System Na2O Research Articles

Modelling of thermal mode for siberian craton by inversion of seismic profiles "Rift" and "Meteorite"

Temperature of the bowels of the earth is one of the most indefinite physical parameters. As seismic velocities are more sensitive to the temperature then to the composition, so the inversion of seismic profiles into thermal models is one of the most perspective way for estimation of the thermal field. On constructing the seismic models all researchers have a problem with quality control of their results, because nobody knows the exact meaning of seismic velocities that can more accurately describe the features of the Earth structure. Seismic data obtained from the raw material by superlong seismic profiles survey within Siberian Craton was processed by different science groups [Oreshin, et al., 2002; Pavlenkova and Pavlenkova, 2006; Egorkin, 1999]. All velocity models differ from each other by structure and absolute values of seismic velocities. According to geodynamics, variation 0.2 km/sec in longitudinal velocity leads to temperature alteration about 500С. Thermal profiles obtained by velocity inversion is the way to estimate the quality of each seismic model. The main target of this research is to reconstruct the composition and thermal mode for archean mantle of Siberian Craton. Our initial data is compositions of garnet peridotite and fertile primitive mantle, also we use seismic velocities [Pavlenkova and Pavlenkova, 2006], thermobarometry [Griffin, et al., 1996] and heatflow research [Aremieva and Mooney, 2001]. We must estimate the influence of chemical composition to seismic velocity and density and compare processed data with P–T valuations and thermal models. Calculation executes by minimization of Gibbsenergy in system Na2O–TiO2–СaO–FeO–MgO–Al2O3–SiO2. P-velocities depend on Р–Т–Х conditions, effects of phase-transformations, anharmonicity and inelasticity. To make an adjustment for inelasticity we estimate Q-factors: QS and QP. For the composition of Siberian Craton we set a model of garnet peridotite (GP) for the depth less than 180 km and primitive mantle (PM) for more than 180 km [Kuskov, et al., 2011]. Deviations in velocities for GP and PM are small – 0.3% for VР and 1% for VS, so geotherms received from seismic models vary within 50С [Kuskov, et al., 2011]. Difference in composition has a little small influence to seismic velocities and can’t be registered by seismic methods [Kuskov, et al., 2006; Kuskov, et al., 2011], but it leads to increasing of density (fig. 1). The density of fertile primitive mantle is more than density of garnet peridotite at about 2– 3%, that is equal to temperature deviance at 500C.

New constraints on UHT metamorphism in the Eastern Ghats Province through the application of phase equilibria modelling and in situ geochronology

High Mg–Al granulites from the Sunki locality in the central portion of the Eastern Ghats Province record evidence for the high-temperature peak and retrograde evolution. Peak metamorphic phase assemblages from two samples are garnet + orthopyroxene + quartz + ilmenite + melt and orthopyroxene + spinel + sillimanite + melt, respectively. Isochemical phase diagrams (pseudosections) based on bulk rock compositions calculated in the chemical system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O–TiO 2–Fe 2O 3 (NCKFMASHTO) and Al contents in orthopyroxene indicate peak UHT metamorphic conditions in excess of 960 °C and 9.7 kbar. Microstructures and the presence of cordierite interpreted to record the post-peak evolution show that the rocks underwent decompression and minor cooling from conditions of peak UHT metamorphism to conditions of ~ 900 °C at ~ 7.5 kbar. In situ U–Pb isotope analyses of monazite associated with garnet and cordierite using the Sensitive High Resolution Ion Microprobe (SHRIMP) yield a weighted mean 207Pb/ 235U age of ca. 980 Ma, which is interpreted to broadly constrain the timing of high-temperature monazite growth during decompression and melt crystallization at ~ 900–890 °C and 7.5 kbar. However, the range of 207Pb/ 235U monazite ages (from ca. 1014 Ma to 959 Ma for one sample and ca. 1043 Ma to 922 Ma for the second sample) suggest protracted monazite growth during the high-temperature retrograde evolution, and possibly diffusive lead loss during slow cooling after decompression. The results of the integrated petrologic and geochronologic approach presented here are inconsistent with a long time gap between peak conditions and the formation of cordierite-bearing assemblages at lower pressure, as proposed in previous studies, but are consistent with a simple evolution of a UHT peak followed by decompression and cooling.

