首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The compositional dependence of the redox ratio (FeO/FeO1.5) has been experimentally determined in K2O-Al2O3-SiO2-Fe2O3-FeO (KASFF) and K2O-CaO-Al2O3-SiO2-Fe2O3-FeO (KCASFF) silicate melts. Compositions were equilibrated at 1,450° C in air, with 78 mol % SiO2. KASFF melts have from 1 to 5 mol % Fe2O3 and include both peraluminous (K2O2O3) and peralkaline (K2O>Al2O3) compositions. KCASFF melts have 1 mol % Fe2O3 encompassing peraluminous, metaluminous (CaO+K2O>Al2O3) and peralkaline compositions. Peralkaline KASFF melts with 1 mol % Fe2O3 have low and constant values for the redox ratio, whereas in peraluminous melts the redox ratio increases with increasing (K2O/Al2O3). Increasing total iron concentration increases the redox ratio in peraluminous melts and slightly decreases the redox ratio in peralkaline melts. Substituting CaO for K2O at fixed total iron (1 mol %) increases the redox ratio in both peraluminous and metaluminous KCASFF melts; however, the redox ratio in peralkaline KCASFF melts is not affected by this exchange. These data indicate that Fe3+ is in four-fold coordination, with K+ or Ca2+ providing local charge balance. The tetrahedral ferric species is most stable in peralkaline melts and least stable in peraluminous melts, due to the competition between Al3+ and Fe3+ for charge balancing cations in the latter melt. Tetrahedral Fe3+ is also less stable when Ca2+ provides local charge balance. The data are consistent with a network modifying role for Fe2+ in the melt.The data are interpreted to reflect the effects of melt composition on the partitioning of K+ and Ca2+ and Fe3+ and Al3+ between various species in the melt. These relationships are discussed in terms of homogeneous equilibria between various iron-bearing and iron-free melt species. The results also reflect the effect of liquid composition on the exchange potentials Fe3+ Al–1 and Ca0.5K–1. The exchange potentials are relatively constant in peralkaline melts, but decrease in metaluminous and peraluminous melts as both (CaO+K2O)/(CaO+K2O+Al2O3) and K2O/CaO decrease. These qualitative observations imply that minerals exhibiting these exchanges will also be similarly affected as liquid composition changes. Present address: Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, USA  相似文献   

2.
The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe3+/Fe2+ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe3+/Fe2+ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO2) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.  相似文献   

3.
The effect of TiO2 and P2O5 on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts at air conditions in the temperature range 1,400–1,550 °C at 1-atm total pressure. The base composition of the anorthite–diopside eutectic composition was modified with 10 wt % Fe2O3 and variable amounts of TiO2 (up to 30 wt %) or P2O5 (up to 20 wt %). Some compositions also contained higher SiO2 concentrations to compare the role of SiO2, TiO2, and P2O5 on the Fe3+/Fe2+ ratio. The ferric/ferrous ratio in experimental glasses was analyzed using a wet chemical technique with colorimetric detection of ferrous iron. It is shown that at constant temperature, an increase in SiO2, TiO2, and P2O5 content results in a decrease in the ferric/ferrous ratio. The effects of TiO2 and SiO2 on the Fe3+/Fe2+ ratio was found to be almost identical. In contrast, adding P2O5 was found to decrease ferric/ferrous ratio much more effectively than adding silica. The results were compared with the predictions from the published empirical equations forecasting Fe3+/Fe2+ ratio. It was demonstrated that the effects of TiO2 are minor but that the effects of P2O5 should be included in models to better describe ferric/ferrous ratio in phosphorus-bearing silicate melts. Based on our observations, the determination of the prevailing fO2 in magmas from the Fe3+/Fe2+ ratio in natural glasses using empirical equations published so far is discussed critically.  相似文献   

4.
This report presents a model predicting activities for NiO in a wide range of silicate melts that include the components SiO2, TiO2, Al2O3, MgO, FeO, CaO, Na2O, and K2O. The conceptual simplicity of this model, combined with its success in modeling complex variations in activity with melt composition, suggests that the approach may provide insight into the character of trace components in the melt. The model presented in this report considers NiO to exist as Ni2+ and O2? in the melt, and predicts the activity of NiO by modeling variations in both aNi2+ and aO2?. Activities of Ni2+ are modeled assuming that NiO mixes randomly with a hypothetical ‘mixing pool’ of cations dominated by cations of similar size and charge to Ni2+, mainly Fe2+, Mg2+, Ca2+, and Ni2+. aO2? is modeled as a function of total oxygen ? 2·network-forming cations, with the understanding that O2? in silicate melts exists in equilibrium with bridging and non-bridging oxygens through reactions of the type Si–O–Si + O2? → 2 Si–O. For illustration, the model is applied to reduced mafic lunar samples that may have equilibrated with a Ni-bearing metal phase.  相似文献   

