Mixing properties of solid solutions of the post-spinel phases with a composition of CaCr 2 O 4 –CaAl 2 O 4 , CaCr 2 O 4 –CaFe 2 O 4 , MgCr 2 O 4 –MgAl 2 O 4 , and MgCr 2 O 4 –MgFe 2 O 4 in the temperature range of 1873–2223 K and at a pressure of 18–25 GPa were studied by the semi-empirical modeling. These PT parameters were used to estimate a formation energy of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites. The study results: (1) the studied binary solid solutions are characterized by a complete miscibility; (2) the incorporation of Fe 3+ impurity ion into the post-spinel phases of MgCr 2 O 4 and CaCr 2 O 4 is less favorable in terms of energy than that of Al 3+ ions over the entire pressure and temperature range under study. The results obtained were used to interpret compositions of the post-spinel phases forming inclusions in lower mantle diamonds.

High-pressure high-temperature (HPHT) crystalline monosulfide solid solution (Mss) phases (FexNi1−xS, x = 0.90, 0.75, 0.50, 0.25), troilite (FeS I), and α-NiS of the Fe-Ni-S system were synthesized at 7 GPa and 900–1550 °C. The structural parameters of the obtained phases were refined by XRD using the Rietveld method. Factor group analysis revealed the number of active Raman modes for FeS I and α-NiS. Raman spectra of troilite, α-NiS, and Mss phases were obtained. It was shown that the Raman spectra of Mss phases and α-NiS have a similar topology. The Raman spectra of the experimental phases in the Fe-Ni-S system were analyzed with non-negative matrix factorization, which provided a meaningful concentration dependence of the spectral patterns. The spectral components were assigned to the FeS I and α-NiS structures. The structural and spectroscopic studies show linear dependencies of unit cell parameters and spectral components on composition and confirm the existence of a series of monosulfide solid solution FexNi1−xS.

The paper reports the detailed studies of drilling cores from peat deposits of the Vydrino bog with a thickness of 4.4 m and an age of 13 100 cal. years. The peat is composed of fen, transitional, and raised types. The early-diagenetic transformations of peat sediments are considered, and the distribution of elements, the formation of authigenic minerals, and the chemical composition of bog waters are studied. The destruction of organic matter begins from the upper intervals of peat at the early diagenetics stage. Pyrograms do not have clearly defined high-temperature peaks, “rudiments” of the macromolecular structure of kerogen, which indicates a low degree of transformation of peat organic matter. The high abundance of organotrophic, ammonifying, nitrifying, and phosphate-mobilizing microorganisms, and the low abundance of Fe- and Mn-oxidizing microorganisms, and sulfate-reducing bacteria are revealed. The presence of organotrophic microorganisms throughout the section indicates that the biogeochemical processes of the carbon cycle span the entire peat sequence. The low S(II) content indicates the low intensity of sulfate reduction. The fen peat is characterized by the high contents of Si, Al, Fe, Ca, Sr, Ba, Zr, La and anomalous contents of Cu, Zn, which is caused by the peatland formation under conditions of rich mineral nutrition. The ash part of the transitional peat demonstrates a decrease in the contents of Si, Fe, Sr, Br, K Si, Ca, Ba, Cu, Zn and La, which reflects the gradual weakening of the connection of the peat deposit with the underlying rocks. The near-surface horizon of raised peat is characterized by an increase in the contents of K, Mn, Zn, Hg, Pb and As, which is accompanied by an increase in atmospheric dust and anthropogenic impact on the bog ecosystem in the 20th and 21st centuries. The bog waters of the fen peat are characterized by the high contents of the main ions, Al, Fe, Mn, Sr, while the transitional peat shows a decrease in DOC, SO_4^2 - , HCO_3^ - , Al, Fe, Ni, Ca, Mg. The oligotrophic stage peat deposit layer is characterized by the development of Fe oxides and hydroxides, the presence of vivianite is noted for transitional peats, and the eutrophic stage layer includes rhodochrosite and sulfides of Fe, Cu, and Zn.

