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1.
Beginning of melting and subsolidus relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K2O-CaO-Al2O3-SiO2-H2O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al2O3-SiO2-H2O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al2O3-SiO2-H2O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al2SiO5 polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.  相似文献   

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3.
中-低压泥质岩在KFMASH体系中的相平衡关系   总被引:3,自引:0,他引:3  
张翠光  魏春景 《岩石学报》2004,20(3):725-736
利用内部一致热力学数据库、可靠的固溶体活度模型,用有关程序THERMOCALC 3.1计算了KFMASH(K2O-FeO-MgO-Al2O3-SiO2-H2O)体系和亚体系KMASH、KFASH中的岩石成因格子。温压范围为P=0.05~1.2GPa,T=450~900℃.包括黑云母、白云母、钾长石、绿泥石、硬绿泥石、十字石、堇青石、斜方辉石、石榴石、尖晶石、红柱石、蓝晶石、矽线石、石英(过量)、熔体和水(固相线以下水过量、固相线以上水不过量)..利用这些成因格子以及所计算的AFM图、P-T视剖面图,可以很好地阐明泥质岩石中低压变质作用的相平衡关系及P-T条件。所计算的结果与岩石学研究非常吻合,能解释从绿片岩相至麻粒岩相的一系列变化。尤其是熔体的引入,使我们能够定量计算高角闪岩相以上出现的混合岩化过程。  相似文献   

4.
Dumortierite, generally simplified as Al7BSi3O18, was synthesized in the pure system Al2O3–B2O3–SiO2–H2O (ABSH) using gels with variable Al/Si ratios mixed with H3BO3 and H2O in known proportions as starting materials. Synthesis conditions ranged from 3 to 5 and 15 to 20 kbar fluid pressure at 650° to 880°C. On the basis of analyses, synthetic dumortierite shows relatively narrow homogeneity ranges with regard to Al/Si which, however, vary as a function of pressure: at low pressures (3–5 kbar) Al/Si is 2.77–2.94 versus 2.33–2.55 at high pressures (15–20 kbar). Outside of these homogeneity limits, dumortierite was found to coexist with quartz or corundum, depending on the starting composition. Whereas synthetic dumortierite invaribly contains 1.0 boron atom per formula unit (p.f.u.) based on 18 oxygens, the water contents vary drastically as a function of pressure and temperature (1.32–2.30 wt.% H2O or 0.85–1.47 H p.f.u.). H2O is an essential component in dumortierite. Structural formulae based on complete chemical analyses of the dumortierites synthesized reveal that there is invariably an Si-deficiency against the ideal number of 3.0 p.f.u. In the calculation procedure used here, this deficiency is balanced by assuming tetrahedral Al. The remaining Al, taken to occupy the octahedral sites, is always below the ideal number of 7.0 p.f.u. Charge-balancing the structure with the hydrogen found analytically leads to two different mechanisms of H incorporation: (1) 3H+ + octahedral vacancy for Al[6]; (2) H+ + tetrahedral Al for Si[4]. Dumortierite synthesized at high fluid pressure contains little Al[4] and, thus, little H+ of type 2; its hydrogen is predominantly present as type 1. Conversely, dumortierite formed at low fluid pressures is high in Al[4] and hydrogen type 2. The amounts of hydrogen type 1 in low-pressure dumortierites decrease with rising temperatures of synthesis. Typical structural formulae are: (Al6.670.33)[Al0.49Si2.51–O13.53(OH)1.47](BO3) for a low-pressure product, and (Al6.680.32)[Al0.09Si2.91O13.94(OH)1.06](BO3) for a high-pressure product. Independently of the synthesis conditions, dumortierite was found always to be orthorhombic, with b0/a0 deviating slightly, but significantly from the valid for hexagonal lattice geometry. As a function of increasing Al/Si in the synthetic crystals, their a0, c0, and V0 rise, whereas b0 decreases. Thus b0/a0 decreases most sensitively with rising Al/Si and also with growing Al[4]. More experimentation is required before the compositional variations of dumortierite found here can be applied successfully to geothermobarometry of natural rocks.  相似文献   

