Institute of Experimental Mineralogy

Abstract Using surface titration and electrokinetic measurements in aqueous solutions at 25°C, the surface of natural talc and cadmium adsorption were studied. Using the Mular – Roberts titration method, the point of zero charge of talc was determined to be pH pzc = 8.1 ± 0.1, while the isoelectric point pH iet is approximately 2, which corresponds to the predominance of SiO 2 in the surface layer due to the preferential removal of Mg in the acidic region. When titrating from pH 3 to the alkaline region, ζ-potential of the talc surface exhibits strong hysteresis, becoming positive, and pH iet shifts toward the alkaline region due to the retardation of the reverse exchange reaction Mg 2+ → 2H + . This circumstance suggests that proton adsorption (desorption) occurs only at the outer adsorption sites of the edges, making it possible to apply the theory of surface complexation to adsorption modeling. Cadmium adsorption on talc is described by the Freundlich isotherm, and the adsorption kinetics obey a pseudo-second-order equation. Almost complete adsorption of cadmium is observed in neutral and slightly alkaline solutions (pH 7.5-8). Coupled modeling of proton and cadmium adsorption using the Extended Constant-|Capacitance Model showed that the best agreement with experimental data on both cadmium and proton adsorption is provided by a model with two inner-sphere surface cadmium complexes, >MgOCdOH and >SiOHCd 2+ . Cadmium adsorption (at pH > 6) is primarily determined by the surface complex >MgOCdOH, while minor absorption in the range 4 < pH < 6 is associated with the formation of the silanol complex >SiOHCd 2+ .

Abstract Two new geodynamic concepts are considered on the example of the Maksyutov eclogite-blueschist complex (MC), Southern Urals, for their use as global schemes for the origin and development of subduction of eclogite complexes. One of them - the " Subduction Initiation Rule” (SI), represents the process of subduction initiation in pre-arc basins in conjunction with the formation of ophiolite complexes and the formation of a diagnostic magmatic chemostratigraphic sequence of rocks with a certain change in the content of high-charge (HFSE) and lithophile (LILE) elements. The second is the geodynamic classification of oceanic basalts, which make it possible, based on their geochemical characteristics, to consider large structural fragments of the Earth's crust as participants in the subduction process, to determine their magma-generating role as a source of the initial mantle-crustal material, and to evaluate the geochemical interaction between them and with the mantle substrate. The Maksyutov complex is considered a unique facility for testing such developments as general models of formation and development of subduction processes, since the main features and criteria of the SI are presented quite fully and clearly implemented. However, the individual features of the Maksyutov complex - geochemical, petrological and geodynamic - complicate the perception of the SI Rules as a universal model, raising a number of controversial issues related more likely to the features of the orogen, than to the general SI criteria. It is assumed that these features do not affect the established subduction rules, but require further study and clarification, since they affect the established rules of subduction, but require further study and refinement, since they affect the fundamental points of complex formation. These are the questions of the time of origin and duration of the existence of MC, the subduction polarity of its development, the features of the evolution of metamorphism of immersion and exhumation, the source and origin of the UHP index minerals - diamond, etc.

Experimental data on the solubility of niobium and tantalum oxides and oxyfluorides in fluoride solutions were reviewed. Experimental solubility data for pyrochlore (CaNa)Nb2O6F and microlite (CaNa)Ta2O6F were used to calculate their thermodynamic properties at 300–800°C. The thermodynamic properties of albite, andalusite, muscovite, paragonite, and pyrophyllite were refined in the temperature range 300–550°C. The influence of solution composition and aluminosilicate mineral association on the solubility of pyrochlore and microlite in the supercritical region of physicochemical parameters was modeled by thermodynamic calculations. The calculations showed that the solubility of the minerals is very low, and niobium and tantalum cannot be removed by metamorphosed solutions. Mechanisms were proposed for HF accumulation, which could play an important role in niobium and tantalum dissolution, as well as recrystallization and replacement of ore mineral phases over limited distances.