5.
The Fe3+/Fetot of all Fe-bearing minerals has been analysed by Mössbauer spectroscopy in a suite of biotite-rich to biotite-free graphitic metapelite xenoliths, proxies of an amphibolite-granulite transition through progressive biotite melting. Biotite contains 9 to 16% Fe3+/Fetot, whereas garnet, cordierite and ilmenite are virtually Fe3+ -free (0–1% Fe3+/Fetot) in all samples, regardless of biotite presence. Under relatively reducing conditions (graphite-bearing assemblages), biotite is the only carrier of Fe3+ during high-temperature metamorphism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorporate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO2, according to the redox reaction:
Depending on the nature of the peritectic Fe-Mg mineral produced (garnet, cordierite, orthopyroxene), the CO2 can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxidation of graphite by iron reduction can account for the in situ generation of CO2, implying a consequential rather than causal role of CO2 in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO2 is present in some granulites although it is not required for their formation.  相似文献   

6.
The relationship between the redox ratio Fe+2/(Fe+2+Fe+3) and the K2O/(K2O + Al2O3) ratio (K2O*) were experimentally investigated in silicate melts with 78 mol% SiO2 in the system SiO2-Al2O3-K2O-FeO-Fe2O3, in air at 1,400° C. Quenched glass compositions were analyzed by electron microprobe and wet chemical microtitration techniques. Minimum values of the redox ratio were obtained at K2O*0.5. The redox ratio in peralkaline melts (K2O*>0.5) increases slightly with K2O* whereas this ratio increases dramatically in peraluminous melts (K2O*<0.5) as K2O is replaced by Al2O3. These data indicate that all Fe+3 (and Al+3) occur as tetrahedral species charge balanced with K+ in peralkaline melts. In peraluminous melts, Fe+3 (and Al+3) probably occur as both tetrahedral species using Fe+2 as a charge-balancing cation and as network-modifying cations associated with non-bridging oxygen.  相似文献   

7.
A novel complex continuous system of solid solutions involving vauquelinite Pb2Cu(CrO4)(PO4)(OH), bushmakinite Pb2Al(VO4)(PO4)(OH), ferribushmakinite Pb2Fe3+(VO4)(PO4)(OH), and a phase with the endmember formula Pb2Cu(VO4)(PO4)(H2O) or Pb2Cu(VO4)(РО3ОН)(ОН) is studied based on samples from the oxidation zone of the Berezovskoe, Trebiat, and Pervomaisko-Zverevsky deposits in the Urals, Russia. This is the first natural system in which chromate and vanadate anions show a wide range of substitutions and the most extensive solid solution system involving (CrO4)2– found in nature. The major couple substitution is Cr6+ + Cu2+ ? V5+ + M3+, where M = Fe, Al. The correlation coefficients calculated from 125 point analyses are: 0.96 between V and (Fe + Al), 0.96 between Cr and (Cu + Zn),–0.96 between V and (Cu + Zn),–0.97 between Cr and (Fe + Al), and–0.97 between (Fe + Al) and (Cu + Zn). The substitutions V5+ ? Cr6+ (correlation coefficient–0.98) and to a lesser extent P5+ ? As5+ (correlation coefficient–0.86) occur at two types of tetrahedral sites, whereas the metal–nonmetal/metalloid substitutions, i.e., V or Cr for P or As, are minor. The substitution Fe3+ ? Al3+ is also negligible in this solid solution system.  相似文献   