Structural study of Mg‐bearing sodosilicate glasses by Raman spectroscopy

AbstractThe presence of magnesium in glasses of geological, medical, and technological interests influences their physicochemical and durability properties. However, the understanding of the role of magnesium is dependent on the combined knowledge of the structural environment of magnesium in the glass or melt and of the silicate network connectivity of the studied systems. In this article, we present a Raman spectroscopic study of the network connectivity of 10 ternary silicate glasses in the system Na2OMgOSiO2 and one Mg‐free binary silicate glass Na2OSiO2. Results obtained at constant polymerization suggest the existence of various Qn units according to the nature of the modifying cation. As polymerization decreases for Na2OMgOαSiO2 glasses (labeled as NMSα with α decreasing from 10 to 2), the band associated with SiOSi bending in fully polymerized region disappears being gradually replaced by a band attributed to SiOSi bending in region containing mainly Q2 and Q3 species. For highly polymerized glasses (NMS10‐NMS4), the coexistence of these two bands suggests the presence of two interconnected networks. Concomitantly, the signal associated with Q3 species first increases. For a further decrease of the polymerization, the high wavenumber part of the signal associated with Q3 species decreases, while the intensity of the high wavenumber part of the band related to Q2 species increases. This result strongly suggests that magnesium charge‐balances preferentially Q2 species rather than Q3 species. Copyright © 2010 John Wiley & Sons, Ltd.

Ultrahigh-temperature metamorphism in Daqingshan, Inner Mongolia Suture Zone, North China Craton

The Inner Mongolia Suture Zone marks the Paleoproterozoic collisional boundary between the Yinshan Block and Ordos Block in the North China Craton. The ultrahigh-temperature (UHT) orogen developed within this collisional suture reflects the extreme thermal conditions associated with the tectonic processes during the incorporation of the North China Craton within the Columbia supercontinent. Here we report the results from a systematic petrological and phase equilibria modeling study of sapphirine-bearing granulites from Daqingshan, a region within the central domain of the Inner Mongolia Suture Zone. The sapphirine–spinel–garnet–sillimanite–biotite-bearing granulites in Daqingshan occur typically as dark lenses or layers within spinel–garnet–sillimanite–biotite-bearing gneisses, and are associated with mafic granulites carrying orthopyroxene + clinopyroxene + plagioclase + calcic amphibole. The sapphirine in these rocks is magnesian ( X Mg = 0.73–0.79) and occurs in various microstructural associations such as: (1) matrix coarse-grain mineral in plagioclase aggregates, (2) subidioblastic mineral mantling spinel or in direct contact with spinel, (3) fine-grained phase in sillimanite, and (4) vermicular symplectitic aggregates with plagioclase. Garnet + plagioclase + sapphirine + sillimanite + ilmenite + inferred melt is considered to represent the peak assemblage in these rocks. We apply systematic phase equilibria analysis in the system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O–TiO 2–Fe 2O 3 (NCKFMASHTO) to constrain the peak P–T conditions of the Daqingshan Mg–Al granulites. Our pseudosection analysis defines the stability of the peak assemblage at T > 970 °C at P > 11 kbar. The peak temperature derived from pseudosection analysis is broadly consistent with the temperature estimate from conventional thermometry of sapphirine–spinel pairs (ca. 940 °C). The extreme metamorphism recorded from the Mg–Al granulites from Daqingshan is also consistent with the occurrence of dry orthopyroxene + clinopyroxene + plagioclase, and orthopyroxene + pargasite + plagioclase in the associated mafic granulites, although conventional thermometry yields lower temperatures from these rocks (830–840 °C) marking the post-peak retrograde stage. Our data confirm UHT conditions in Daqingshan and indicate that the extreme metamorphism is of regional extent within the Inner Mongolia Suture Zone, in agreement with the model of ultra-hot metamorphic orogen developed within a Paleoproterozoic subduction–collision system.