We investigated the element-specific electronic structure and charge-carrier dynamics of a single-crystal ferromagnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>CoS</a:mi><a:mn>2</a:mn></a:msub></a:math> with complementary x-ray spectroscopy techniques. Hard x-ray photoemission (HAXPES) is used to provide crucial information on the bulk electronic structure and chemical bonding in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>CoS</b:mi><b:mn>2</b:mn></b:msub></b:math> that is compared against the isoelectronic paramagnet <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>CoSe</c:mi><c:mn>2</c:mn></c:msub></c:math>. The Co <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>1</d:mn><d:mi>s</d:mi></d:mrow></d:math> core-level line shows several satellite features for <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>CoS</e:mi><e:mn>2</e:mn></e:msub></e:math>, showing explicit charge-transfer processes and local screening of the core hole by S ligands, whereas no such features are observed in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>CoSe</f:mi><f:mn>2</f:mn></f:msub></f:math>. The satellite structures indicate the electronic configuration of divalent <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msup><g:mrow><g:mi>Co</g:mi></g:mrow><g:mrow><g:mn>2</g:mn><g:mo>+</g:mo></g:mrow></g:msup></g:math> as a combination of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msup><h:mi>d</h:mi><h:mn>8</h:mn></h:msup><h:mi mathvariant="normal">Ḻ</h:mi></h:mrow></h:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:msup><j:mi>d</j:mi><j:mn>9</j:mn></j:msup><j:msup><j:mi mathvariant="normal">Ḻ</j:mi><j:mn>2</j:mn></j:msup></j:mrow></j:math> in addition to the nominal ionic <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msup><l:mrow><l:mi>d</l:mi></l:mrow><l:mn>7</l:mn></l:msup></l:math> state, where <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mi mathvariant="normal">Ḻ</m:mi></m:math> represents an S <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mn>3</o:mn><o:mi>p</o:mi></o:mrow></o:math> hole. We employ resonant Auger spectroscopy across the S <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mi>K</p:mi></p:math>-edge for <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:msub><q:mi>CoS</q:mi><q:mn>2</q:mn></q:msub></q:math> to obtain electron delocalization times to adjacent Co atomic sites. The fast carrier dynamics are attributed to strongly screened Coulomb interactions and hence a facile carrier delocalization. The strong hybridization formed between the Co <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mn>3</r:mn><r:mi>d</r:mi></r:mrow></r:math> and S <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:mn>3</s:mn><s:mi>p</s:mi></s:mrow></s:math> states with pronounced charge-transfer character reflects a self-doped system with a finite density <t:math xmlns:t="http://www.w3.org/1998/Math/MathML"><t:mi>n</t:mi></t:math> of holes at the sulfur site (<u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:msup><u:mi mathvariant="normal">Ḻ</u:mi><u:mi>n</u:mi></u:msup></u:math>), in line with recent models that indicate a negative charge-transfer energy for <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:msub><w:mi>CoS</w:mi><w:mn>2</w:mn></w:msub></w:math>. In addition to HAXPES data, we also report on experimental and theoretical <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"><x:mi>L</x:mi></x:math>-edge x-ray absorption and x-ray magnetic circular dichroism data for <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"><y:msub><y:mi>CoS</y:mi><y:mn>2</y:mn></y:msub></y:math> that demonstrate multiconfiguration effects in the excitation process. To enable a direct comparison of the experimental spectra, we used density functional theory calculations to obtain the projected density of states to describe the ground-state electronic structure. The existence of fast carrier dynamics and strong charge-transfer properties, demonstrated in this study, highlights the unique nature of <z:math xmlns:z="http://www.w3.org/1998/Math/MathML"><z:msub><z:mi>CoS</z:mi><z:mn>2</z:mn></z:msub></z:math> with a wide potential in topological spintronics applications and integration in energy-related device platforms.

Abstract The solubility of phenakite (Be2SiO4) in granite melts was experimentally studied at temperatures of 1000 and 1100°C and pressures of 1 and 4 kbar in dry conditions and in the presence of 10 wt % H2O. The starting materials were granite glasses with agpaitic coefficient of 1–2.5 and natural phenakite. It was found that the solubility of phenakite increases with increasing agpaitic index (Na + K)/Al of the melt, and the solubility of BeO in hydrous melts is higher than in dry ones. The solubility of phenakite also increases with pressure. The obtained experimental data were generalized with the previous data in the form of an equation describing the solubility of BeO in alkaline-granite melts coexisting with crystalline Be phases depending on the agpaitic index, temperature, and pressure. The results of the experiments and their generalizations support the model of Be concentration in alkaline hydrous melts—the products of differentiation of granite magmas.