8.
The saturation surface of pseudobrookite (Fe2TiO5) was determined for melts in the system SiO2-Al2O3-K2O-FeO-Fe2O3-TiO2 at 1400° C and 1 atm. The variation in concentrations of Fe2O3, TiO2 and Fe2TiO5 in liquids can be used to infer relative changes in activity coefficients of these components with changing K2O/(K2O+Al2O3) of the melts. Saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing K2O/(K2O+Al2O3) in peralkaline liquids. The activity coefficients of Fe2O3 and TiO2 and Fe2TiO5, therefore, are higher in peraluminous liquids than in peralkaline liquids in this system. In addition, the iron redox ratio was measured as a function of K2O/(K2O+Al2O3) for liquids just below the saturation surface; was fixed so all variations in redox ratio are entirely due to changes in melt composition. The redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible. The homogeneous equilibrium experiments indicate that the activity coefficient of Fe2O3 relative to that of FeO is significantly greater in peraluminous melts than peralkaline melts. Both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids K+in excess of that required to charge balance tetrahedral Al+3 is used to stabilize both Fe+3 and Ti+4. Calculations show that ferric iron and titanium compete equally effectively for charge-balancing potassium but neither can outcompete aluminum. The observed changes in solution properties of Fe2O3 and TiO2 in the synthetic melts are used to explain variations in Fe-Ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites. Since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids, Fe-Ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites. In addition, iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable P, T, and . Finally, observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies.  相似文献   

9.
Mossbauer spectroscopy has been used to determine the redox equilibria of iron and structure of quenched melts on the composition join Na2Si2O5-Fe2O3 to 40 kbar pressure at 1400° C. The Fe3+/ΣFe decreases with increasing pressure. The ferric iron appears to undergo a gradual coordination transformation from a network-former at 1 bar to a network-modifier at higher (≧10 kbar) pressure. Ferrous iron is a network-modifier in all quenched melts. Reduction of Fe3+ to Fe2+ and coordination transformation of remaining Fe3+ result in depolymerization of the silicate melts (the ratio of nonbridging oxygens per tetrahedral cations, NBO/T, increases). It is suggested that this pressure-induced depolymerization of iron-bearing silicate liquids results in increasing NBO/T of the liquidus minerals. Furthermore, this depolymerization results in a more rapid pressure-induced decrease in viscosity and activation energy of viscous flow of iron-bearing silicate melts than would be expected for iron-free silicate melts with similar NBO/T.  相似文献   

10.
Cordierite–orthoamphibole gneisses and rocks of similar composition commonly contain low‐variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO–MgO–Al2O3–SiO2–H2O (FMASH), Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) and Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite–orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calculations in NCKFMASH and NCKFMASHTO highlight the role of minor constituents such as Ca, Na and Fe3+ in the mineral assemblage evolution of such rocks and these systems are more appropriate for interpreting the evolution of natural examples.  相似文献   

11.
Sludge samples taken from different sources and times may have different characteristics that could affect dewatering performance. In this study, 20 sludge samples from five wastewater treatment plants and different seasons in 1 year were characterized. Pearson correlation analysis indicated that solid content (SC), total suspended solid (TSS), polysaccharides and proteins contents had positive correlations with the capillary suction time (CST), whereas volatile suspended–solid/total suspended solid (VSS/TSS) exhibited negative correlations with CST. Moreover, no correlations between CST and specific resistance to filtration were found among these different sludge samples. The principal component analysis confirmed that only two group variables could represent most of the sludge characteristic parameters. The first set of variables represents the particulate nature of the biotic factors (SC, VSS/TSS, SCOD, TSS, polysaccharides and proteins), and the second set is the pH. CST could not be a reasonable indicator of dewaterability in sludge deep dewatering by Fe2+/S2O8 2?-phosphogypsum composite conditioning. Furthermore, the results of diaphragm filter press dewatering showed that initial SC and VSS/TSS were the most dominant sludge characteristics affecting the solid content of dewatered cake (R p = 0.610, p = 0.016; R p = ?0.838, p = 0.000, respectively) with Fe2+/S2O8 2?-phosphogypsum composite conditioning. Results from this study suggest that dewatering performance is predictable by sludge characteristics parameters for Fe2+/S2O8 2?-phosphogypsum conditioning.  相似文献   