The transition from blueschist to greenschist facies modeled by the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite

The reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite is proposed to model the transition from the blueschist to greenschist facies. This reaction was investigated experimentally over the range of 1.0–2.1 GPa and 500–800°C using synthetic phases in the chemical system Na2O–CaO–MgO–Al2O3–SiO2–H2O. Reversals of this reaction were possible at 500 and 550°C and growth of the low-pressure assemblage at 600°C; however, at temperatures of 600°C and higher and at pressures above 1.6 GPa omphacite nucleation (at the expense of diopside and albite) became quite strong and prevented attaining clear reversals of this reaction. Compositional changes in the amphiboles were determined by both electron microprobe analyses and correlations between unit-cell dimensions and composition. Glaucophane and particularly tremolite showed clear signs of compositional re-equilibration and merged to a single amphibole of winchite composition by about 754°C. These data were used to model the miscibility gap between glaucophane and tremolite using either the asymmetric multicomponent formulism parameters of W TR,GL of 68 kJ with αTR of 1.0 and αGL of 0.75 or a simple two-site asymmetric thermodynamic mixing expression with Margules parameters W NaCa of 13.4 kJ and W CaNa of 19.3 kJ. Combination of these thermodynamic models of the miscibility gap with extant thermodynamic data for the other phases yields a calculated location of the above reaction, involving pure diopside and albite, that is in good agreement with the observed experimental reversals and amphibole compositions over the range of 0.94–1.93 GPa and 400–754°C. The calculated effect of jadeite solid solution into diopside is to reduce the dP/dT slope from 0.0028 to 0.0021 GPa/°C and decrease the pressure by 0.28 GPa at 754°C. The dP/dT slope of this reaction boundary lies close to a linear geotherm of 13°C/km and is consistent with the slopes of other solid–solid reactions that have been used to model the blueschist-to-greenschist facies transition.

New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine granulites

AbstractThe sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>900°C). Diagnostic UHT assemblages in these rocks include sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz. The stability of spinel + quartz, sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet–orthopyroxene is indicative of a subsequent high P–UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10 wt% Al2O3) coexisting with garnet. Peak P–T conditions of ∼970°C and 9.5 kbars are calculated from conventional garnet–orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet–orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P–T estimates indicate a three‐stage evolution of the sapphirine granulites and associated garnet–orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low‐pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet–orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene–cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet–orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo‐barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P–T evolution of this sector of the EGB. Copyright © 2010 John Wiley & Sons, Ltd.

Effect of composition and structural factors on the viscosity of borosilicate glasses and melts

The features of the effect of the composition and structure of glasses of the systems Na2O – CaO – B2O3 – SiO2 and Na2O – CaO – B2O3 – Al2O3 – SiO2 on the temperature curve of the viscosity in the 1010 – 104 Pa · sec range were established. The important effect of phase separation processes—liquation and crystallization—on the viscosity indexes in the temperature region of the transition from the plastic to the liquid state was demonstrated.

Effect of CaF2 and Li2O Additives on the Viscosity of CaO–SiO2–Na2O Slags

High aluminum containing steels react vigorously with silica-based mold fluxes to form alumina. The change in alumina content increases the viscosity and thus silica-based mold fluxes are compensated with significant amounts of Na2O, CaF2, and Li2O to ensure both lubrication and heat transfer control. Detailed viscosity studies using the rotating spindle method showed that additions of CaF2 up to 8 wt% in the CaO–SiO2–12wt%Na2O system at a constant CaO/SiO2 ratio of 0.8 decreases the viscosity by breaking the network structure of molten fluxes, but is negligible above 8 wt%. A similar modification of the network was observed for Li2O up to 2 wt%. The viscosity data was correlated with the XPS analysis and verified CaF2 and Li2O as effective in modifying complex silicate structures into simpler silicate structures depending on the availability of complex silicates and thus was limited to below certain concentrations.

