Phase equilibria in the system lamprophyllite-nepheline

Experimental studies of the melting diagram for the lamprophyllite-nepheline system and data on the phases crystallizing in this system indicate that lamprophyllite incongruently melts with the origin of melt and titanium oxide. The maximum temperature at which nepheline and lamprophyllite can occur in equilibrium is estimated at 833 ± 6°C. Our pioneering data on lamprophyllite crystallization from melt prove that this mineral can be of magmatic genesis. The distribution coefficients between lamprophyllite and melt are evaluated for K (0.1–0.25), Mn (0.82–1.06), Fe (0.13–0.40), and Mg (0.82–1.5); and the Sr/Ba lamprophyllite/melt exchange coefficient is estimated at 1.8–3.7.     

Phase transition and mixing behaviour of the cummingtonite–grunerite solid solution

 Natural amphiboles with composition close to the binary join cummingtonite–grunerite and crystals of the same samples annealed at 700 °C for 55.5 h, in order to obtain different degrees of non-convergent cation order, have been characterised by means of X-ray single-crystal diffraction and IR spectroscopy. Long-range order parameters describing the non-convergent order of Mg/Fe among the different octahedral sites have been calculated from the site occupancies of the investigated samples. Values of the O6-O5-O6 angles and of the 〈M4-O〉 mean bond distances depend on the C2/m → P2₁/m phase transition for a given degree of order. In the IR spectra, only two phonon lines dominated by the bending of the tetrahedral chains are sensitive to the displacive phase transition and to the different degree of cation order; all the other wavenumber shifts are correlated with compositional changes only. The local strains arising from the cation substitution, ordering and phase transition have been quantified by means of the autocorrelation function. Very small local heterogeneities are associated with the Mg/Fe substitution and disordering in samples at intermediate composition. The displacive phase transition seems to occur in order to reduce local distortions and the P2₁/m samples are as homogeneous as orthorhombic anthophyllites. The orthorhombic structure, however, appears less flexible than the monoclinic in accommodating cations larger than Mg at the octahedral sites. Received: 9 February 2000 / Accepted: 30 September 2000     

Phase equilibria and physical properties

The average physical properties of multiphase aggregates, such as rocks, depend on the properties of the individual phases as well as the statistical spatial distribution of the phases, such as the relative concentrations, and the shape, orientation, and distribution of the grains of the individual phases. If the properties of the phases are similar, the aggregate properties depend mainly on volume averages of properties of the phases; this is the case for elastic properties of rocks. If the properties of the phases differ significantly, the geometric distribution of the phases becomes important; this is the case if a fluid phase is present, which may have a large effect on elastic and electrical properties. Laboratory measurements of properties of individual phases and aggregates, used with theoretical treatments of aggregate properties, permits the interpretation of seismic velocities, attenuation and electrical conductivity in the earth in terms of possible compositions, phases and distribution of phases, such as intergranular fluids.     

Phase equilibria in the system calcium carbonate-water

High pressure experiments and thermodynamic calculations have explored the pattern of phase equilibria in the system CaCO₃-H₂O with particular emphasis on the phase CaCO₃.H₂O. The data suggest that CaCO₃.H₂O is unstable with respect to argonite plus water everywhere in the region considered (0–140°C and 1 bar–20 kb).     

高级变质岩中深熔作用的相平衡研究

深熔作用在高级变质岩中非常普遍并受到广泛关注。自20世纪90年代以来,随着变质相平衡研究的突破性发展,利用THERMOCALC程序和视剖面图方法可以定量研究固相线以上的熔体形成、熔体分馏和退变质反应。变质沉积岩中的熔融作用主要有三种机制饱和水固相线上的熔融、白云母脱水熔融和黑云母脱水熔融。在模拟泥质岩石的KFMASH体系和NCKFMASH体系中的相平衡计算表明,NCKFMASH体系中铁镁矿物的相平衡关系受KFMASH亚体系中矿物相平衡关系的控制,但KFMASH亚体系中固相线位置要比实际的高50~60℃。因此,模拟泥质岩石的固相线以上的相平衡关系最好在NCKFMASH或组分更多的体系中进行。相平衡研究表明麻粒岩相岩石的保存与熔体丢失有关;混合岩的形成过程包括部分熔融作用、不同程度熔体分凝与汲取和不同程度的逆反应和退变反应。     

Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex

The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C.Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO₂ (gas or liquid or both at room temperature) through CO₂ + H₂O ± CH₄ mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism.The density and fluid composition data are derived from direct observations of phase changes between ? 180 and + 380°C and from the application of published experimental data in the system CH₄-CO₂-H₂O-NaCl. The most dense, pure CO₂ inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P-T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO₂ + H₂O inclusions. There is evidence of immiscible separation of CO₂-rich and H₂O-rich fluids at temperatures at least as high as 375°C.     

The system Ag-Fe-S: Phase equilibria and mineral assemblages

Phase relations in the Ag-Fe-S system were determined from 700 to 150 °C by quench experiments with the use of evacuated, sealed, silica tubes as reaction vessels; these data were then used to interpret various aspects of natural occurrences of Ag-Fe-S minerals (e.g. argentiferous pyrite). The assemblages Ag₂S+Fe_1–x S and Ag₂S+FeS₂ become stable, with decreasing temperature, at 622±2 ° and 607±2 °C, respectively; their establishments involve ternary invariant conditions. The three condensed phases Ag₂S+Fe_1–x S+FeS₂ become stable together at 532±2 °C through a ternary eutectic reaction near Ag₂S in composition. An invariant reaction at 248±8 °C results in the formation of the Ag+FeS₂ pair from the Ag₂S+Fe₇S₈ assemblage, which is stable at higher temperatures. The associations of native silver and pyrite are found in certain massive sulfide deposits, whereas natural coexistence of argentite and pyrrhotite has not been documented. Experiments demonstrate the feasibility of retrograde reequilibration in ores to produce the silver+pyrite pair from argentite+pyrrhotite. Less than 0.05 and 0.1 at. % Ag are soluble in FeS₂ and Fe_1–x S, respectively, at 600 °C and less than 0.8 at. % Fe in Ag₂S at 500 °C. Silver does not measurably affect the d _10.2 values of Fe_1–x S or the cell dimension of FeS₂ (a _{25 °C}=5.4175±0.0001 Å). This study also demonstrates that at low temperatures the binary fugacity data are applicable to ternary assemblages of the Ag-Fe-S system because of these very limited solubilities. The presence of Fe lowers the fcc bcc inversion temperature of Ag₂S more than 50 °C; the exact amount of lowering is dependent on the associated Ag-Fe-S phases. The bcc mono. inversion temperature, however, is not measurably affected. No ternary solid phases were encountered above 150 °C. Heating of sternbergite and argentopyrite (both AgFe₂S₃) mineral samples shows instability at 152 °C (e.g. partial breakdown of sternbergite in 405 days); rate studies show that a 10 °C temperature increase results in approximately a 5-fold increase in breakdown rate.

