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.     

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.     

Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

Fluid-bearing and fluid-absent invariant points in the system CaO-MgO-Al2O3-SiO2-CO2-H2O for a greenschist facies assemblage

An outline for a metamorphic grid involving a greenschist facies assemblage is presented in Watts (1973). This grid is derived from a theoretical determination of all possible P-T dependent (solid-solid) reactions and invariant points as well as those in which a fluid, containing CO₂, H₂O and other unspecified components, takes part. The thermodynamic data in Watts (1973) were accidentally calculated at 425° K and not at 425° C. Since 425° K is an unreasonably low temperature for greenschist facies equilibria, these data have been recalculated at 700° K (427° C) and a new topology of fluid-absent (solid-solid) reactions and invariant points in P-T space results, since the slopes of such reactions change under conditions at the higher temperature. The slopes of the fluid-bearing reactions are independent of P, T and remain unchanged. However, for each different P-T grid of solid-solid reactions, a new set of chemographic arrangements is valid for the fluidbearing invariant points. A set of μ_H2O-μ_CO2 diagrams consistent with, and dependent on the new P-T grid is presented.     

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).     

Two-pyroxene thermobarometry with new experimental data in the system CaO-MgO-Al2O3-SiO2

In the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), equilibrium alumina contents of orthopyroxene and clinopyroxene, both coexisting with spinel + forsterite or spinel + anorthite, have been reversed in 16 runs at 1,300–1,400°C and 10.2–20.8 kbar, using PbO flux. The present data and the data of Perkins and Newton (1980) have been modeled using the Redlich-Kister equation. The resulting model satisfies most of the reversed data in the CMAS system, agrees very well with thermochemical measurements, and is consistent with the model for the enstatite-diopside join of Lindsley et al. (1981) and with the system MgO-Al₂O₃-SiO₂ of Gasparik and Newton (1984). The present data, however, do not confirm the negative slopes of Al-isopleths in the spinel lherzolite field suggested by Dixon and Presnall (1980). The new model has been used to calculate a graphical two-pyroxene thermobarometer applicable to natural two-pyroxene assemblages closely approaching in composition the CMAS system.     

X-ray determination and equilibrium composition of clinopyroxenes in the system CaO-MgO-Al2O3-SiO2

Ternary clinopyroxenes have been synthesized in the plane Di-CaTs-En. The variation of their crystallographic parameters has allowed the development of three determinative grids, which utilize b-, 2 _22¯1–2 ₃₁₀ and 2 ₃₃₀ –2 ₂₀₂ respectively. These grids show significant differences in comparison with the previously proposed ones. Present results have been used to review some data on clinopyroxenes equilibria in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) (Biggar 1969; Bruno and Facchinelli 1978; Herzberg 1978; O'Hara and Schairer 1963; Boyd 1969). In particular the petrogenic grid correlating P, T and CaTs content of clinopyroxenes in spinel-lherzolite assemblage (Herzberg 1978) has been revised, and consequently equilibrium temperatures rise by one hundred degrees.     

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.     

Gehlenite stability in the system CaO-Al2O3-SiO2-H2O-CO2

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.     

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.     

Relationship between fluid-bearing and fluid-absent invariant points and a petrogenetic grid for a greenschist facies assemblage in the system CaO-MgO-Al2O3-SiO2-CO2-H2O