Synthesis, growth and some physical properties of new orthoborates ScBaNa(BO3)2 and YBaNa(BO3)2

Powder samples of ScBaNa(BO3)2 and YBaNa(BO3)2 were prepared by two-step solid-phase synthesis and by heat treatment after mechanochemical activation. Crystal growth was performed from high temperature solutions of MBO3–BaO–B2O3–Na2O (M=Sc, Y) systems. By means of DSC and X-ray analysis some thermal properties of the crystals were described.

Clinopyroxene–melt trace element partitioning and the development of a predictive model for HFSE and Sc

Clinopyroxene–melt trace element partitioning experiments were carried out in the system Na2O–CaO–MgO–Al2O3–SiO2 at pressures of 1, 2.3 and 3 GPa and temperatures of 1508 to 1811 K, to investigate the effects of temperature (T), pressure (P) and composition (X) on partition coefficients. Of particular interest were elements entering the octahedral M1-site. Ion probe analyses of run products produced crystal–melt partition coefficients (D) for 16 elements (Na, Ca, Al, Cl, Sc, Ti, Fe, Zr, In, La, Ce, Nd, Sm, Ho, Yb and Hf). With the exception of D Na, partition coefficients for all elements studied decrease with increased P and T, despite the concomitant increase in the Al content of the T-site. Fitting partition coefficients for isovalent series of cations to the elastic strain model of Blundy and Wood (1994) produced values for the site radius (r 0), effective elastic modulus (E) and strain-free partition coefficient (D 0). At each pressure, E values for the M1 and M2-sites increase with increasing Al concentration in the T-site $$ \left( {X_{\text{Al}}^{T} } \right) $$ . For a given bulk composition, E values decrease with increased T. The decrease in E with increasing T accounts for the remarkable constancy of the degree of fractionation between chemically similar elements, e.g. $$ D\left( {{\frac{\text{Zr}}{\text{Hf}}}} \right) $$ , over the range of pressures studied here. $$ E_{{{\text{M}}1}}^{4 + } $$ for our experiments is found to be higher than predicted by the Hazen and Finger (1979) relationship between elastic moduli and interatomic distance. This is explained by deformation of the M1-site polyhedron leading to relative displacement and kinking of the clinopyroxene T-site chains. We developed expressions for $$ E_{{{\text{M}}1}}^{4 + } $$ , $$ r_{{0,{\text{M}}1}}^{4 + } $$ , D Sc and D Ti as functions of P, T and composition. We show the feasibility of using calculated D Ti values in the prediction of D Zr and D Hf. Scandium and Ti partition coefficients were modelled based on the thermodynamic description for the crystal–melt exchange reaction and in terms of the energetics of the different charge-imbalanced configurations produced by insertion of a heterovalent trace cation. The resulting equations produce values of D Sc and D Ti that are within a factor of 2 of other experimentally determined values. Fits of the equations along the lherzolite solidus show that D Sc remains compatible in clinopyroxene at high pressure and that ratios of Zr/Hf and Ti/Eu should vary subtly with the pressure at which melting occurred.

Hydrothermal formation and conversion of calcium titanate species in the system Na 2O–Al 2O 3–CaO–TiO 2–H 2O

The hydrothermal formation and conversion of titanate species in the system Na 2O–Al 2O 3–CaO–TiO 2–H 2O was investigated based on the thermodynamic calculation and systematic experiments. Experimental results show that the reaction product of anatase with aluminate solution is sodium tri-titanate (Na 2O·3TiO 2), and that kassite (CaO·2TiO 2·H 2O) is formed by the reaction of sodium tri-titanate with calcium hydroxide (Ca(OH) 2) and sodium aluminate solution with high free caustic concentration at elevated temperatures. The results also show that there are two reaction paths for the formation of perovskite (CaTiO 3), one is the direct reaction of sodium tri-titanate with calcium hydroxide, and the other is the conversion of kassite with calcium hydroxide and sodium aluminate solution at elevated temperatures. In addition, the aluminum species in the sodium aluminate solution play a very important role in the process of kassite formation and its conversion to perovskite. The findings provide a potential method for the efficient preparation of titanate species and enhance the understanding of the scaling of heat exchanger surfaces in the Bayer process alumina refinery which is a result of titanate formation.