12.
Zinclipscombite, a new mineral species, has been found together with apophyllite, quartz, barite, jarosite, plumbojarosite, turquoise, and calcite at the Silver Coin mine, Edna Mountains, Valmy, Humboldt County, Nevada, United States. The new mineral forms spheroidal, fibrous segregations; the thickness of the fibers, which extend along the c axis, reaches 20 μm, and the diameter of spherulites is up to 2.5 mm. The color is dark green to brown with a light green to beige streak and a vitreous luster. The mineral is translucent. The Mohs hardness is 5. Zinclipscombite is brittle; cleavage is not observed; fracture is uneven. The density is 3.65(4) g/cm3 measured by hydrostatic weighing and 3.727 g/cm3 calculated from X-ray powder data. The frequencies of absorption bands in the infrared spectrum of zinclipscombite are (cm?1; the frequencies of the strongest bands are underlined; sh, shoulder; w, weak band) 3535, 3330sh, 3260, 1625w, 1530w, 1068, 1047, 1022, 970sh, 768w, 684w, 609, 502, and 460. The Mössbauer spectrum of zinclipscombite contains only a doublet corresponding to Fe3+ with sixfold coordination and a quadrupole splitting of 0.562 mm/s; Fe2+ is absent. The mineral is optically uniaxial and positive, ω = 1.755(5), ? = 1.795(5). Zinclipscombite is pleochroic, from bright green to blue-green on X and light greenish brown on Z (X > Z). Chemical composition (electron microprobe, average of five point analyses, wt %): CaO 0.30, ZnO 15.90, Al2O3 4.77, Fe2O3 35.14, P2O5 33.86, As2O5 4.05, H2O (determined by the Penfield method) 4.94, total 98.96. The empirical formula calculated on the basis of (PO4,AsO4)2 is (Zn0.76Ca0.02)Σ0.78(Fe 1.72 3+ Al0.36)Σ2.08[(PO4)1.86(AsO4)0.14]Σ2.00(OH)1. 80 · 0.17H2O. The simplified formula is ZnFe 2 3+ (PO4)2(OH)2. Zinclipscombite is tetragonal, space group P43212 or P41212; a = 7.242(2) Å, c = 13.125(5) Å, V = 688.4(5) Å3, Z = 4. The strongest reflections in the X-ray powder diffraction pattern (d, (I, %) ((hkl)) are 4.79(80)(111), 3.32(100)(113), 3.21(60)(210), 2.602(45)(213), 2.299(40)(214), 2.049(40)(106), 1.663(45)(226), 1.605(50)(421, 108). Zinclipscombite is an analogue of lipscombite, Fe2+Fe 2 3+ (PO4)2(OH)2 (tetragonal), with Zn instead of Fe2+. The mineral is named for its chemical composition, the Zn-dominant analogue of lipscombite. The type material of zinclipscombite is deposited in the Mineralogical Collection of the Technische Universität Bergakademie Freiberg, Germany.  相似文献   

13.
Abstract: Five groups of the Phanerozoic granitoids in South Korea can be deduced from their temporal and spatial distributions: (1) Jurassic granitoids in the Gyeonggi massif, (2) Permo-Jurassic granitoids in the Ogcheon belt, (3) Permo-Jurassic granitoids in the Yeongnam massif, (4) Cretaceous granitoids in the Ogcheon belt, and (5) Cretaceous granitoids in the Gyeongsang basin. Though the granitoids of each group generally show calc-alkaline and orogenic natures, the petrological, geochemical and genetical features are different with each other. The Permo-Jurassic granitoids in the Ogcheon belt have lower contents of Al2O3, Fe2O3, CaO, P2O5, but higher of FeO, FeOT, MgO, K2O than those in the Yeongnam massif. From higher feature of K2O, Na2O+K2O and K2O/Na2O, the Ogcheon belt seems to have been located at closer continent side relative to the Yeongnam massif during Permo-Jurassic time. From lower values of Fe2O3/FeO and magnetic susceptibility the granitoids of the Ogcheon belt had been solidified under more reducing environment than those of the Yeongnam massif. The Cretaceous granitoids in the Ogcheon belt have lower contents of TiO2, Fe2O3, FeO, FeOT, CaO and P2O5, but higher of MgO, K2O, Na2O+K2O and K2O/Na2O than those in the Gyeongsang basin. This feature indicates that the Ogcheon belt would correspond to the continental environment of magma genesis during Cretaceous time. Higher values of Fe2O3/FeO and magnetic susceptibility in the Cretaceous granitoids in the Gyeongsang basin suggest that the granitoids had been solidified under highly oxidizing environment. From the particular chemical features of K2O, Na2O+K2O and K2O/Na2O, the Permo-Jurassic granitoids in the Ogcheon belt, the Yeongnam massif as well as the Cretaceous ones in the Gyeongsang basin can be categorized to the continental margin type granite. The Jurassic granitoids in the Gyeonggi massif are possibly of collision type, and the Cretaceous granitoids in the Ogcheon belt of post–orogenic, intra–conti–nent type. The Jurassic granitoids in the Gyeonggi massif had been possibly generated by crustal melting during the collision of Gyeonggi massif to the northern Pyeongnam basin block. The Cretaceous granitoids in the Ogcheon belt had been emplaced at the hinterland of the continental margin during post-orogenic stage of the Honam Shear Zone. The Cretaceous granitoids in the Gyeongsang basin are often compared with Japanese Cretaceous?Paleogene granitoids in their geochemical and genetical natures. For the granitoid composition, the granitoids in the Gyeongsang basin are higher in Fe2O3, Fe2O3/FeO, Na2O, K2O, Na2O+K2O and K2O/Na2O, but lower in Al2O3, FeO, MnO, CaO and P2O5 than the Japanese granitoids. The contents of TiO2, FeOT and MgO are similar in both granitoids. This geochemical contrast would imply that the Cretaceous granitoid magmas in the Gyeongsang basin had been originated at closer place to the continent side under more tensional field, and solidified under more oxidizing environment than the coeval Japanese granitoid magmas.  相似文献   

