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1.
The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join and with less polymerized melts along the join and O_5. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q1, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity, diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further, discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport properties. Received: 26 September 1996 / Accepted: 18 October 1996  相似文献   

2.
The structure of glasses and melts of Na2O· 0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 compositions have been measured using high temperature Raman spectroscopy. For the oxidized sample it has been demonstrated that there is a close structural relationship between melt and glass. No coordination changes of Fe3+ with temperature and no new anionic species have been observed in the oxidized melt. The Raman spectra of the reduced sample clearly show a decrease in the degree of polymerization, as determined by the observation of the polarization character of the spectra and the details of the change of the Raman intensities during heating in hydrogen. Mössbauer spectra suggest that Fe3+ is tetrahedrally coordinated in the oxidized glass and part of the Fe2+ is tetrahedrally coordinated in the reduced glass.  相似文献   

3.
The viscosity-temperature relationships of five melts on the join Na2Si2O2-Na4Al2O5 (5, 10, 20, 30 and 40 mole percent Na4Al2O5) have been measured in air, at 1 atm and 1000–1350°C with a concentric cylinder viscometer. All the melts on this join of constant bulk polymerization behave as Newtonian fluids, in the range of shear rates investigated, and the melts exhibit Arrhenian viscosity-temperature relationships.Isothermal viscosities on this join initially decrease and then increase with increasing mole percent Na4Al2O5. The minimum viscosity occurs near 20 mole percent Na4Al2O5 at 1000°C and moves to higher Na4Al2O5 content with increasing temperature.The observation of a viscosity minimum along the join Na2Si2-O5-Na4Al2O5 is not predicted based on earlier viscosity data for the system Na2O-Al2O3-SiO2 (RlEBLlNG, 1966) or based on calculation methods derived from this and other data (Bottinga and Weill, 1972). This unexpected behavior in melt viscosity-temperature relations emphasizes the need for a more complete data set in simple silicate systems.Previous spectroscopic investigation of melts on the join Na22Si2O5-Na4Al2O5 offer a structural explanation for the observed viscosity data in terms of a disproportionation reaction involving polyanionic units. Macroscopically, the viscosity data may be qualitatively reconciled with the configurational entropy model for viscous flow (Richet, 1984).  相似文献   

4.
Quench products of melts synthesized at 5 GPa and 1500°C in model system CaMgSi2O6–Na2CO3(±CaCO3)–KCl, were studied using vibrational (IR and Raman) and X-ray absorption spectroscopy (XANES). Correlations between structural peculiarities of the quenches with chemical composition are established. Increase of the CaMgSi2O6 content of the melts results in gradual substitution of the Са-bearing carbonate groups by Na-bearing, whereas Ca is progressively more bounded with silicate structural units. XANES spectra reveal that chlorine is predominantly present as (K x Na1–x )Cl complexes. XANES spectra also indicate distribution of potassium cations between chloride and silicate groups, although its partial bonding with carbonate groups in the melt is not excluded.  相似文献   

5.
The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO2-SiO2] has been studied with Raman spectroscopy. Fluorine is dissolved to form F– ions that are electrically neutralized with Na+ or Al3+. There is no evidence for association of fluorine with either Si4+ or Al3+ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si4+ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na+) reacting to form fluoride complexes. In this process an equivalent amount of Na+ (orginally required for Al-3+charge-balance) or Al3+ (originally required Na+ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.  相似文献   

6.
The effect of composition and temperature on the relaxed adiabatic bulk modulus of melts in the P2O5-Al2O 3-Na2SiO3 system have been investigated in the temperature range of 1140 to 1450 °C using ultrasonic interferometric methods at frequencies of 3, 5 and 7 MHz. The density of these melts was determined using Pt-double-bob Archimedean densitometry techiques. P2O5 is known to dramatically affect the structure and the chemical and physical properties of granitic and pegmatitic melts as a function of the peralkalinity of the melt. The physical results of the structural changes occurring in Na2O-Al2O3-SiO2 melt upon the addition of P2O5 are observed by variations in the properties such as density and compressibility. For the present peralkaline melts, the bulk modulus and density decrease with addition of 15 mol% P2O5, and increase with the addition of 15 mol% Al2O3. The addition of P2O5 to the present melts results in a larger increase in melt compressibility than that observed with increasing polymerization between Na2SiO3 and Na2Si2O5 melts. This would suggest that not only is the polymerization of the melt increasing with the addition of P2O5 (Mysen et al. 1981; Nelson and Tallant 1984; Gan and Hess 1992), but that the tetrahedrally co-ordinated phosphorus complexes are influencing the bond lengths and energies within the melt structure; resulting in the structure becoming more compressible than expected, although incompressible (Vaughan and Weidner 1987) tetrahedral P2O5 polyhedra (Mysen et al. 1981; Gan and Hess 1992; Toplis et al. 1994) are being added to the melt structure.  相似文献   