ChemInform Abstract: Synthesis, X-Ray Diffraction and MAS NMR Characteristics of Nitrate Cancrinite Na7.6 [AlSiO4] 6(NO3)1.6(H2O)2.

Abstract Investigations on the hydrothermal formation and the crystal structure of nitrate cancrinite have been carried out in the system Na2O–xSiO2–Al2O3–NaNO3–H2O, 1

Speciation of reduced C–O–H volatiles in coexisting fluids and silicate melts determined in-situ to ∼1.4 GPa and 800 °C

The structure of silicate melts in the system Na 2O·4SiO 2 saturated with reduced C–O–H volatile components and of coexisting silicate-saturated C–O–H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800 °C) and pressure (up to 1435 MPa). Redox conditions in the HDAC were controlled with the reaction, Mo + H 2O = MoO 2 + H 2, which is slightly more reducing than the Fe + H 2O = FeO + H 2 buffer at 800 °C and less. The dominant species in the fluid are CH 4 + H 2O together with minor amounts of molecular H 2 and an undersaturated hydrocarbon species. In coexisting melt, CH 3 – groups linked to the silicate melt structure via Si–O–CH 3 bonding may dominate and possibly coexists with molecular CH 4. The abundance ratio of CH 3 – groups in melts relative to CH 4 in fluids increases from 0.01 to 0.07 between 500 and 800 °C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400 °C and 730 MPa, respectively. A schematic solution mechanism is, Si–O–Si + CH 4⇌Si–O–CH 3+H–O–Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C–O–H) components will, therefore, affect melt properties in a manner resembling dissolved H 2O.

Structural study of the NaCdVO 4–Cd 3V 2O 8 and CdO–V 2O 5 sections of the ternary system Na 2O–CdO–V 2O 5

The NaCdVO 4–Cd 3V 2O 8 and CdO–V 2O 5 sections of the ternary system Na 2O–CdO–V 2O 5 have been studied and the crystal structures of Cd 3V 2O 8 and Cd 18V 8O 38 compounds were determined from single-crystal X-ray diffraction data. Cd 3V 2O 8 crystallizes with the maricite-type structure in space group Pnma, a=9.8133(10) Å, b=6.9882(10) Å, c=5.3251(10) Å and Z=4, whereas Cd 18V 8O 38 crystallizes in space group P1 with a new-type structure, a=8.5761(14), b=8.607(3), c=12.896(2) Å, α=95.64(1), β=102.45(1), γ=108.42(1)° and Z=1. The Cd 3V 2O 8 structure is made up of Cd1O 4 infinite chains of edge-sharing Cd1O 6 octahedra which are parallel to the b direction. The Cd1O 4 chains are linked together by VO 4 tetrahedra and strongly distorted Cd2O 4 tetrahedra. The structure of Cd 18V 8O 38 is based on an ordered three-dimensional framework of cadmium and vanadium polyhedra that share corners. The distorted CdO 6 octahedra, CdO 5 trigonal bipyramids and CdO 5 square pyramids share corners, edges or faces.

Thermal analysis of the Na 2O-rich concentration region of the quasi-binary system Na 2O–SiO 2

The phase diagram of the Na 2O-rich concentration region of the binary system Na 2O–SiO 2 was derived from STA measurements. The eutectic was established at about 1134 ± 9 K between 78 and 80 mole per cent of sodium oxide. The XRD measurements showed the existence of a crystal structure which was unknown so far. Its composition is most likely Na 10SiO 7 which was also deducted from the shape of the liquidus line obtained for all samples measured in the high Na 2O concentration range of the binary system Na 2O–SiO 2. A second eutectic is placed between 85 and 87 mole per cent of sodium oxide at 1118 ± 15 K. The structure change of Na 2O ( β → α) should take place at 1243 K but was not observed in any sample in the Na 2O-rich concentration region of the binary system Na 2O–SiO 2.

