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1.
High-pressure, low-temperature metamorphic Mn-rich quartzites from Andros and Evvia (Euboea) islands, Greece, situated in the Eocene blueschist belt of the Hellenides, reveal different Mn-Al-Ca-Mg-silicate assemblages in response to variable metamorphic grade. On Evvia, piemontite- and/or braunite-rich quartzites which are associated with low-grade blueschists (T<400° C, P> 8 kbar) show the principle mineral assemblage quartz + montite + sursassite + braunite + Mg-chlorite + hematite + rutile + titanite. The Mn-Al-silicate sursassite, basically (Mn2+, Ca)4 Al2(Al, Fe3+, Mn3+, Mg)4Si6O21(OH)7, thus far reported as a rare mineral, locally occurs as a rockforming mineral in cm- to m-thick layers. On Andros, higher-grade quartzites (T450–500° C, P>10 kbar) of similar composition contain the assemblage quartz + piemontite + spessartine + braunite + Mg-chlorite+hematite + phengite+ phlogopite + rutile. Rare sursassite is present only as a relict phase. Additional, mostly accessory minerals in quartzites from Evvia and Andros are ardennite, Na-amphibole, acmitic clinopyroxene, albite, apatite, and tourmaline. The chemical composition of the main phases is characterized in detail.Disequilibrium textures and mineral compositions in some samples from Andros and Evvia imply the reactions sursassite + braunite + quartz = spessartine+clinochlore±hematite + H2O + O2 (1) sursassite + braunite + phengite + quartz = spessartine + phlogopite±hematite + H2O + O2 (2) and in braunite-free assemblages sursassite + Mn3+Fe –1 3+ [hematite, piemontite] + hematite + quartz = spessartine + clinochlore + H2O+O2 (3) Reactions (1) to (3) have positive P-T slopes. They are considered to account for the breakdown of sursassite and the formation of spessartine during prograde metamorphism of the piemontite quartzites and related rocks. P-T data from Andros and Evvia and geological data from few other occurrences reported suggest sursassite+ quartz±braunite to be stable at T<400–450° C over a considerable pressure interval at least up to 10 kbar. Theoretical phase relations among Mn3+-Mn2+-silicates in the pseudoquaternary system Al-Mn-Ca-Mg with excess quartz, H2O, and O2 indicate that low-grade assemblages containing sursassite (±braunite±pumpellyite±viridine±piemontite + quartz) are likely precursors of higher-grade assemblages including spessartine, Mg-chlorite, braunite, viridine, and piemontite reported from greenschist-, amphibolite-, and high-grade blueschist-facies rocks of appropriate composition.  相似文献   

2.
 Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO2=NNO+2.3. The space group at room T is P21/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)∘, V=415.2 ?3]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi2O6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding M2(Li0.85Mg0.09Fe2+ 0.06) M1(Fe3+ 0.85Mg0.15)Si2O6. Li is ordered at the M2 site and Fe3+ is ordered at the M1 site, whereas Mg (and Fe2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P21/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P21/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes. Received: 7 May 2002 / Accepted: 23 October 2002  相似文献   

3.
The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe2O3 (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe2O3‐saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe2+‐Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.  相似文献   