The mixing properties of solid solutions of post-spinel phases of the composition CaCr2O4–CaAl2O4, CaCr2O4–CaFe2O4, MgCr2O4–MgAl2O4 and MgCr2O4–MgFe2O4 in the temperature range 1873– 2223 K and pressures 18–25 GPa were studied using the method of semi-empirical modeling. With these PT parameters, the energies of formation of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites were estimated. It is shown that (1) the studied binary solid solutions are characterized by complete miscibility, (2) the incorporation of the Fe3+ impurity ion into the post-spinel phases of MgCr2O4 and CaCr2O4 is less favorable in terms of energy than the incorporation of Al3+ ions over the entire range of pressures and temperatures under study. The results obtained were used to interpret the composition of post-spinel phases forming inclusions in lower mantle diamonds.

The elemental composition of attic dust in rural houses in the southern part of Western Siberia serves as a reliable indicator of anthropogenic and technological pollution in certain regions and individual structures. In comparison with the soils of the surrounding areas, attic dust is contaminated with volatile elements (Cd, Pb, Zn, Hg) and other elements of anthropogenic origin. Enrichment factors for many elements in attic dust are an order of magnitude higher than those in soils.

High pressure crystalline Mss phases (Fe0.9Ni0.1S, Fe0.5Ni0.5S, Fe0.25Ni0.75S) and end members troilite (FeS I) and crowningshieldite (α-NiS) of Fe-Ne-S system were synthesized at 7 GPa and 900-1500 °C. The unit cell parameters of studied phases were determined by powder X-ray diffraction analysis; space group of Mss phases is P63/mmc. Factor group analysis revealed the number of active Raman modes for FeS I and α-NiS. Raman spectra of troilite and crowningshieldite and Mss phases were obtained. It was shown that the Raman spectra of Mss phases and α-NiS have a similar topology with a slight shift in the main bands. Raman spectra of our experimental phases in the Fe-Ne-S system were analyzed with non-negative matrix factorization, which provided a meaningful concentration dependence of the spectral patterns. The spectral components were assigned to the FeS I and α-NiS structures.

Modeling of eight mechanisms for the incorporation of Ti 4+ and Cr 3+ impurity components into phlogopite was carried out by a semi-empirical method using the GULP (General Utility Lattice Program) software. The calculation of thermodynamic mixing properties in the range of 1–7 GPa and 373–1573 K and the analysis of the structure geometry for the simulated solid solutions provided the following energy-preferred schemes of isomorphic substitution: VI (Mg 2+ ) + 2 IV (Si 4+ ) = VI (Ti 4+ ) + 2 IV (Al 3+ ) and VI (Mg 2+ ) + 2 IV (Al 3+ ) = VI (□) + 2 IV (Ti 4+ ), VI (Mg 2+ ) + IV (Si 4+ ) = VI (Cr 3+ ) + IV (Al 3+ ), and 3 VI (Mg 2+ ) = VI (Al 3+ ) + VI (Cr 3+ ) + VI (□). It is shown the scheme 2 VI (Mg 2+ ) = VI (Ti 4+ ) + VI (□) illustrating entrance of Ti with the formation of a vacancy is realized in the case of microconcentrations of Ti only. Accumulation of high-Ti contents associates with the formation of a vacancy in the octahedral site. This provides incorporation of Ti via the schemes VI (Mg 2+ ) + 2 IV (Al 3+ ) = VI (□) + 2 IV (Ti 4+ ) and (Mg, Fe 2+ ) + 2OH − = Ti 4+ + 2O 2− only. It is shown that incorporation of high-Cr concentrations (> 5.5 wt % Cr 2 O 3 ) is accompanied by an increase in the number of vacancies in the octahedral site with an increase in the proportion of the dioctahedral component K(Al, Cr, □) 2 AlSi 3 O 10 (OH) 2 .