MAS-NMR studies of glasses and glass-ceramics based on a clinopyroxene–fluorapatite system

Glasses and glass-ceramics based on the ternary system CaMgSi2O6–NaAlSi2O6–Ca5(PO4)3F (diopside (Di)–jadeite (Jd)–fluorapatite (FAp)) have been studied using magic angle spinning (MAS) NMR, powder X-ray diffraction (XRD) and thermal calorimetry. In this silicate system Na2O and Al2O3 have been progressively substituted for CaO and MgO: (CaMgSi2O6)1−x–(NaAlSi2O6)x–(Ca5(PO4)3F)y where 0 ≤ x ≤ 30 mol% and y = 7 mol%. 29Si MAS-NMR of the glasses showed a peak corresponding to a mixed Q2/Q3 silicate species which slightly decreased in chemical shift on increasing Na2O and Al2O3 substitution. After heat treatment of the glasses two major crystalline phases: a silicate solid solution (clinopyroxene) and FAp have been identified by XRD, with the silicate phase crystallising out first. A glassy phase coexisted with these crystalline phases and became more evident with increasing Na2O and Al2O3 content. As expected, no crystalline Jd has been found from XRD or MAS-NMR. 27Al MAS-NMR spectra of the glasses shows only Al(IV) to be present on addition of Al2O3. Upon heat treatment, a new, sharp Al(VI) peak at −4 ppm assigned to the solid solution was observed, which corresponds to Al3+ in a six coordinate state, replacing Mg2+ along the crystalline silicate chain structure with the replacement of either an additional Mg2+ or Ca2+ by Na+ to maintain the charge balance. The amount of Al(VI) formed on heat treatment grows linearly with addition of Al2O3, but does not exceed ca. 20% from the total aluminium concentration present in the system. The signal from the Al(IV) in the heat treated material belongs to the residual aluminosilicate glassy phase. 31P MAS-NMR shows the presence of mixed calcium/magnesium/sodium amorphous orthophosphate species in the glass, which upon heat treatment, changes to crystalline Ca5(PO4)3F. 23Na MAS-NMR of the heat treated glass-ceramics shows a Jd-like environment at the lowest Na2O and Al2O3 content. This site gets increasingly disordered with increasing Na2O and Al2O3 content, which is reflected in the changing 23Na MAS-NMR shift.

Synthetic amphiboles and triple-chain silicates in the system Na2O-MgO-SiO2-H2O: phase characterization, compositional relations and excess H

AbstractThe presence of structural OH in amphiboles in excess of the usual two OH per formula has been debated for over 40 years (Gier et ah,1964; Leake et ah,1968). However, the reality of the excess-OH phenomenon is still an open question, because accurate water analyses of amphiboles are rarely available. In this study, we review the data available on the chemically simple synthetic system Na2O—MgO-SiO2-H2O (NMSH) and present new results from NMR, infrared spectroscopy, and X-ray-diffraction that allow re-interpretation of previous studies of NMSH amphiboles along the pseudobinary join between the two end-member compositions Na2Mg6Si8O22(OH)2 and Na3Mg5Si8O21(OH)3.We show that there is extensive solid solution involving excess H at 650—750°C, but also document the presence of a wide miscibility gap below 600°C. This miscibility gap is defined by amphiboles very close to the end-member composition Na3Mg5Si8O22(OH)3 coexisting with amphiboles with compositions near the ‘normal’ Na2Mg6Si8O22(OH)2 end member.We also report the characterization of triple-chain silicates (TCS) in the NMSH system and their phase relations with NMSH amphiboles. The upper thermal stability field of the key TCS Na2Mg4Si6O16(OH)2relative to its decomposition to two NMSH amphiboles with a combined equivalent composition has been determined and a pronounced backbend of the transformation boundary documented. Phase relations observed in synthesis experiments suggest that at 550—650°C all TCSs have compositions close to Na2Mg4Si6Oi6(OH)2. Infrared spectroscopy indicates that the TCS synthesized on this composition, studied in detail here, vary from end-member Na2Mg4Si6Oi6(OH)2 to binary solid solutions with less than ∼6 mol.% clinojimthompsonite component. No clear spectroscopic evidence for a ‘Drits’ component NaMg4Si6Oi5(OH)3 (Drits et al.,1975) has been found. Analysis of H2O by vacuum extraction and Karl-Fischer titration indicates large excesses of H2O in all the TCSs studied here that clearly exceed the amounts expected from (OH) groups alone. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicate that this excess H2O is structural. We propose that the excess H2O is likely to be molecular H2O located in the ^4-site channels. The observed backbend of the triple-chain decomposition curve is in agreement with a reaction involving dehydration and loss of this molecular H2O. However, the absolute amount of analysed molecular H2O exceeds that expected from the change in Clapeyron slope alone.While demonstrating the reality of excess OH in amphiboles, the evidence presented in this paper also points to interesting avenues for future research on both amphiboles and TCSs, such as understanding the dynamics and enhanced crystal chemistry of excess OH and molecular H2O in pyriboles.