---

Zusammenfassung Für die Interpretation von natürlichen Ag-Fe-S-Mineralen (z. B. silberhaltiger Pyrit) werden die Phasenbeziehungen im System Ag-Fe-S durch quenching Experimente bei Temperaturen von 700 ° bis 150 °C untersucht. Evakuierte und zugeschmolzene Quarzglasröhren dienen dabei als Reaktionsgefäße. Die Phasenassoziationen Ag₂S+Fe_1–x S (Argentit+Magnetkies) und Ag₂S+FeS₂ (Argentit+Pyrit) werden mit fallender Temperatur bei 622±2 °C und 607±2 °C stabil. Ihre Bildung ist nur unter ternären, invarianten Bedingungen möglich. Bei 532±2 °C bilden sich durch eutektische Reaktion (nahe der Ag₂S-Zusammensetzung) als stabile Phasen Ag₂S+Fe_1–x S+FeS₂. Bei 248±8 °C reagieren Ag₂S+Fe₇S₈, die zwischen Temperaturen von 292 °C bis 248 °C stabil sind, zu Ag+FeS₂. Paragenesen von gediegen Silber+Pyrit kommen in einigen massiven Sulfidlagerstätten vor; die Paragenese Argentit+Magnetkies ist dagegen noch nicht beobachtet worden. Die bisherigen experimentellen Ergebnisse machen eine retrograde Wiedereinstellung des Gleichgewichtes von gediegen Silber+Pyrit aus Argentit+Magnetkies wahrscheinlich. Bei 600 °C sind <0,05% bzw. 0,1% Ag in FeS₂ und Fe_1–x S löslich. Bei 500 °C lösen sich <0,8% Fe in Ag₂S. Die Zellkonstanten von Magnetkies (gemessen als d _10,2) und von Pyrit (a _{25 °C}=5,4175±0,0001 Å) werden durch die Aufnahme von Ag nicht meßbar beeinflußt. Die vorliegenden Ergebnisse zeigen, daß die Fugazitäten bei niederen Temperaturen entlang den binären Schnitten Fe-S und Ag-S auch auf das ternäre System Ag-Fe-S angewendet werden können, weil nur sehr beschränkte Mischbarkeiten existieren. Die Gegenwart von Fe erniedrigt die Inversions-temperatur fcc bcc für Ag₂S um mehr als 50 °C. Die genaue Inversions-temperatur wird durch die assozierten Ag-Fe-S Phasen festgelegt. Die bcc mono. Inversionstemperatur wird dagegen nicht meßbar beeinflußt. Oberhalb 150 °C werden keine ternären Phasen beobachtet. Sternbergit und Argentopyrit (beides AgFe₂S₃-Minerale) werden oberhalb 152 °C instabil (z. B. bricht Sternbergit teilweise nach 405 Tagen zusammen). Eine Temperaturerhöhung um ca. 10 °C erhöht die Zerfallsrate um ein Fünffaches.

     

Phase equilibria of several tungsten deposits in southern China

This paper reviews the features of several tungsten ore deposits in southern China, including the Shizhuyuan, Yaogangxian, Xihuashan, Dangping, Piaotang and Dajishan mines. The authors think that the formation depth of the greisen-type deposits is intermediate between the pegmatite-and porphyry-type deposits. Using phase diagrams of the model system K₂O-Al₂O₃-SiO₂-H₂O-HF, the formation mechanism of greisens has been argued. The variation of mineral assemblages from K-feldspar, muscovite to topaz reflects a gradual increase in acidity. This variation can be used as a criterion to appreciate the depth of mineralization. Phase equilibria of tungsten minerals in grcisen and skarn deposits can be depicted on chemical potential diagrams of the model system CaO-FeO-WO₃-CO₂-F₂O_?1. The assemblage wolframite + fluorite is shown to have ben transformed to scheelite + magnetite with decreasingμ _HF. Depending on CO₂ activity in the fluids, there are two distinct phase diagrams. One shaws that wolframite is incompatible with calcite, and the other demonstrates that scheelite is incompatible with rhodochrosite. On the phase diagrams of the model system FeO-MnO-WO₃-F₂S_?1, huebnerite is transformed to ferberite with increasingμF₂O_?1 orμF₂S_?1. During the replacement of wolframite by scheelite, the wolframite residue must have been enriched in Mn.     

Phase equilibria involving humite minerals in impure dolomitic limestones

The natural occurrence of critical assemblages among the phases clinohumite, calcite, dolomite, tremolite, forsterite, diopside, chlorite, and spinel in metamorphosed impure limestones, together with experimental and thermodynamic data, permits the calculation of phase equilibria governing the stability of clinohumite in terms of the variables P, T, and composition of a CO₂-H₂O-HF fluid. Equilibrium constant expressions are given for 23 equilibria that describe the stable phase relations between the above phases. Pure OH-clinohumite is considered to be metastable at relatively low pressures. The occurrence of clinohumite in natural marbles is the result of nonideal fluorine substitution which increases the stability of clinohumite. The stability field for clinohumite +calcite, governed primarily by the equilibrium 4forsterite+dolomite+H₂O = clinohumite+calcite +CO₂, expands to more CO₂-rich fluid compositions with increasing fluorine contents and decreasing total pressure. The F/(F+OH) ratio of clinohumite coexisting with calcite, dolomite, and forsterite is a sensitive indicator of the composition of the mixedvolatile fluid phase. The thermodynamic model is in good agreement with observed phase relations and can be used to gain useful information concerning the P-T-X _fluid conditions responsible for the formation of clinohumite.     

