Phase equilibria in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-CO2 and the stability limit of stilpnomelane in metamorphosed Precambrian iron-formations

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.     

Zur Stabilität von Margarit im System CaO-Al2O3-SiO2-H2O

An increasing number of occurrences of margarite have been reported in the last years. However, previous experimental investigations in the system CaO-Al₂O₃-SiO₂-H₂O are limited to the synthesis of margarite and to the upper stability limit according to the reaction (1) 1 margarite?1 anorthite +1 corundum +1 H₂O (Chatterjee, 1971; Velde, 1971). Since margarite often occurs together with quartz, the upper stability limit of margarite in the presence of quartz is of special interest. Therefore, the reactions (5) 1 margarite +1 quartz ?1anorthite +1 kyanite/andalusite +1 H₂O and (6) 4 margarite+3 quartz ? 2 zoisite+5 kyanite+3 H₂O were investigated experimentally using mixtures of natural margarite (from Chester, Mass., USA), quartz, kyanite, andalusite, zoisite, and synthetic anorthite. The indicated equilibrium temperatures at water pressures equal to total pressure are: 515± 25°C at 4 kb, 545 ±15°C at 5 kb, 590±10°C at 7 kb, and 650±10°C at 9 kb for reaction (5), and 651±11°C at 10 kb, 648 ± 8°C at 12.5kb, and 643±13°C at 15kb for reaction (6), respectively. Besides this, additional brackets for equilibrium temperatures were determined for the above cited reaction (1): 520±10°C at 3 kb, 580±10°C at 5 kb, and 640± 20°C at 7 kb. On the basis of these experimentally determined reactions (1), (5), and (6) and of the reactions (3) 2 zoisite +1 kyanite? 4 anorthite +1 corundum +1 H₂O (7) 2 zoisite +1 kyanite +1 quartz ? 4 anorthite +1 H₂O and (10) 1 pyrophyllite ? 1 andalusite/kyanite+3 quartz+1 H₂O for which experimental or, in the case of reaction (3), calculated data were already available, a pressure-temperature diagram with 3 invariant points and 11 univariant reactions was developed using the method of Schreinemakers. This diagram, summarizing both experimental and phase relation studies, allows conclusions about the conditions under which margarite has been formed in nature. Margarite is limited to low grade metamorphism at water pressures up to approximately 3.5 kb; in the presence of quartz, margarite is even limited to low grade metamorphism at water pressures up to 5.5 kb. Only at water pressures higher than the values stated before margarite, and margarite+quartz, respectively, can occur in medium grade metamorphism (as defined by Winkler, 1970 and 1973). For the combined occurrence of margarite+quartz and staurolite as reported by Harder (1956) and Frey (personal communication, 1973) it may be estimated that water pressure has been greater than approximately 5.5 kb, wheras temperature has been in the range from 550 to 650°C. Furthermore, the present study shows that the assemblage zoisite+kyanite (+ H₂O) is an indicator of both pressure [P _{H 2 O}> approximately 9kb]and temperature [T> approximately 640 to 650° Cat water Pressures up to 15 kb].     

High-pressure,high-temperature stability of surinamite in the system MgO-BeO-Al2O3-SiO2-H2O

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     

Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-Al2O3-SiO2-H2O-CO21

Abstract Chemical analysis (including H₂, F₂, FeO, Fe₂O₃) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca_18.92Mg_1.88Fe³⁺_0.40Al_10.97Si_17.81- O_69.0.1(OH)_8.84F_0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca₁₉Mg(MgAl₇)Al₄Si₁₈O₆₉(OH)₉, which is related to Fe^3+-vesuvianite by the substitutions Mg + OH = Fe³⁺+ O in the B and O(10) sites and Fe³⁺= Al in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂ shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH₂O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH₂O > 0.97).     