4.
A new thermodynamic formulation of the Fe–Ti oxide geothermometer/oxygen barometer is developed. The method is based upon recently calibrated models for spinel solid solutions in the quinary system (Fe2+, Mg)(Al,Fe3+,Cr)2O4–(Fe2+, Mg)2TiO4 by Sack and Ghiorso, and rhombohedral oxides in the quaternary system (Fe2+,Mg,Mn)TiO3–Fe2O3 (this paper). The formulation is internally consistent with thermodynamic models for (Fe2+,Mg)-olivine and -orthopyroxene solid solutions and end-member thermodynamic properties tabulated by Berman. The constituent expressions account for compositional and temperature dependent cation ordering and reproduce miscibility gap features in all of the component binaries. The calibration does not account for the excess Gibbs energy resulting from compositional and temperature dependent magnetic ordering in either phase. This limits application of the method to assemblages that equilibrated at temperatures above 600° C. Practical implementation of the proposed geothermometer/oxygen barometer requires minimal use of projection algorthms in accommodating compositions of naturally occurring phases. The new formulation is applied to the estimation of temperature and oxygen fugacity in a wide variety of intermediate to silicic volcanic rocks. In combination with previous work on olivine and orthopyroxene thermodynamics, equilibration pressures are computed for a subset of these volcanics that contain the assemblage quartz, oxides and either ferromagnesian silicate. The calculated log10 f O 2-T relations are reflected in coexisting ferromagnesian mineral assemblages. Volcanics with the lowest relative oxygen fugacity (log10 f O 2) are characterized by the assemblage olivine-quartz, those with slightly higher log10 f O 2 s, by the assemblage orthopyroxene-quartz. The sequence proceeds with the necessary phases biotite-feldspar, then hornblende-quartz-clinopyroxene, and finally at the highest log10 f O 2 s, sphene-quartz-clinopyroxene. Quantitative analysis of these trends, utilizing thermodynamic data for the constituent phases, establishes that, in most cases, the T-log10 f O 2value computed from the oxides is consistent with the compositions of coexisting silicate phases, indicating that phenocryst equilibrium was achieved prior to eruption. There is, however, considerable evidence of oxide-silicate disequilibrium in samples collected from more slowly cooled domes and obsidians. In addition, T-log10 f O 2trends from volcanic rocks that contain biotite and orthopyroxene are interpreted to imply a condition of Fe2+–Mg exchange disequilibrium between orthopyroxene and coexisting ferromagnesian silicates and melt. It is suspected that many biotite-feldspar-quartz-orthopyroxene bearing low temperature volcanic rocks inherit orthopyroxene xenocrysts which crystallized earlier in the cooling history of the magma body.The problem is probably at least as complex as that of the feldspars... A.F. Buddington (1956)  相似文献   

5.
We define and calibrate a new model of molar volume as a function of pressure, temperature, ordering state, and composition for spinels in the supersystem (Mg, Fe2+)(Al, Cr, Fe3+)2O4 ? (Mg, Fe2+)2TiO4. We use 832 X-ray and neutron diffraction measurements performed on spinels at ambient and in situ high-P, T conditions to calibrate end-member equations of state and an excess volume model for this system. The effect on molar volume of cation ordering over the octahedral and tetrahedral sites is captured with linear dependence on Mg2+, Al3+, and Fe3+ site occupancy terms. We allow standard-state volumes and coefficients of thermal expansion of the end members to vary within their uncertainties during extraction of the mixing properties, in order to achieve the best fit. Published equations of state of the various spinel end members are analyzed to obtain optimal values of the bulk modulus and its pressure derivative, for each explicit end member. For any spinel composition in the supersystem, the model molar volume is obtained by adding excess volume and cation order-dependent terms to a linear combination of the five end-member volumes, estimated at pressure and temperature using the high-T Vinet equation of state. The preferred model has a total of 9 excess volume and order-dependent parameters and fits nearly all experiments to within 0.02 J/bar/mol, or better than 0.5 % in volume. The model is compared to the current MELTS spinel model with a demonstration of the impact of the model difference on the estimated spinel-garnet lherzolite transition pressure.  相似文献   