Experiments aimed at the synthesis of Cr- and Ti-bearing phlogopite in the silicate-carbonate systems peridotite—K2CO3 + H2O and basalt—K2CO3 + H2O at 7 GPa and 900–1200°С were carried out. It is shown that the crystallization of titanium-bearing phlogopite requires subducted crustal material at mantle depths. However, the mantle peridotite should predominate over basalt for Ti-phlogopite crystallization; otherwise, dioctahedral mica (aluminoceladonite) with (Mg + Fe)/VIAl > 1 is formed via the scheme 2VIAl = VITi4+ + VI(Mg + Fe). The competitive behavior of Ti and Cr upon incorporation into phlogopite is considered. It is shown that the presence of >1.3 wt

—A 6-meter core of the Holocene sediments in Lake Dukhovoe is studied. The material composition of the core is heterogeneous and has clear bedding. Lacustrine sediments are represented by the upper 293 cm. Each bed of the sediments is subjected to microbiological analysis. A high total number of heterotrophic bacteria is revealed in the upper (0–15 cm) and deeper (110–150 cm) sapropel intervals. Sulfate-reducing bacteria are identified only in Chrysophyceae cysts. The CaO/Fe2O3 ratio shows the degree of “carbonate content” of bottom sediments. It is low in the lake sapropel (0.2), which indicates the geochemical specificity (carbonate-free sapropel). Na, Mg, Al, Si, Al, Ca, Fe, Mn, Zr, and Cr concentrations in sapropel are within the average concentrations for the Earth’s crust, shale, and oceanic pelagic clays, while U and Mo concentrations exceed them. Phytoplankton is enriched in phosphorus (biogenic element) and chalcophile elements (Zn, Cd, Sn, Sb, Hg, Pb, and Cu), which characterize the pollution of the modern atmosphere of the Baikal region. Diagenetic processes result in the transformation of the pore waters, namely an increase in mineralization as compared to lake water, an increase in the concentrations of biogenic components (HCO3-, NH4+, NO3-, and PO43-) and a decrease in SO42-. Pyrite framboid accumulations are revealed in organomineral sediments below the horizon of 200 cm, and iron phosphates represented by vivianite are identified in sapropel.

Experimental studies of melting relations in the MgSiO3-Na2CO3 (+/- Al2O3, Fe2O3) multicomponent alkaline carbonate-silicate system were carried out at 23-24 GPa and 1100-1700 degrees C to shed light on the conditions and mechanisms of formation of Na-bearing bridgmanite. At 1100-1300 degrees C, an assemblage of low-Na bridgmanite (<0.6 wt% Na2O), ringwoodite, periclase (ferropericlase), alkaline carbonate with composition close to Na2Mg (CO3)(2), and sodic carbonate-silicate melts is formed. With increase in temperature up to 1700 degrees C, carbonate disappears and the content of Na2O increases in all mantle phases (up to 1.6, 4.4, and 2.7 wt% in bridgmanite, ringwoodite, and ferropericlase, respectively), which indicates that Na-bearing bridgmanite may crystallize at moderate degrees of partial melting. The clear positive correlation between the contents of Na and Al, and negative correlation between Na and Fe in bridgmanite, as well as the preferable structural incorporation of Al and Na with temperature, provide evidence for the predominant crystallization of Na-rich bridgmanite via the vacancy mechanism Si-B(4+) + Mg-A(2+) + O-O(2-) = Al3+ (B) + Na-A(+) + V-O from Al-bearing alkaline carbonate-silicate melts, with composition similar to those detected as inclusions in lower-mantle diamonds.