The physicochemical conditions of diamond formation in the mantle matter: experimental studies

Experimental studies of diamond formation in the alkaline silicate-carbon system Na 2O–K 2O–MgO–CaO–Al 2O 3–SiO 2–C were carried out at 8.5 GPa. In accordance with the diamond nucleation criterion, a high diamond generation efficiency (spontaneous mass diamond crystallization) has been confirmed for the melts of the system Na 2SiO 3–carbon and has been first established for the melts of the systems CaSiO 3–carbon and (NaAlSi 3O 8) 80(Na 2SiO 3) 20–carbon. It is shown that in completely miscible carbonate-silicate melts oversaturated with dissolved diamond-related carbon, a concentration barrier of diamond nucleation (CBDN) arises at a particular ratio of carbonate and silicate components. Study of different systems (eclogite–K-Na-Mg-Ca-Fe-carbonatite–carbon, albite–K 2CO 3–carbon, etc.) has revealed a dependence of the barrier position on the chemical composition of the system and the inhibiting effect of silicate components on the nucleation density and rate of diamond crystal growth. In multicomponent eclogite-carbonatite solvent, the CBDN is within the range of carbonatite compositions (<50 wt.% silicates). Based on the experimental criterion for the syngenesis of diamond and growth inclusions in them, we studied the syngenesis diagram for the system melanocratic carbonatite–diamond and determined a set of the composition fields and physical parameters of the system that are responsible for the cogeneration of diamond and various mineral and melt parageneses. The experimental results were applied to substantiate a new physicochemical concept of carbonate-silicate (carbonatite) growth media for most of natural diamonds and to elaborate a genetic classification of growth mineral, melt, and fluid inclusions in natural diamonds of mantle genesis.

Preparation and properties of porous glass using fly ash as a raw material

The porous glasses were prepared by a conventional phase separation method using coal fly ash as a raw material, and the properties of these porous glasses were investigated. The composition of coal fly ash is basically composed of SiO 2–Al 2O 3–Fe 2O 3–CaO system and the SiO 2–B 2O 3–Al 2O 3–CaO–Na 2O system of glass was chosen as base glass composition. The pore diameter increases proportional to cube root of heating time ( t 1/3), however, the early stage of phase separation is not clear. It is estimated that the rate determining step may be the diffusion process of structural units involving oxygen ions and the phase separation may take place by the nucleation and growth mechanism, and the relatively larger pores of above 1 μm can be obtained easily. The chemical composition of porous glasses is SiO 2–B 2O 3–Al 2O 3(–CaO–Na 2O). A relatively large amount of fly ash (>40%) can be used successfully for the preparation of porous glass.

Structural particulars and formation mechanisms of glass coatings belonging to the system Na2O – B2O3 – ZnO – TiO2 – SiO2

It is established on the basis of investigations of the particulars of the structure and phase distribution in glasses belonging to the system Na 2 O -B 2 O 3 - ZnO - TiO 2 - SiO 2 that spinodal liquation followed by crystallization of rutile with a uniform distribution of the crystalline phase is energetically favored over stable crystallization, which makes it possible to obtain a finely dispersed, silky coating texture with 11 - 12% luster. The composition of the glass matrix is optimized to obtain a mat, glass crystal, coating with high technical - performance and decorative indicators.