6.
Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.  相似文献   

7.
The reaction chloritoid (ctd)=almandine (alm)+diaspore+H2O (CAD) has been reversed using Fe3+-free synthetic chloritoid and almandine, under fO2 conditions of the solid oxygen buffer Fe/FeO (CADWI), and using partially oxidized synthetic minerals under fO2 conditions of the solid oxygen buffer Ni/NiO (CADNNO). Experiments have been conducted between 550 and 700°C, 25 and 45 kbar. The equilibrium pressure and temperature conditions are strongly dependent on the fO2 conditions (CADNNO lies some-what 50°C higher than CADWI). This can be explained by a decrease in aH2O for experiments conducted on the Fe/FeO buffer, and a decrease in actd and aalm (through incorporation of ferric iron preferentially in chloritoid) for experiments conducted on the Ni/NiO buffer. The H2O activity has been calculated using the MRK equation of state, and the values obtained checked against the shift of the equilibrium diaspore=corundum+H2O bracketed on the Fe/FeO buffer and under unbuffered fO2 conditions. For fO2 buffered by the assemblage Fe/FeO, aH2O increases with pressure from about 0.85 at 600°C, 12 kbar to about 0.9 at 605°C, 25 kbar and 1 above 28 kbar. For fO2 buffered by the assemblage Ni/NiO, aH2O=1. The aH2O decrease from Ni/NiO to Fe/FeO is, however, too small to be entirely responsible for the temperature shift between CADNNO and CADWI. In consequence, the amount of ferric iron in almandine and chloritoid growing in the CADNNO experiments must be significant and change along the CADNNO, precluding calculation of the thermodynamic properties of chloritoid from this reaction. Our experimental data obtained on the Fe/FeO buffer are combined, using a thermodynamic analysis, with Ganguly's (1969) reversal of the reaction chloritoid=almandine+corundum +H2O (CAC) on the same oxygen buffer. Experimental brackets are mutually consistent and allow extraction of the thermodynamic parameters H o f,ctd and S octd. Our thermodynamic data are compared with others, generally calculated using Ganguly's bracketing of CACNNO. The agreement between the different data sets is relatively good at low pressure, but becomes rapidly very poor toward high pressure conditions. Using our thermodynamic data for chloritoid and KD=(Fe3+/Al)ctd/(Fe3+/Al)alm estimated from natural assemblages, we have calculated the composition of chloritoid and almandine growing from CADNNO and CACNNO. The Fe3+ content in chloritoid and almandine increases with pressure, from less than 0.038 per FeAl2SiO5(OH)2 formula unit at 10 kbar to at least 0.2 per formula unit above 30 kbar. This implies that chloritoid and almandine do contain Fe3+ in most natural assemblages. The reliability of our results compared to natural systems and thermodynamic data for Mg-chloritoid is tested by comparing the equilibrium conditions for the reaction chloritoid+quartz=garnet (gt)+kyanite+H2O (CQGK), calculated for intermediate Fe–Mg chloritoid and garnet compositions, from the system FASH and from the system MASH. For 0.65<(XFe)gt<0.8, CQKG calculated from FASH and MASH overlap for KD=(Mg/Fe)ctd/(Mg/Fe)gt=2. This is in good agreement with the KD values reported from chloritoid+garnet+quartz+kyanite natural assemblages.  相似文献   

8.
Summary The principal mineral component in the matrix of the Cochabamba carbonaceous chondrite is a phyllosilicate, which is identified as cronstedtite mainly on the basis of its chemical composition. Its approximate idealized formula is given by M6 Fe 0.7 3+ Al0.5Si2.7O10 (OH)8 with M=Fe2+, Fe3+, and Mg in somewhat variable amounts. TEM studies reveal the presence of three polytypes, and show a high degree of stacking disorder parallel to (001) with the displacement vector ±b/3 or ±2b . Crumpled amorphous masses in the matrix may contain structural building blocks of phyllosilicates. They, rather than the anhydrous minerals, seem to be the most likely progenitors of cronstedtite. Some constraints on its origin are reviewed. In addition to cronstedtite, observations on some other matrix phases are also reported.
Chemische und kristallographische Untersuchung von Cronstedtit in der Matrix des kohligen Chondrits (CM2) Cochabamba
Zusammenfassung Der Hauptbestandteil der Matrix im kohligen Chondrit Cochabamba ist ein Schichtsilikat, das hauptsächlich aufgrund seiner chemischen Zusammensetzung als Cronstedtit identifiziert wurde. Die idealisierte Formel entspricht ungefähr M6Fe 0.7 +3 Al0.5Si2.7O10(OH)8 mit M=Fe2+, Fe3+ und Mg in wechselnden Mengen. TEM-Untersuchungen zeigen das Vorkommen von drei Modifikationen, sowie einen hohen Grad von Versetzungsfehlern parallel zu (001), mit dem Versetzungsfaktor ±b/3 oder ±2b/3. Deformierte amorphe Aggregate in der Matrix scheinen primitive Bausteine der Schichtsilikate zu sein. Sie (und nicht die wasserfreien Mineralien) dürften das Material darstellen, aus dem Cronstedtit gebildet wurde. Die Bildungsbedingungen von Cronstedtit werden diskutiert. Außerdem wird über Beobachtungen an anderen Matrixmineralien berichtet.