Stabilitätsbedingungen Grossular-führender Paragenesen im System CaO-Al2O3-SiO2-CO2-H2O

Ohne Zusammenfassung

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Stability conditions of grossularite-bearing parageneses in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O

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Herrn Prof. Dr. H. G. F. Winkler danke ich vielmals für sein Interesse an dieser Arbeit, für anregende Diskussionen und die kritische Durchsicht des Manuskriptes. Auch den Herren Doz. Dr. P. Metz, Dr. K.-H. Nitsch und Doz. Dr. V. Trommsdorff danke ich herzlich für wertvolle Diskussionen und Hinweise. Herrn Dr. E-an Zen danke ich für einen Hinweis zur Phasentheorie und Herrn Prof. Dr. E. Schwarzmann für die Durchführung von IR-Aufnahmen. — Der Deutschen Forschungsgemeinschaft gilt mein Dank für die Arheitsmöglichkeiten an den Herrn Prof. Dr. Winkler zur Verfügung gestellten Apparaturen.     

The influence of the fluid phase composition on the solidus temperatures in the haplogranite system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2

The solidus temperatures in the haplogranite-system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O-CO₂ have been determined up to 15 kbar for a constant molar ratio of sodium to potassium of 11 and for fluid compositions ranging from pure water to pure carbon dioxide. The data for the water-saturated solidus are virtually identical with those of previous studies. At constant pressure, the solidus curve as a function of the fluid phase composition exhibits a point of inflection in the range of the water-rich compositions. This phenomenon is attributed to chemical interactions between the CO₂ and the H₂O in the silicate melt. The point of inflection disappears if the CO₂ in the gas phase is replaced by molecular nitrogen. The CO₂-saturated solidi have been measured at 2 and 5 kbars. The data at 5 kbar indicate a melting point depression in the order of 40° C compared to the dry solidus of Huang and Wyllie (1975). The experimental data can be used to estimate the melting temperatures of common quartz and feldspar bearing crustal rocks under the conditions of granulite facies metamorphism. Since for most fluid phase compositions, the solidus curves are very steep in the P, T-diagram, the beginning of melting is nearly exclusively determined by the fluid composition and almost independent of pressure between about 2 and more than 10 kbar. Therefore, the onset of partial melting in quartz and feldspar containing rocks under granulite facies conditions can be used to estimate the composition of a coexisting H₂O-CO₂ fluid phase if geothermometric data are available. The temperature range between the beginning of granulite facies metamorphism and the initiation of melting expands with increasing carbon dioxide content in the H₂O-CO₂ fluid phase. At a CO₂ molar fraction of 0.9, this range extends from about 600° C to 900° C and is almost independent of pressure.     

An experimental investigation on the P-T stability of Mg-staurolite in the system MgO-Al2O3-SiO2-H2O

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     

Internally-Consistent Thermodynamic Data for Minerals in the System Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2

Internally consistent standard state thermodynamic data arepresented for 67 minerals in the system Na₂O-K₂O-CaO-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-TiO₂-H₂O-CO₂.The method of mathematical programming was used to achieve consistencyof derived properties with phase equilibrium, calorimetric,and volumetric data, utilizing equations that account for thethermodynamic consequences of first and second order phase transitions,and temperature-dependent disorder. Tabulated properties arein good agreement with thermophysical data, as well as beingconsistent with the bulk of phase equilibrium data obtainedin solubility studies, weight change experiments, and reversalsinvolving both single and mixed volatile species. The reliabilityof the thermodynamic data set is documented by extensive comparisons(Figs. 4–45) between computed equilibria and phase equilibriumdata. The high degree of consistency obtained with these diverseexperimental data gives confidence that the refined thermodynamicproperties should allow accurate prediction of phase relationshipsamong stoichiometric minerals in complex chemical systems, andprovide a reasonable basis from which activity models for mineralsmay be derived.     

Stability of the assemblage orthopyroxene-sillimanite-quartz in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O

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.     

Synthesis of Ca-tourmaline in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl

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.

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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.

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

P CO2 in low-grade metamorphism; zeolite,carbonate, clay mineral,prehnite relations in the system CaO-Al2O3-SiO2-CO2-H2O

Equilibria for several reactions in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O have been calculated from the reactions calcite+quartz=wollastonite+CO₂ (5) and calcite+Al₂SiO₅+quartz=anorthite+CO₂ (19) and other published experimental studies of equilibria in the systems Al₂O₃-SiO₂-H₂O and CaO-Al₂O₃-SiO₂-H₂O.The calculations indicate that the reactions laumontite+CO₂=calcite+kaolinite+2 quartz+2H₂O (1) and laumontite+calcite=prehnite+quartz+3H₂O+CO₂ (3) in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O, are in equilibrium with an H₂O-CO₂ fluid phase having -0.0075 for P _fluid=P _total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO₂.Using the above reactions as model equilibria, several probelms of p CO₂ in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).     