5.
The stability of coexisting orthopyroxene, sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O as a function of pressure, mainly at 1,000° C, and at oxygen fugacities defined mostly by the hematite-magnetite buffer. The upper stability of the assemblage is terminated at 17 kbars, 1,000° C, by the reaction opx+Al-silicate gar+qz, proceeding toward lower pressures with increasing Fe/(Fe+Mg) ratio in the system. The lower stability is controlled by the reaction opx+sill+qz cord, which occurs at 11 kbars in the iron-free system but is lowered to 9 kbars with increasing Fe/(Fe+Mg). Spinel solid solutions are stabilized, besides quartz, up to 14 kbars in favour of garnet in the iron-rich part of the system (Fe/(Fe+Mg)0.30). Ferric-ferrous ratios in orthopyroxene are increasing with increasing ferro-magnesian ratio. At least part of the generally observed increase in Al content with Fe2+ in orthopyroxene is not due to an increased solubility of the MgAlAlSiO6 component but rather of a MgFe3+AlSiO6 component. The data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz.  相似文献   

6.
P, T, \(X_{{\text{CO}}_{\text{2}} }\) relations of gehlenite, anorthite, grossularite, wollastonite, corundum and calcite have been determined experimentally at P f =1 and 4 kb. Using synthetic starting minerals the following reactions have been demonstrated reversibly
  1. 2 anorthite+3 calcite=gehlenite+grossularite+3 CO2.
  2. anorthite+corundum+3 calcite=2 gehlenite+3 CO2.
  3. 3anorthite+3 calcite=2 grossularite+corundum+3CO2.
  4. grossularite+2 corundum+3 calcite=3 gehlenite+3 CO2.
  5. anorthite+2 calcite=gehlenite+wollastonite+2CO2.
  6. anorthite+wollastonite+calcite=grossularite+CO2.
  7. grossularite+calcite=gehlenite+2 wollastonite+CO2.
In the T, \(X_{{\text{CO}}_{\text{2}} }\) diagram at P f =1 kb two isobaric invariant points have been located at 770±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.27 and at 840±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.55. Formation of gehlenite from low temperature assemblages according to (4) and (2) takes place at 1 kb and 715–855° C, \(X_{{\text{CO}}_{\text{2}} }\) =0.1–1.0. In agreement with experimental results the formation of gehlenite in natural metamorphic rocks is restricted to shallow, high temperature contact aureoles.  相似文献   

7.
Summary. ?Ca-tourmaline has been synthesized hydrothermally in the presence of Ca(OH)2 and CaCl2-bearing solutions of different concentration at T = 300–700 °C at a constant fluid pressure of 200 MPa in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl. Synthesis of tourmaline was possible at 400 °C, but only above 500 °C considerable amounts of tourmaline formed. Electron microprobe analysis and X-ray powder data indicate that the synthetic tourmalines are essentially solid solutions between oxy-uvite, CaMg3- Al6(Si6O18)(BO3)3(OH)3O, and oxy-Mg-foitite, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O. The amount of Ca ranges from 0.36 to 0.88 Ca pfu and increases with synthesis temperature as well as with bulk Ca-concentration in the starting mixture. No hydroxy-uvite, CaMg3(MgAl5)(Si6O18)(BO3)3(OH)3(OH), could be synthesized. All tourmalines have < 3 Mg and > 6 Al pfu. The Al/(Al + Mg)-ratio decreases from 0.80 to 0.70 with increasing Ca content. Al is coupled with Mg and Ca via the substitutions Al2□Mg−2Ca−1 and AlMg−1H−1. No single phase tourmaline could be synthesized. Anorthite ( + quartz in most runs) has been found coexisting with tourmaline. Other phases are chlorite, tremolite, enstatite or cordierite. Between solid and fluid, Ca is strongly fractionated into tourmaline ( + anorthite). The concentration ratio D = Ca(fluid)/Ca(tur) increases from 0.20 at 500 °C up to 0.31 at 700 °C. For the assemblage turmaline + anorthite + quartz + chlorite or tremolite or cordierite, the relationship between Ca content in tourmaline and in fluid with temperature can be described by the equation (whereby T = temperature in °C, Ca(tur) = amount of Ca on the X-site in tourmaline, Ca( fluid) = concentration of Ca2+ in the fluid in mol/l). The investigations may serve as a first guideline to evaluate the possibility to use tourmaline as an indicator for the fluid composition.
Zusammenfassung. ?Synthese von Ca-Turmelin im System CaO-MgO-Al 2 O 3 -SiO 2 -B 2 O 3 -H 2 O-HCl Im System CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl wurde Ca-Turmalin hydrothermal aus Ca(OH)2 and CaCl2-haltigen L?sungen bei T = 300–700 °C und einem konstanten Fluiddruck von 200 MPa synthetisiert. Die Synthese von Turmalin war m?glich ab 400 °C, aber nur oberhalb von 500 °C bildeten sich deutliche Mengen an Turmalin. Elektronenstrahl-Mikrosondenanalysen und R?ntgenpulveraufnahmen zeigen, da? Mischkristalle der Reihe Oxy-Uvit, CaMg3Al6(Si6O18)(BO3)3(OH)3O, und Oxy-Mg-Foitit, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O gebildet wurden. Der Anteil an Ca variiert zwischen 0.36 und 0.88 Ca pfu und nimmt mit zunehmender Synthesetemperatur und zunehmender Ca-Konzentration im System zu. Hydroxy-Uvit, CaMg3(MgAl5) (Si6O18)(BO3)3(OH)3(OH), konnte nicht synthetisiert werden. Alle Turmaline haben < 3 Mg und > 6 Al pfu. Dabei nimmt das Al/(Al + Mg)- Verh?ltnis mit zunehmendem Ca-Gehalt von 0.80 auf 0.70 ab. Al ist gekoppelt mit Mg und Ca über die Substitutionen Al2□Mg−2Ca−1 und AlMg−1H−1. Einphasiger Turmalin konnte nicht synthetisiert werden. Anorthit (+ Quarz in den meisten F?llen) koexistiert mit Turmalin. Andere Phasen sind Chlorit, Tremolit, Enstatit oder Cordierit. Ca zeigt eine deutliche Fraktionierung in den Festk?rpern Turmalin (+ Anorthit). Das Konzentrationsverh?ltnis D = Ca(fluid)/Ca(tur) nimmt von 0.20 bei 500 °C auf 0.31 bei 700 °C zu. Für die Paragenese Turmalin + Anorthit + Quarz mit Chlorit oder Tremolit oder Cordierit gilt folgende Beziehung zwischen Ca-Gehalt in Turmalin und Fluid und der Temperatur: (wobei T = Temperatur in °C, Ca(tur) = Anteil an Ca auf der X-Position in Turmalin, Ca(fluid) = Konzentration von Ca2+ im Fluid in mol/l). Die Untersuchungen dienen zur ersten Absch?tzung, ob Turmalin als Fluidindikator petrologisch nutzbar ist.