With 4 Figures  相似文献   

9.
A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.  相似文献   

10.
Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO4 solutions. Single-crystal XRD refinement with R1=0.0232 for the unique observed reflections (|Fo| > 4F) and wR2=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, Vo=160.11(4) cm3, and fractional positions for all atoms except the H in the H3O groups. The chemical composition of this sample is described by the formula (H3O)0.91Fe2.91(SO4)2[(OH)5.64(H2O)0.18]. The enthalpy of formation (Hof) is –3694.5 ± 4.6 kJ mol–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H2O, and -MgSO4. The entropy at standard temperature and pressure (So) is 438.9±0.7 J mol–1 K–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (Cp) data between 273 and 400 K were fitted to a Maier-Kelley polynomial Cp(T in K)=280.6 + 0.6149T–3199700T–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol–1. Speciation and activity calculations for Fe(III)–SO4 solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the Cp data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in Cp, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H3O)Fe3(SO4)2(OH)6 was estimated: Gof= –3226.4 ± 4.6 kJ mol–1, Hof=–3770.2 ± 4.6 kJ mol–1, So=448.2 ± 0.7 J mol–1 K–1, Cp (T in K)=287.2 + 0.6281T–3286000T–2 (between 273 and 400 K).  相似文献   

11.
The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg2Si2O6, CaMgSi2O6, CaFeSi2O6, and Fe2Si2O6. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi2O6-Mg2Si2O6 and CaFeSi2O6-Fe2Si2O6, (3) calorimetric data for CaMgSi2O6-Mg2Si2O6 pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe2+-Mg2+ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe2+ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.  相似文献   

12.
Experiments at high pressures and temperatures reveal the stability of a Fe4O5-type structured phase in several simple chemical systems. On the one hand, the Fe4O5 end-member is stable in the presence of SiO2-rich phases, including stishovite, but contains ≤0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the Fe4O5 phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P–T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% Mg2Fe2O5 component. The Fe4O5 phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing Fe4O5 has been synthesised with up to 46 mol% Fe2Cr2O5 component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for Fe2+ and Fe3+, respectively, leads to variations in Fe3+/∑Fe from the ideal value of 0.5 for the Fe4O5 end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that Fe4O5-type structured phases may be stable over a range of P–T–fO2 conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth’s transition zone. Thus, the presence of even small amounts of Fe3+ could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase.  相似文献   

13.
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   

14.
Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe4O5–Mg2Fe2O5 solid solutions and (2) to explore the phase relations involving (Mg,Fe)2Fe2O5 (denoted as O5-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O5-phase coexists with Fe2O3, hp-(Mg,Fe)Fe2O4, (Mg,Fe)3Fe4O9 or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O5-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe3+-bearing phase B. However, (Mg,Fe)2Fe2O5 does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe2+ and Fe3+ (at least ~?1.0 cations Fe per formula unit). Fe-L2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe3+/Fetot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si4+?+?[6]Mg2+?=?2Fe3+. The ability to accommodate Fe2+ and Fe3+ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg2Fe2O5 have been refined, yielding H°1bar,298?=???1981.5 kJ mol??1. Solid solution is complete across the Fe4O5–Mg2Fe2O5 binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe2+ with silicates indicates that (Mg,Fe)2Fe2O5 has a strong preference for Fe2+. Modelling of partitioning with olivine is consistent with the O5-phase exhibiting ideal mixing behaviour. Mg–Fe2+ partitioning between (Mg,Fe)2Fe2O5 and ringwoodite or wadsleyite is influenced by the presence of Fe3+ and OH incorporation in the silicate phases.  相似文献   