7.
Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl2Si2O8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al2O3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well.  相似文献   

8.
The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na2O-Fe2O3-Al2O3-SiO2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na2O + FeO) < (Al2O3 + Fe2O3)] to peralkaline [(Na2O + FeO) > (Al2O3 + Fe2O3)] were determined over a temperature range 560-890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 108.8 to 1012.0 Pa s. The compositions are based on addition of FeF3 and FeCl3 to aluminosilicate melts with a fixed amount of SiO2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts. Am. Mineral.91, 344-352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.  相似文献   

9.
Information about the state of sulfur in silicate melts and glasses is important in both earth sciences and materials sciences. Because of its variety of valence states from S2− (sulfide) to S6+ (sulfate), the speciation of sulfur dissolved in silicate melts and glasses is expected to be highly dependent on the oxygen fugacity. To place new constraint on this issue, we have synthesized sulfur-bearing sodium silicate glasses (quenched melts) from starting materials containing sulfur of different valence states (Na2SO4, Na2SO3, Na2S2O3 and native S) using an internally heated gas pressure vessel, and have applied electron-induced SKα X-ray fluorescence, micro-Raman and NMR spectroscopic techniques to probe their structure. The wavelength shift of SKα X-rays revealed that the differences in the valence state of sulfur in the starting compounds are largely retained in the synthesized sulfur-bearing glasses, with a small reduction for more oxidized samples. The 29Si MAS NMR spectra of all the glasses contain no peaks attributable to the SiO4-nSn (with n > 0) linkages. The Raman spectra are consistent with the coexistence of sodium sulfate (Na2SO4) species and one or more types of more reduced sulfur species containing S-S linkages in all the sulfur-bearing silicate glasses, with the former dominant in glasses produced from Na2SO4-doped starting materials, and the latter more abundant in more reduced glasses. The 29Si MAS NMR and Raman spectra also revealed changes in the silicate network structure of the sulfur-bearing glasses, which can be interpreted in terms of changes in the chemical composition and sulfur speciation.  相似文献   

10.
The effect of CaO, Na2O, and K2O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe3+/Fe2+ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe3+/Fe2+ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO2–TiO2–Al2O3–FeO–Fe2O3–MgO–CaO–Na2O–K2O–P2O5 melts at air conditions.  相似文献   

11.
The solubility behavior of phosphorus in glasses and melts in the system Na2O-Al2O3-SiO2-P2O5 has been examined as a function of temperature and Al2O3 content with microRaman spectroscopy. The Al2O3 was added (2, 4, 5, 6, and 8 mol% Al2O3) to melts with 80 mol% SiO2 and ∼2 mol% P2O5. The compositions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorous-bearing glasses and melts between 25 and 1218 °C. The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm−1 assigned to P=O stretching in orthophosphate complexes together with a weaker band near 1000 cm−1 assigned P2O7 complexes. With increasing Al content, the proportion of P2O7 initially increases relative to PO4 and is joined by AlPO4 complexes which exhibit a characteristic P-O stretch mode slightly above 1100 cm−1. The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses. When P-bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO4/P2O7 so that in the molten state, PO4 units are barely discernible. The P2O7/AlPO4 abundance ratio in peralkaline compositions increases with increasing temperature. This decrease in PO4/P2O7 with increasing temperature results in depolymerization of the silicate melts. Dissolved P2O5 in peraluminous glass and melts forms AlPO4 complexes only. This solution mechanism has no discernible influence on the aluminosilicate melt structure. There is no effect of temperature on this solution mechanism. Received: 7 October 1997 / Accepted: 11 May 1998  相似文献   

12.
The enthalpies of solution of La2O3, TiO2, HfO2, NiO and CuO were measured in sodium silicate melts at high temperature. When the heat of fusion was available, we derived the corresponding liquid-liquid enthalpies of mixing. These data, combined with previously published work, provide insight into the speciation reactions in sodium silicate melts. The heat of solution of La2O3 in these silicate solvents is strongly exothermic and varies little with La2O3 concentration. The variation of heat of solution with composition of the liquid reflects the ability of La(III) to perturb the transient silicate framework and compete with other cations for oxygen. The enthalpy of solution of TiO2 is temperature-dependent and indicates that the formation of Na-O-Si species is favored over Na-O-Ti at low temperature. The speciation reactions can be interpreted in terms of recent spectroscopic studies of titanium-bearing melts which identify a dual role of Ti4+ as both a network-former end network-modifier. The heats of solution of oxides of transition elements (Ni and Cu) are endothermic, concentration-dependent and reach a maximum with concentration. These indicate a charge balanced substitution which diminishes the network modifying role of Na+ by addition of Ni2+ or Cu2+. The transition metal is believed to be in tetrahedral coordination, charge balanced by the sodium cation in the melts.  相似文献   