Received July 24, 1998;/revised version accepted October 21, 1999  相似文献   

8.
Melting relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been reinvestigated using Schreinemakers analysis and hydrothermal experiments. The reaction sanidine+muscovite+zoisite+quartz+vapor =melt has been bracketed at 10, 15, and 20 kbars and 670–680, 680–690, and 690–700° C, respectively and it marks the lowest solidus temperatures in the system investigated.Below 10 kbars, experimental data on the beginning of melting in zoisite- or muscovite-bearing anorthite+sanidine assemblages have been obtained, which are not showing any differences and therefore point to melt compositions close to the feldspar-quartz join.  相似文献   

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Boron-bearing kornerupine was synthesized in the simplest possible model system at fluid pressures and temperatures both within and outside the stability field of boron-free kornerupine. Best conditions for synthesis of single-phase products are 7 kb and 830 °C. Microprobe and wet chemical analyses as well as X-ray studies indicate compositional variations of kornerupines regarding all five constituent components: Increasing B-contents (from 0.37 to 3.32 wt% B2O3) are correlated with decreasing OH? values largely according to the Eq. B3+?3 H+; the ratio MgO∶Al2O3SiO2 varies from 4∶3∶4 in the direction towards 1∶1∶1. Thus kornerupine exhibits an at least ternary range of solid solution in the system studied. Crystallochemically speaking it is significant that, although the Mg∶Al∶Si ratio of kornerupine may remain constant with increasing boron contents, the total number of cations per formula unit increases beyond the ideal number of 14.0 as given by Moore and Bennett (1968). Considering the presence of an additional structural site at (000) it is suggested that the introduction of boron initiates a sequence of substitutions such as $$B^{[4]} \to Si^{[4] } \to A1^{[4]} \to Mg^{[6]} \to \square$$ . The filling of this site, empty in boron-free kornerupine, by Mg is connected with a loss of hydrogen located near this site. Petrologically speaking an exchange reaction relation exists between kornerupine and its coexisting fluid according to the equation Boron-free kornerupine+B2O3=boron-kornerupine+H2O. The molar fractions $$X_{B_2 O_3 } = B_2 O_3 /\left( {B_2 O_3 + H_2 O} \right)$$ of kornerupines exceed those of their coexisting fluids by about one order of magnitude. Fluids with relatively low XB 2 O 3 lead to the coexistence of kornerupine with boron-free minerals such as enstatite and sapphirine, fluids with relatively high XB 2 O 3 produce the boron-minerals grandidierite, sinhalite, and tourmaline (in the present system without Na!) in addition to kornerupine.  相似文献   