Problems of evolved gabbroic associations and local granitoid magmatism in an ocean spreading ridge system are of key importance for the understanding the planetary differentiation during the ocean crust formation. The paper presents new data on plutonic associations, which are exposed in two different types of tectonic “windows” of the Mid-Atlantic Ridge: the footwall of a large-offset detachment fault (Ashadze complex, 13° N) and an uplifted wall of a transform fault (Peyve Seamount, 8° N, Vernadsky Fault). Regular features of crystal differentiation with fractionation of a significant amount of Fe–Ti oxides up to the formation of silicic residual melt composition are shown on the basis of mineral compositional trends and crystallization modeling of MORB-type melts in COMAGMAT 3.75 program. The simulated decrease in oxygen fugacity to QFM level is a result of closed-system oxygen consumption by crystallization of a large volume of magnetite. This is indirectly confirmed by regular formation of reactive fayalite in most evolved gabbroids as a result of redox interaction of reduced residual melt with early magnetite. The earliest zircon crystallized in the apatite-bearing oxide microgabbro has a low-Hf composition (<1.3 wt % HfO2). Experimental study of melt inclusions in apatite and zircon yields the real major-element composition of the residual granitoid melts. Further differentiation of silicic residual melts in gabbro-hosted felsic injections occurs as zircon enrichment in HfO2 up to 2.3 wt % (Ashadze complex) and up to 3.5 wt % (Peyve Seamount).

Electrum dendrites and associated minerals from the Ametistovoe Au-Ag epithermal deposit (Kamchatka) were studied by optical and electron microscopy, X-ray computed tomography, electron microprobe and electron back scattered diffraction techniques.Electrum forms dendrites up to 1 mm in size within sphalerite, or, rarely, within galena and chalcopyrite and random particles within quartz.It has a very homogeneous chemical composition (fineness 603 to 615) which correlates neither with the host mineral nor with the electrum morphology.X-ray computed microtomography showed electrum dendrites are 2-dimensional and comprise acicular "trunk" hosting numerous "branches".3-dimensional and "bush-like" dendrites are rare.Angles between "trunk" and "branches" are usually 90 and, less commonly, 60 suggesting twinning along (100) and (111) planes respectively.Electron back scattered diffraction evidenced the dendrites are single crystals with no defects detected while electrum within quartz has a mosaic texture with low pattern quality.A model proposed for the observed phenomena includes: 1) growth of single crystal electrum dendrites within silica gel; 2) overgrowth of dendrites by sphalerite crystals; 3) recrystallization of the opaline silica to crystalline quartz which resulted in destruction of electrum * * Corresponding author.

The paper presents experimental data on melting model compositions of basaltic komatiite (BK) and enstatite chondrite (ECH) at a temperature of T = 1300°C and hydrogen pressure $${{P}_{{{{{\text{H}}}_{{\text{2}}}}}}}$$ = 100 MPa. The experiments modeled interaction between the magma ocean and the early Earth’s hydrogen atmosphere. The experimental products consist of silicate glass (quenched melts), which is notably depleted in FeO but enriched in lithophile oxides and H2O, and iron with minor Si and O admixtures. The equilibrium oxygen fugacity in the experimental runs was approximately two logarithmic units below the Fe−FeO buffer. Calculation of the fractional crystallization of the melts indicates that the complete crystallization products are granodiorite, which consists of two feldspars, clinopyroxene, and quartz with a minor amount of black mica (for the starting composition obtained in the run with BK), or quartz−two feldspars granite with minor amounts of biotite and muscovite (for the starting composition obtained in the run with ECH). Crystallization of zircon from the ECH melt might occur at T = 730−750°C. Our proposed model is the first that explains generation of melts enriched in SiO2 and H2O by internal processes of planetary evolution and does not invoke pre-hydrated upper crust for generating the Earth’s first felsic material.