14.
The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al2O3–K2O–FeO–Fe2O3–P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3–O–P+5 and Al+3–O–P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K–O–Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.  相似文献   

15.
Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi2O6 and LiCrSi2O6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe2+(M1, M2) and intervalence Fe2+/Fe3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr3+ in the crystal structures. The positions of the spin-allowed bands of Cr3+, especially of the low energy one caused by the electronic 4 A 2g → 2 T 1g transition, are found to be in accordance with mean M1–O distances. The local relaxation parameter ε calculated for limCr 3+ → 0 from the spectra and interatomic á Cr - O ñ \left\langle {Cr - O} \right\rangle and á Mg - O ñ \left\langle {Mg - O} \right\rangle distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the “guest” ion, Cr3+, deviates greatly from the “diffraction” value, ε = 0, than in any other known Cr3+-bearing systems studied so far. Under pressure the spin-allowed bands of Cr3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO6 octahedral modulus, k\textpoly\textloc k_{\text{poly}}^{\text{loc}} , derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi2O6 kosmochlor are very close, ~203 and ~196 GPa, respectively, being, however, nearly twice higher than that of MgO6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (2008). Such a strong stiffening of the structural octahedron, i.e. twice higher value of k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} comparing with that of k\textMg2 + \textloc k_{{{\text{Mg}}^{2 + } }}^{\text{loc}} , may be caused by simultaneous substitution of Ca2+ by larger Na+ in the neighboring M2 sites at so-called jadeite-coupled substitution Mg2+ + Ca2+ → Cr3+ + Na+. It is also remarkable that the values of CrO6 octahedral modulus of NaCrSi2O6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement by Origlieri et al. (2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility of M1- and M2-sites, our k\textCr3 + \textloc k_{{{\text{Cr}}^{3 + } }}^{\text{loc}} -value, ~196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa.  相似文献   

16.
The dependence of water concentration in synthetic (Mg, Fe2+)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe2+/(Fe2+ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H2O molecules in the structural channels of alkali-free (Mg, Fe2+)-cordierite indicate that the H-H vector of some H2O molecules (H2O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe2+)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.  相似文献   

17.
Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18 .x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000?°C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm?1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.  相似文献   

18.
Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by 57Fe Mössbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. 57Fe Mössbauer spectra are consistent with the assumption that high-spin Fe3+ substitutes for Al in the octahedrally coordinated site. The Mössbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with 57Fe hyperfine parameters typical for octahedrally coordinated high-spin Fe2+. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe3+ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe3+ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe2+. However, it cannot be excluded that Fe2+ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H2O molecule.  相似文献   

19.
Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.  相似文献   

20.
Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10?3 SI), molar Al2 O 3/(CaO + Na2 O + K 2O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An10–An31) and EPG (An15–An33) represents oligoclase to andesine and TLg (An2–An15) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or88 to Or95 in GGn, Or86 to Or97 in EPG and Or87 to Or94 in TLg). Biotites in GGn (Mg/Mg + Fet= 0.34–0.45), EPG (Mg/Mg + Fet= 0.27–0.47), and TLg (Mg/Mg + Fet= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe\(\rightleftharpoons \)2Al, 3Mg\(\rightleftharpoons \)2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fet=0.58–0.66), EPG (Mg/Mg + Fet=0.31?0.59), and TLg (Mg/Mg + Fet=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (～13, ～17 and ～13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号