11.
The MgAl surinamite end member, (Mg3Al3)[6]O[AlBeSi3O15], was synthesized in the requisite system with and without water. The new phase is monoclinic, space group P2/n, with a=9.881(1)Å; b=11.311(1) Å; c=9.593(1) Å; =109.52(2)°. Refractive indices are n x=1.7015(20); n y=1.7035(20); n z=1.7055(20). The infrared spectrum shows characteristic differences against the structurally related and optically extremely similar phase sapphirine.Using the seeding technique, the preliminary stability field for MgAl surinamite was found to lie at high temperatures (650 °C) and high pressures (4 kbar). At lower temperatures breakdown takes place to hydrous assemblages of chlorite, talc, and chrysoberyl with kyanite or yoderite; at lower pressures chrysoberyl forms parageneses with sapphirine and cordierite. In crystal chemical terms the underlying principle for the stability of surinamite versus that of the low-pressure assemblages is the higher proportion of octahedrally coordinated Al in surinamite (75%). Following the same principle surinamite itself decomposes at still higher pressures to a paragenesis, in which all Al enters octahedral coordination (pyrope+a chrysoberyl-type phase and some unidentified X-ray peaks).The stability field of synthetic MgAl surinamite is in good agreement with P, T-estimates of some 8–12 kbar, 800°–950° C as taken from the literature for the few occurrences of natural, Fe-bearing surinamite in granulite and upper amphibolite facies environments. The incorporation of iron in surinamite must be limited, because this mineral is known to coexist with its more iron-rich breakdown assemblage almandine-rich garnet+chrysoberyl. As the minimum melting curve of granite under hydrous conditions lies outside the surinamite field up to a water pressure of about 20 kbar, the absence of surinamite in normal granitic pegmatites can already be explained by physical constraints. However, there are probably also chemical constraints in the generally high Fe/Mg bulk chemistry of the pegmatite environments.Now at Institut für Kristallographie, Technische Hochschule, Templergraben 55, D-5100 Aachen, FRG  相似文献   

12.
The assemblage Mg-cordierite — corundum is formed stably through the reaction chlorite+Al-silicate=cordierite+corundum+H2O at 535° C, 2kb; 615°, 5 kb; and 665° C, 7 kb water pressure. In the order of increasing pressure andalusite, sillimanite, and kyanite participate as stable phases in this equilibrium. A spinel-Al-silicate tie-line is only stable at high temperatures not likely to be attained in rocks. The natural assemblage spinel-Al-silicate is, however, to be explained by the additional presence of FeO in these rocks.  相似文献   

13.
The pressure-temperature stability field of Mg-staurolite, ideally Mg4Al18Si8O46(OH)2, was bracketed for six possible breakdown reactions in the system MgO-Al2O3-SiO2-H2O (MASH). Mg-staurolite is stable at water pressures between 12 and 66 kbar and temperatures of 608–918 °C, requiring linear geotherms between 3 and 18 °C/km. This phase occurs in rocks that were metamorphosed at high-pressure, low-temperature conditions, e.g. in subducted crustal material, provided they are of appropriate chemical composition. Mg-staurolite is formed from the assemblage chlorite + kyanite + corundum at pressures <24 kbar, whereas at pressures up to 27 kbar staurolite becomes stable by the breakdown of the assemblage Mg-chloritoid + kyanite + corundum. Beyond 27 kbar the reaction Mg-chloritoid + kyanite + diaspore = Mg-staurolite + vapour limits the staurolite field on its low-temperature side. The upper pressure limit of Mg-staurolite is marked by alternative assemblages containing pyrope + topaz-OH with either corundum or diaspore. At higher temperatures Mg-staurolite breaks down by complete dehydration to pyrope + kyanite + corundum and at pressures below 14 kbar to enstatite + kyanite + corundum. The reaction curve Mg-staurolite = talc + kyanite + corundum marks the low-pressure stability of staurolite at 12 kbar. Mg-staurolite does not coexist with quartz because alternative assemblages such as chlorite-kyanite, enstatite-kyanite, talc-kyanite, pyrope-kyanite, and MgMgAl-pumpellyite-kyanite are stable over the entire field of Mg-staurolite. Received: 16 April 1997 / Accepted: 24 September 1997  相似文献   