13.
Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt.The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations (NBOT) between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall NBOT, change as a function of the bulk composition of the melt.It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients (Kcrystal?liqi = CcrystaliCliqi) decrease linearly with increasing NBOT. The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to NBOT, so that the degree of light REE enrichment of the melts would depend on their NBOT.The solution mechanisms of minor oxides such as CO2, H2O, TiO2, P2O5 and Fe2O3 in silicate melts are known. These data have been recast as changes of NBOT of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated.  相似文献   

14.
Hydrothermal experiments were carried out at 2 kbar water pressure, 700 °–800 ° C, with the objective of determining the level of dissolved Zr required for precipitation of zircon from melts in the system SiO2-Al2O3-Na2O-K2O. The saturation level depends strongly upon molar (Na2O + K2O)/Al2O3 of the melts, with remarkably little sensitivity to temperature, SiO2 concentration, or melt Na2O/ K2O. For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation. In peralkaline melts, however, zircon solubility shows pronounced, apparently linear, dependence upon (Na2O + K2O)/Al2O3, with the amount of dissolvable Zr ranging up to 3.9 wt.% at (Na2O + K2O)/Al2O3 = 2.0. Small amounts (1 wt.% each) of dissolved CaO and Fe2O3 cause a 25% relative reduction of zircon solubility in peralkaline melts.The main conclusion regarding zirconium/zircon behavior in nature is that any felsic, non-peralkaline magma is likely to contain zircon crystals, because the saturation level is so low for these compositions. Zircon fractionation, and its consequences to REE, Th, and Ta abundances must, therefore, be considered in modelling the evolution of these magmas. Partial melting in any region of the Earth's crust that contains more than 100 ppm Zr will produce granitic magmas whose Zr contents are buffered at constant low (< 100 ppm) values; unmelted zircon in the residual rock of such a melting event will impart to the residue a characteristic U- or V-shaped REE abundance pattern. In peralkaline, felsic magmas such as those that form pantellerites and comendites, extreme Zr (and REE, Ta) enrichment is possible because the feldspar fractionation that produces these magmas from non-peralkaline predecessors does not drive the melt toward saturation in zircon.Zircon solubility in felsic melts appears to be controlled by the formation of alkali-zirconosilicate complexes of simple (2:1) alkali oxide: ZrO2 stoichiometry.  相似文献   

15.
The structure of H2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na2O·4SiO2–H2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H2O at nominal densities, ρfluid, (from EOS of pure H2O) of 0.90 and 0.78 g/cm3. With ρfluid = 0.78 g/cm3, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρfluid = 0.90 g/cm3, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na2O·4SiO2 starting material.Molecular H2O (H2O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H2O)-ratio is positively correlated with temperature. The OH/(H2O)° in melts is greater than in coexisting fluids. Structural units of Q3, Q2, Q1, and Q0 type are observed in all phases under all conditions. An expression of the form, 12Q3 + 13H2O2Q2 + 6Q1 + 4Q0, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol.  相似文献   

16.
57Fe Mössbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M+-cations are available to balance the charge of both M+Fe3+O2 and M+AlO2 complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe3+ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe2+. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe2+ are depolymerized for increasing mean Z/r 2 of the network modifiers, which do not stabilize M3+ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe2+ among these sites and thereby to an ordering in the network of melts.  相似文献   

17.
The investigation of rocks, minerals, and melt inclusions showed that porphyritic alkaline picrites and meimechites crystallized from different parental magmas. At a similar ultrabasic composition, the alkaline picrite melts were enriched in K2O relative to Na2O, and contained up to 0.12–0.13 wt % F and less Cr, Ni, and H2O (only 0.01–0.16 wt % H2O, versus 0.6–1.6 wt % in the meimechite melts) compared with the meimechite magmas. The crystallization of alkaline picrite melts occurred under stable conditions at relatively low temperatures without abrupt changes: olivine and clinopyroxene crystallized at 1340–1285 and 1230–1200°C, respectively, as compared with 1600–1450 and 1230–1200°C in the meimechites. The alkaline picrite melts evolved toward melanephelinite, nephelinite, tephrite, and trachydolerite; whereas the meimechite magmas gave rise to subalkaline picritic rocks. The partitioning of vanadium between olivine and melt suggests that the meimechite magma crystallized under more oxidizing conditions compared with the alkaline picrite melts: the KDV values for the meimechite melts (0.011–0.016) were three times lower than those for the alkaline picrite melts (0.045–0.052). The parental magmas of the alkaline picrites and meimechites were enriched in trace elements relative to mantle levels by factors of tens to hundreds. The alkaline picrite magma showed lower LILE and LREE contents compared with the meimechite magma. The magmas had also different indicator ratios of incompatible elements, including those immobile in aqueous fluids. It was concluded that the meimechite and alkaline picrite melts were derived from different mantle sources. The former were generated at lower degrees of melting of an undepleted mantle source, and the meimechite melts were produced by high-degree melting of a probably lherzolite-harzburgite source.  相似文献   

