7?C300 Hz, are presented.     

Different-scale tendencies of the evolution of the composition of volcanogenic pyrite deposit ores

The changing contents and ratios of certain primary and secondary elements in the composition of volcanogenic pyrite deposit ores reveal two tendencies that have different scales of their manifestation in geological time and space. The first one is expressed in terms of the evolution of geochemical differences of deposit ores during relatively short periods of pyrite formation in some ore provinces. It is determined by the changing stages of tectonic development and corresponding alterations of magma composition within the provinces. The other tendency is global; it is expressed in terms of century-old changes of ore geochemical characteristics in the deposits that have developed in different pyrite formation epochs over the last 3.5 billion years, probably as a result of the differentiation of Earth material, a gradual temperature drop in the upper mantle, and the associated development of tectonic conditions and magma-formation mechanisms.     

Ca- and Mg-perovskite phases in the Earth’s mantle as a probable reservoir of Al: Computer-simulation evidence

Semi-empirical and quantum chemical studies of Al atom energy in CaSiO₃ and MgSiO₃ with the perovskite-type structure at pressures and temperatures of the Earth’s mantle are reported. The phase diagram for CaSiO₃ is reproduced and refined. Probable mechanisms of Al incorporation in the structures studied are considered. According to the results of the calculations, Al is preferably incorporated into MgSiO₃, rather than into CaSiO₃. Evaluation of the isomorphic capacity of perovskite phases in relation to Al shows that the Al content in MgSiO₃ may reach 2.4 mol % at 120 GPa and 2400 K. CaSiO₃ cannot be a source of Al atoms in the Earth’s mantle.