14.
The pressure temperature stability of the phase Mn-cordierite hitherto not recorded as a mineral has been determined at temperatures ranging from 400° C up to the melting mainly using standard hydrothermal techniques at the oxygen fugacities provided by the buffering power of the bomb walls. Manganocordierite is a pronounced low-pressure phase with a maximum pressure stability of about 1 kb near 400° C and decreasing pressure limits at higher temperatures. Throughout the temperature range investigated the stable high-pressure breakdown assemblage of Mn-cordierite is spessartine, an Al-silicate, and a SiO2-polymorph. Due to the variable water contents of Mn-cordierite and spessartine there is a pronounced curvature in the negative dP/dT-slope of the requisite upper pressure breakdown curve of Mn-cordierite. Only theoretical deductions were possible concerning the stable hydrous low-temperature breakdown assemblage of Mn-cordierite below about 400° C.The manganocordierites synthesized are orthorhombic low-cordierites with distortion indices increasing with temperature, water pressure, and duration of heating. Their mean refractive indices increase with rising contents of absorbed water in the structural channels. Based on experiments with natural material the upper temperature stability limit of the mineral carpholite must lie at temperatures below about 400° C for water pressures up to 2.5 kb.The absence of Mn-cordierite from natural rocks studied thus far cannot be explained on chemical grounds, but must be due to its narrow pressure temperature stability range. The phase may yet be discovered as a mineral in manganiferous metasediments formed by lowpressure contact metamorphism.  相似文献   

15.
New experiments on melting reactions of muscovite + quartz obtained for a mixture using a synthetic muscovite confirm he experimental data obtained for a mixture using a natural muscovite which have been reported recently. [  相似文献   

16.
The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K?1 mol?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K?1 mol?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10?3T ? 15.27 × 105T?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10?3T ? 30.24 × 105T?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10?3T ? 25.02 × 105T?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10?3T ? 24.08 × 105T?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al2O3-SiO2-H2O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H2O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.  相似文献   

17.
Various members of the KAlSi3O8-BaAl2Si2O8 feldspar series are hydrothermally synthesized. Cellparameters of these are calculated from diffractometer patterns and found to be similar to those of Gay and Roy. A variation diagram is constructed correlating Cn-content and values of ΔFeKα(2θ(111)CaF2—2θ(004)Fsss), which gives $${\text{Mol}}\% {\text{ Cn = 229}}{\text{.83}}\Delta {\text{2}}\theta ---{\text{190}}{\text{.81}}$$ by a least square regression fitting. Phase equilibria relation in the solidus-liquidus-region for the KAlSi3O8-BaAl2Si2O8-H2O system at 1000 kg/cm2 are investigated. It is found to be a case of simple solid solution in a binary system, with reservations at the potassium-rich side of the system. Goranson (1938) gives a temperature of about 1000°C at 1000 kg/cm2 \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) for the incongruent melting of sanidine, but the authors prefer a value around 930°C at the same \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) . Reaction products of starting materials on the join KAlSi2O6-BaAl2Si2O8 and KAlSiO4-BaAl2Si2O8 gave no experimental hint for replacement of K+ by Ba++.  相似文献   

18.
The phase relations of Al- and Fe-bearing silicates in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-CO2, in the presence of quartz and magnetite, are discussed on the basis of mineralogic and petrologic data from Precambrian iron-formations and blueschist facies meta-ironstone from the Franciscan Formation, California. These relations allow an estimation of the physiochemical conditions during low-grade metamorphism of iron-formations. Petrologic data together with available experimental and predicted thermodynamic data on the associated minerals place the upper stability limit of stilpnomelane in iron-formations at about 430–470° C and 5–6 kilobars. Fe-end member stilpnomelane can persist to a maximum temperature of 500° C and pressures up to 6–7 kilobars, although it is unlikely to occur in metamorphosed iron-formations. In iron-formation occurrences the stilpnomelane stability field is bordered by four equilibrium reactions with the assemblages stilpnomelane-zussmanite-chlorite-minnesotaite, stilpnomelane-zussmanite-chlorite-grunerite, stilpnomelane-biotite-chlorite-grunerite, and stilpnomelane-biotite-almandine-grunerite. The stability field is reduced by increasing X(CO2) and X Mg Stil , and is also a function of a(K +)/ a(H +) in the metamorphic fluid. If the value of a(K +)/ a(H +) is smaller than that defined by the above assemblages, stilpnomelane decomposes to chlorite, but if larger, it is replaced by biotite. At pressures less than 4 kilobars, the zussmanite field is restricted to a very high value of a(K +)/a(H +) (> 5.0 in log units at 1.0 kilobar) where iron-formation assemblages are not stable.  相似文献   

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