This paper presents results of biogeochemical investigation of the Holocene organic-rich sapropelic sediments of Lake Dukhovoe situated not far from the Baikal. The main source of organic matter in Lake Dukhovoe is phytoplankton, which serves as a biogeochemical barrier where elements are accumulated from water and then buried in the bottom sediments. Microorganisms (e.g., bacteria, micromycete and actinomycetes) play significant role in decomposition of organic matter and formation of organic-mineral complexes in four distinguished types of sapropels: organic, organic-mineral, mineral-organic and mineral. Diagenetic transformations of the Lake Dukhovoe sapropels under influence of mechanical, biochemical, microbiological and physicochemical processes lead to transformation of element speciation. A boundary between oxidizing and reducing diagenesis conditions is at the depth of 167 cm in the sapropel core. Change of the conditions forms geochemical barrier in bottom sediments and control the formation of secondary phases (organic-mineral complexes, diatomite, framboidal pyrite, vivianite, carbonates, etc.) as well as processes of accumulation and leaching of chemical elements. Additionally, high contents of Fe, Mn, Cu, Zn, Pb, Cd, and Mo in the upper 10 cm of the sapropel might have been caused by forest fires and anthropogenic factors. The fact of marcasite formation in the stomatocysts of Chrysophyte algae (golden algae) is new and was established for the first time. Based on the results and geochemical equilibrium modelling, the stomatocysts of Chrysophyte algae can be considered as a microreactor for iron sulfide formation.

An Erratum to this paper has been published: https://doi.org/10.1134/S001670292218001X

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on a new CaFe1.2Al0.8O4 phase at ambient temperature and up to a maximum pressure of 61 GPa. Crystals of CaFe1.2Al0.8O4 were synthesized at 24 GPa and 1500 degrees C. This phase has orthorhombic unit cell parameters (a = 8.9785(1) angstrom, b = 2.9158(1) angstrom, c = 10.4253(6) angstrom, V = 286(3) angstrom(3), Z = 8), crystallizes with a space group Pnma, and exhibits K-0,K-T of 195(2) GPa. At pressures similar to 50 GPa, unit cell volume of CaFe1.2Al0.8O4 drops by about 7% over a small pressure interval. By combining results from synchrotron-based high-pressure single-crystal X-ray diffraction and electronic structure calculations, it is shown that these changes in the compression behaviour are associated with changes of the electronic state of iron in the octahedral site. Thus, CaFe1.2Al0.8O4 is stable under the conditions of the transition zone and lower mantle and, therefore, may be considered as a likely Al carrier in the Earth's deep geospheres.

The post-spinel phases include compounds with stoichiometry $${{A}^{{2 + }}}B_{2}^{{3 + }}{{{\text{O}}}_{4}}$$ $$\left( {A_{2}^{{2 + }}{{B}^{{4 + }}}{{{\text{O}}}_{4}}} \right)$$ and structures of the calcium ferrite CaFe2O4, calcium titanate CaTi2O4, and marokite CaMn2O4 types. The structures with a centered Cmcm (Bbmm) and primitive Pnma (Pmcn) and Pbcm (Pmab) cells are distinguished in this family of topologically related compounds with a “marokite” channel formed by six octahedra. The sites A and B are occupied by various cations, in particular, Cr, Al, Mg, Fe, Ca, Ti, Fe, Na, and Si, which implies the formation of solid solutions of a wide compositional range. In nature, such high-pressure phases were found in meteorites, as inclusions in diamond crystals, and in rocks from some metamorphic complexes. This review provides a characterization of natural mineralogical finds, the results of an experimental study of post-spinel phases of various compositions and their solid solutions, as well as crystal chemical simulation and assessment of likely compositions and the areas of stability of compounds with a “marokite” channel. The discrepancy between the results of individual studies indicates the necessity to clarify the stability parameters and probable isostructural transitions, and, ultimately, to improve the classification of post-spinel phases.

The mechanism of the formation of solid solutions of titanium bridgmanite in the MgSiO3–MgTiO3 system was tested by the method of atomistic modeling. The data obtained were compared with the experimental results. It was shown that the titanium isomorphic capacity of bridgmanite under the P–T parameters of the cold subduction zone is systematically higher than that for the conditions of the mantle geotherm and noticeably increases with depth.