In the 6 component system CaO-MgO-Al₂O₃-SiO₂-CO₂-H₂ with 9 solid phases (quartz, plagioclase, epidote, tremolite, talc, chlorite, magnesite, calcite, dolomite) and a fluid phase, all 17 possible fluid-absent reactions have been set up and balanced. Using molar entropy and volume data for the solid phases, these reactions are arranged in P-T space about the 8 possible fluid-absent invariant points after the method of Schreinemakers. Field observations in Ordovician greenschist facies basic volcanics at Sofala N.S.W., indicate that neither talc+epidote nor magnesite+calcite are stable under the conditions of metamorphism. Assuming these conditions to apply to the theoretical study here, the fluid-absent invariant points are arranged in a relative fashion with fluid-absent reactions subdividing P-T space into smaller areas.A scheme which permits a fluid of composition (i.e. a fluid containing CO₂ and H₂O together with other components), is modeled by treating H₂O as a mobile component independent of CO₂, and by allowing values that lie off the locus of binary H₂O-CO₂. Taking into account that neither talc+epidote nor magnesite +calcite is to be permitted, the fluid scheme is used to set up and balance all 39 possible fluid-bearing reactions. These are then arranged about 20 valid fluid-bearing invariant points in space after the method of Korzhinskii and Sehreinemakers.A characteristic solid phase assemblage is defined for each P-T area using chemographic relations inherent from the fluid-absent boundary reactions. The fluid-bearing invariant points that have a solid assemblage compatible with the characteristic assemblage in a particular P-T area are stable within the P-T regime of that area. When these stable fluidbearing invariant points are arranged in a relative fashion in space, they outline a fluid grid which can be used to study the possible effects of local variation in X _fluid over the particular P-T regime.Symbols Used U chemical potential - S entropy - V molar volume - n coefficient of a phase in a reaction - X mole fraction - T temperature - P pressure - F number of degrees of freedom - C number of components - p number of phases - s solid - slope of reaction - 1 quartz - 2 plagioclase - 3 epidote - 4 tremolite - 5 talc - 6 chlorite - 7 dolomite - 8 magnesite - 9 calcite     

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     

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.     

Molecular dynamics study of the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2

Molecular dynamics (MD) simulations have been used to calculate the structures and bulk moduli of crystals in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) using an interatomic potential model (CMAS94), which is composed of pairwise additive Coulomb, van der Waals, and repulsive interactions. The crystals studied, total of 27, include oxides, Mg meta- and ortho-silicates, Al garnets, and various Ca or Al bearing silicates, with the coordination number of cations ranging 6 to 12 for Ca, 4 to 12 for Mg, 4 to 6 for Al, and 4 and 6 for Si. In spite of the simplicity of the CMAS94 potential and the diversity of the structural types treated, MD simulations are quite satisfactory in reproducing well the observed structural data, including the crystal symmetries, lattice parameters, and average and individual nearest neighbour Ca-O, Mg-O, Al-O, and Si-O distances. In addition MD simulated bulk moduli of crystals in the CMAS system compare well with the observed values.     

Thermochemical Data on Mineral Phases: The System CaO-MgO-Al2O3-SiO2

Thermochemical data on several phases forming in the systemCaO-MgO-Al₂O₃-SiO₂ have been tested for consistency in reproducingexperimental phase equilibrium relationships. Calorimetric dataon enthalpy, entropy and heat capacity have been adjusted withinexperimental errors and new data on some phases have been estimatedusing phase equilibrium data. The consistency of the data inreproducing phase equilibrium relations in the multicomponent-multiphasesystems has been tested by computing phase diagrams using themethod of minimization of total Gibbs free energy. The recentcalorimetric data on most phases can be used without any significantchange except for phases that show cation disorder. Disordercorrections have been added to the heat capacity data on gehlenite,anorthite, spinel and Ca-Tschermak.     

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     

Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 from 3 to 7 GPa

We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS.CO₂) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I_1A and I_2A, which mark the transition from CO₂-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO₂, we find I_1A at 2.6 GPa/1230 °C and I_2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbonatitic with SiO₂ contents of <6 wt%, CO₂ contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO₂ system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO₂-rich and silica poor. The widespread evidence for the presence of CO₂ in both the oceanic and continental upper mantle implies that such low degree SiO₂-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface. Received: 9 April 1997 / Accepted: 25 November 1997     

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。     

Stability of manganocordierite and related phase equilibria in part of the system MnO-Al2O3-SiO2-H2O

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.