Melting and subsolidus reactions in the system K2O-CaO-Al2O3-SiO2-H2O

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.     

Synthesis and crystal chemistry of kornerupine in the system MgO-Al2O3-SiO2-B2O3-H2O

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∶Al₂O₃∶SiO₂ 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 X_{B 2 O 3} lead to the coexistence of kornerupine with boron-free minerals such as enstatite and sapphirine, fluids with relatively high X_{B 2 O 3} produce the boron-minerals grandidierite, sinhalite, and tourmaline (in the present system without Na!) in addition to kornerupine.     

Heat capacity of minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2: representation,estimation, and high temperature extrapolation

A revised equation is proposed to represent and extrapolate the heat capacity of minerals as a function of temperature: C _P=k₀+k₁ T ^–0.5+k₂ T ^–2+k₃ T ^–3 (where k₁, k₂0).This equation reproduces calorimetric data within the estimated precision of the measurements, and results in residuals for most minerals that are randomly distributed as a function of temperature. Regression residuals are generally slightly greater than those calculated with the five parameter equation proposed by Haas and Fisher (1976), but are significantly lower than those calculated with the three parameter equation of Maier and Kelley (1932).The revised equation ensures that heat capacity approaches the high temperature limit predicted by lattice vibrational theory (C _P=3R+²VT/). For 16 minerals for which and have been measured, the average C _Pat 3,000 K calculated with the theoretically derived equation ranges from 26.8±0.8 to 29.3±1.9 J/(afu·K) (afu = atoms per formula unit), depending on the assumed temperature dependence of . For 91 minerals for which calorimetric data above 400 K are available, the average C _Pat 3,000 K calculated with our equation is 28.3±2.0 J/(afu·K). This agreement suggests that heat capacity extrapolations should be reliable to considerably higher temperatures than those at which calorimetric data are available, so that thermodynamic calculations can be applied with confidence to a variety of high temperature petrologic problems.Available calorimetric data above 250 K are fit with the revised equation, and derived coefficients are presented for 99 minerals of geologic interest. The heat capacity of other minerals can be estimated (generally within 2%) by summation of tabulated oxide component C _Pcoefficients which were obtained by least squares regression of this data base.     

Stability of the assemblage cordierite—Corundum in the system MgO-Al2O3-SiO2-H2O

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.     

Synthetic dumortierite: its PTX-dependent compositional variations in the system Al2O3-B2O3-SiO2-H2O

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: (Al_6.670.33)[Al_0.49Si_2.51–O_13.53(OH)_1.47](BO₃) for a low-pressure product, and (Al_6.680.32)[Al_0.09Si_2.91O_13.94(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.     

The stability of merwinite in the system CaO-MgO-SiO2-H2O-CO2 with CO2-poor fluids

The stability of merwinite (Mw) and its equivalent assemblages, akermanite (Ak)+calcite (Cc), diopside (Di)+calcite, and wollastonite (Wo)+monticellite (Mc)+calcite was determined at T=500–900° C and P _f=0.5–2.0 kbar under H₂O–CO₂ fluid conditions with X _{CO 2}=0.5, 0.1, 0.05, and 0.02. Merwinite is stable at P _f=0.5 kbar with T>700° C and X _{CO 2}<0.1. At P _f=2.0 kbar, the assemblage Di+Cc replaces merwinite at all T and X _{CO 2} conditions. At intermediate P _f=1 kbar, the assemblage Ak+Cc is stable above 707° C and Wo+Mc+Cc is stable below 707° C. The univariant curve for the reaction Di+Cc=Wo+Mc+CO₂ is almost parallel to the T axis and shifts to low P _f with increasing X _{CO 2}, with the assemblage Di+Cc on the high-P _f side. The implications of the experimental results in regard to contact metamorphism of limestone are discussed using the aureole at Crestmore, California as an example.     

Studies in the system KAlSiO4-BaAl2Si2O8-SiO2-H2O

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 Δ_FeKα(2θ₍₁₁₁₎CaF₂—2θ₍₀₀₄₎Fs_ss), 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⁺⁺.     

中-低压泥质岩在KFMASH体系中的相平衡关系

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。