15.
Chemical composition, unit cell parameters, and trace elements of tourmalines from Mesozoic gold-quartz-sulphide and gold-bearing copper-porphyry ore-magmatic systems of the Trans-Baikal area and Mongolia show that they belong to the specific schorl-dravite highly ferruginous oxytourmaline series. They are low in alumina (Al2O3 = 16–33%) and have MgO contents (up to 10%) and Fe2O3 (1%). There is a direct correlation of unit cell parameters (a,c,V) with total iron, which permits composition estimates from X-ray diffraction analyses. As a rule, these tourmalines contain high concentrations of Au, Pb and Cu, which are mainly hosted by inclusions of native gold and ore minerals. The highest As abundances are contained in the tourmalines of the copper-porphyry field.Two trends of isomorphic replacement are related to increasing Fe content of oxyferruginous tourmalines:(1) “Acid leaching” trend (less ferruginous part of the series) Mg + Fe2+ + 4Al + 40 4Fe3+ + 2 + 4(OH,F); and (2) “conjugate deposition” trend Mg + 1.5Fe2+ + 1.5Al + 4(OH,F) 4Fe3+ + 4O.These features distinguish tourmalines from gold-bearing systems from schorl-dravites of tin and rare-metal deposits. They may be used in metallogenic analyses, interpretation of the origin of primary and secondary anomalies, and assessment of the type and zonation of ore fields.  相似文献   

16.
A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO2-Al2O3-Fe2O3-FeO-MgO-CaO-Na2O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe3+,Al3+)Si2O8-Ca(Fe2+,Mg)Si3O8 system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeOt, MgO content and Mg/(Fet+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ǂ.34 wt% (relative error =9%) and ǂ.08 wt% (14%) and ǂ.7 (8%) respectively. Calculated Fe3+-Al exchange chemical potentials of plagioclase, mFe3 + ( Al )- 1 Pl{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} agree with those calculated using reported thermodynamic models for multicomponent spinel, mFe3 + ( Al )- 1 Sp{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} and clinopyroxene, mFe3 + ( Al )- 1 Cpx{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} . The FeOt content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe3+/(Fe3++Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase-liquid oxygen barometer are scattered, this study found that plagioclase-spinel-clinopyroxene-oxygen and plagioclase-olivine-oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and fO2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.  相似文献   

17.
 In Madagascar, hibonite occurs as a rather frequent mineral within thorianite-bearing skarns which are widespread in the Pan African granulitic formations constituting the S-E part of the Island (Tranomaro area). In these skarns, leucocratic segregations made up of CO3-scapolite to meionite (Anequivalent=89–95% which implies T≥850° C), spinel and corundum were formed at stage 1 of metasomatism in a titanite-bearing matrix consisting of scapolite (Aneq=77–88) and aluminous diopside. During stage 2 of metasomatism, scapolite from the lenses were altered to anorthite+calcite while the less calcic scapolite remained stable which indicates T≈800° C. Hibonite crystallized at the expense of corundum and spinel. Expressed as mol% of the CaAl12O19/Ca(Al10TiR2+)O19/REE(Al11R2+)O19 [+Th (Al10R2+ 2)O19] end-members (R 2+=Mg, Fe2+, Zn2+; Al=Al, Fe3+; Ti=Ti, Si), its composition varies from 26/72/2 to 50/23/27. The ideal activity of the CaAl12O19 component is about 0.25. Fluid inclusions in corundum, hibonite and anorthite are composed of nearly pure CO2. In corundum, the isochores for primary inclusions are in agreement with the P-T estimates for regional metamorphism and stage 1 metasomatism (T≈850° C, P≈5 kbar). Inclusions with the highest density in hibonite and anorthite constrain P to about 3–3.5 kbar for T=800° C. Thermodynamic calculations indicate that, in addition to a low activity of CaAl12O19, stability of hibonite in equilibrium with anorthite and calcite implies an extremely low activity of silica (below the zircon-baddeleyite buffer). By contrast the activity of CO2 may be high, in agreement with the observed fluid compositions. These results are corroborated by a short comparison with the other granulite occurrences of hibonite in Tanzania and South India. Received: 18 August 1994 / Accepted: 12 October 1995  相似文献   