18.
The effect of composition on the relaxed adiabatic bulk modulus (K0) of a range of alkali- and alkaline earth-titanosilicate [X 2 n/n+ TiSiO5 (X=Li, Na, K, Rb, Cs, Ca, Sr, Ba)] melts has been investigated. The relaxed bulk moduli of these melts have been measured using ultrasonic interferometric methods at frequencies of 3, 5 and 7 MHz in the temperature range of 950 to 1600°C (0.02 Pa s < s < 5 Pa s). The bulk moduli of these melts decrease with increasing cation size from Li to Cs and Ca to Ba, and with increasing temperature. The bulk moduli of the Li-, Na-, Ca- and Ba-bearing metasilicate melts decrease with the addition of both TiO2 and SiO2 whereas those of the K-, Rb- and Cs-bearing melts increase. Linear fits to the bulk modulus versus volume fraction of TiO2 do not converge to a common compressibility of the TiO2 component, indicating that the structural role of TiO2 in these melts is dependent on the identity of the cation. This proposition is supported by a number of other property data for these and related melt compositions including heat capacity and density, as well as structural inferences from X-ray absorption spectroscopy (XANES). The compositional dependence of the compressibility of the TiO2 component in these melts explains the difficulty incurred in previous attempts to incorporate TiO2 in calculation schemes for melt compressibility. The empirical relationship KV-4/3 for isostructural materials has been used to evaluate the compressibility-related structural changes occurring in these melts. The alkali metasilicate and disilicate melts are isostructural, independent of the cation. The addition of Ti to the metasilicate composition (i.e. X2TiSiO5), however, results in a series of melts which are not isostructural. The alkaline-earth metasilicate and disilicate compositions are not isostructural, but the addition of Ti to the metasilicate compositions (i.e. XTiSiO5) would appear, on the basis of modulus-volume systematics, to result in the melts becoming isostructural with respect to compressibility.  相似文献   

19.
Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite, and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studying by means of their homogenization and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO2 concentrations of the melts vary within a broad range of 45–74 wt %, as also are the concentrations of other major components. According to their SiO2, Na2O, K2O, TiO2, and P2O5 concentrations, the melts are classified into seven groups. The mafic melts (45–53 wt % SiO2) comprise the following varieties: potassic (on average 4.2 wt % K2O, 1.7 wt % Na2O, 1.0 wt % TiO2, and 0.20 wt % P2O5), sodic (3.2% Na2O, 1.1% K2O, 1.1% TiO2, and 0.40% P2O5), and titaniferous with high P2O5 concentrations (2.2% TiO2, 1.1% P2O5, 3.8% Na2O, and 3.0% K2O). The melts of intermediate composition (53–64% SiO2) also include potassic (5.6% K2O, 3.4% Na2O, 1.0% TiO2, and 0.4% P2O5) and sodic (4.3% Na2O, 2.8% K2O, 1.3% TiO2, and 0.4% P2O5) varieties. The acid melts (64–74% SiO2) are either potassic (4.5% K2O, 3.6% Na2O, 0.7% TiO2, and 0.15% P2O5) or sodic (4.5% Na2O, 3.1% K2O, 0.7% TiO2, and 0.13% P2O5). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with very high F concentrations (up to 2.7 wt %, 1.19 wt % on average, 17 analyses) in the Kuriles and Kamchatka. The average F concentration in the sodic melts is 0.16 wt % (37 analyses). The melts are distinguished for their richness in various groups of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features. The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations, because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K2O concentrations (K2O/Na2O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii, Bezymyannyi, Bol’shoi Semyachek, Dikii Greben’, Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and Tumrok Ranges.  相似文献   

20.
Incremental amounts of Na2O and K2O added to immiscible melts in the MgO-CaO-TiO2-Al2O3 SiO2 system cause a decrease in critical temperature, phase separation and change in the pattern of Al2O3 partitioning. Al2O3, which is concentrated in the low SiO2 immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO2 immiscible melt as the alkali/aluminium ratio is increased. However, K2O is more effective than Na2O in stabilizing Al2O2 in the SiO2-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl2O4+2SiOK=2KAlO2+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO4 tetrahedra. The increased stability of the AlO4 species in the highly polymerized SiO2-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO4 species. Element partition systematics indicate that (Na, K)AlO2 species favor the more polymerized, CaAl2O4 and TiO2 species, the less polymerized silicate structure in the melt.  相似文献   

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