18.
Ilvaite, Ca(Fe2+,Fe3+)Fe2+Si2O8(OH) shows two magnetic phase transitions, which have been studied by Mössbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe2+ and Fe3+ ions in octahedral A-sites causes the Fe2+-Fe3+ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe2+ (A) and Fe3+ (A) sites indicates B hf: 328 and 523 kOe respectively at 0 K and T N1= 116K. The corresponding values for Fe2+ (B) site are: B hf 186 kOe and T N2=36K. An additional hyperfine field exists at the Fe2+(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe2+ (B) site occurs on a corner common between two triangles with respect to two sets of Fe2+ (A) and Fe3+ (A) sites with opposite spin directions.  相似文献   

19.
In P - T - logfO2 space, the stability of annite (ideally KFe 3 2+ (OH)2AlSi3O10) at high fO2 (low fH2) is limited by the reaction: annite = sanidine + magnetite + H2. Using the hydrogen-sensor technique, the equilibrium fH2 of this reaction was measured between 500 and 800° C at 2.8 kbar in 50° C intervals. Microbrobe analyses of the reacted annite+sanidine+magnetite mixtures show that tetrahedral positions of annite have a lower Si/Al ratio than the ideal value of 3/1. Silicon decreases from 2.9 per formula unit at low temperatures to 2.76 at high temperatures. As determined by Mössbauer spectroscopy in three experimental runs, the Fe3+ content of annite in the equilibrium assemblage is 11%±3. A least squares fit to the hydrogensensor data gives H R 0 = 50.269 ± 3.987 kJ and S R 0 = 83.01 ± 4.35 J/K for equilibrium (1). The hydrogene-sensor data are consistent with temperature half brackets determined in the classical way along the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers with a mixture of annite+sanidine+magnetite for control. Compared to published oxygen buffer reversals, agreement is only found at high temperature and possible reasons for that discrepancy are discussed. The resulting slope of equilibrium (1) in logfO2 – T dimensions is considerably steeper than previously determined and between 400 and 800°C only intersects with the QFM buffer curve. Based on the hydrogen-sensor data and on the thermodynamic dataset of Berman (1988, and TWEEQ data base) for sanidine, magnetite and H2, the deduced standard-state properties of annite are: H f 0 =-5127.376±5.279 kJ and S 0=422.84±5.29 J/(mol K). From the recently published unit cell refinements of annites and their Fe3+ contents, determined by Mössbauer spectroscopy (Redhammer et al. 1993), the molar volume of pure annite was constrained as 15.568±0.030 J/bar. A revised stability field for annite is presented, calculated between 400 and 800°C.  相似文献   

20.
A new thermodynamic model for multi-component spinel solid solutions has been developed which takes into account thermodynamic consequences of cation mixing in spinel sublattices. It has been applied to the evaluation of thermodynamic functions of cation mixing and thermodynamic properties of Fe3O4–FeCr2O4 spinels using intracrystalline cation distribution in magnetite, lattice parameters and activity-composition relations of magnetite–chromite solid solutions. According to the model, cation distribution in binary spinels, (Fe1-x2+ Fex3+)[Fex2+Fe2-2y-x3+Cr2y]O4, and their thermodynamic properties depend strongly on Fe2+–Cr3+ cation mixing. Mixing of Fe2+–Fe3+ and Fe3+–Cr3+ can be accepted as ideal. If Fe2+, Fe3+ and Cr are denoted as 1, 3 and 4 respectively, the equation of cation distribution is –RT ln(x2/((1–x)(2–2y–x)))= G13* + (1–2x)W13+y(W14–W13–W34) where G13* is the difference between the Gibbs energy of inverse and normal magnetite, Wij is a Margules parameter of cation mixing and G13*, J/mol =–23,000+13.4 T, W14=36 kJ/mol, W13=W34=0. The positive nonconfigurational Gibbs energy of mixing is the main reason for changing activity–composition relations with temperature. According to the model, the solvus in Fe3O4–FeCr2O4 spinel has a critical temperature close to 500°C, which is consistent with mineralogical data.  相似文献   

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