Phase equilibria involving humite minerals in impure dolomitic limestones

The natural occurrence of critical assemblages among the phases clinohumite, chondrodite, norbergite, tremolite, forsterite, brucite, periclase, diopside, calcite and dolomite, together with experimental and thermochemical data, permits the calculation of phase equilibria governing the stability of the humite group minerals in impure dolomitic limestones. The phase relations are described by 29 divariant (OH-F) continuous reactions, and 11 univariant discontinuous reactions. The equilibrium conditions for these reactions have been calculated and plotted in isobaricT-X(OH-F) andT- phase diagrams. Continuous reactions govern the compositions of (OH-F) solid solutions and the consequent movement of three-phase triangles on the chemographic diagram. Discontinuous reactions result in the appearance or disappearance of a distinct phase assemblage. The pure OH-humite minerals are metastable relative to forsterite+brucite. With increasing fluorine content, clinohumite, followed in turn by chondrodite and norbergite, becomes stable. The stability fields for the individual humite minerals expand to more CO₂-rich fluid compositions with increasing fluorine content and decreasing total pressure. At 1,000 bars, clinohumite can contain a maximum of 58 mole percent fluorine before reacting discontinuously to form chondrodite (X _F=0.61) and forsterite. The stability field for clinohumite+calcite is restricted to fluids with <0.40. At temperatures less than 700°C, the minimum fluorine mole fractions required to stabilize chondrodite and norbergite are 0.31 and 0.62 respectively. At the same conditions, chondrodite can contain a maximum of approximately 85 mole % F. The calculated phase equilibrium boundaries, the range of stable humite compositions and the compositions of coexisting (OH-F) phases are in good overall agreement with natural assemblages. Owing to steepdT/d slopes for several of the humite continuous reactions, the F/(F+OH) ratio of a given humite mineral is a useful indicator of the CO₂/H₂O ratio of the fluid phase.     

Dual-thermodynamic estimation of stoichiometry and stability of solid solution end members in aqueous–solid solution systems

The dual-thermodynamic (DualTh) approach is shown to provide a useful alternative to other methods in: (i) forward modelling of equilibrium speciation, activities, and element partitioning in a heterogeneous system involving several variable-composition phases, such as the aqueous–solid solution system; (ii) estimation of interaction parameters of a non-ideal mixing model from known bulk compositions of coexisting aqueous and solid-solution phases; and (iii) retrieval of unknown stoichiometries and apparent standard chemical potentials of trace solid-solution end-members. Inverse-modelling tasks (ii) and (iii) can be performed when the solid solution of interest is shown experimentally to co-exist with the aqueous phase either in the equilibrium or at the minimum stoichiometric saturation state.DualTh calculations exploit the ability of Gibbs energy minimisation (GEM) algorithms to find simultaneously two numerical solutions of the isobaric–isothermal chemical equilibrium speciation problem: (1) primal solution x — a vector of amounts of components (species) in phases; and (2) dual solution u — a vector of chemical potentials of stoichiometry units (usually chemical elements and charge). Conversely, the chemical potential of a phase component can be found in two complementary ways: (i) primal via its standard-state potential, concentration and activity coefficient (the latter two are functions of the x vector); and (ii) dual through its formula stoichiometry multiplied by the u vector. The DualTh methods compare primal and dual values of the chemical potential in simple and straightforward equations that can be easily computed in a spreadsheet, or implemented in GEM geochemical modelling codes.     

大别山南部高压-超高压变质地体中云母片岩和片麻岩的相平衡研究

对于大别山高压一超高压变质带中大面积出露的云母片岩和片麻岩的变质条件和演化历史有不同的认识：如中低压绿帘角闪岩相,高压角闪岩相和超高压榴辉岩相。通过对大别山南部超高压榴辉岩区、高压榴辉岩区和宿松变质杂岩带中的云母片岩和片麻岩进行详细的岩相学及相平衡研究表明：它们的变质条件分别为610℃～690℃,1．2～1．4GPa;590～600℃、1．3～1．5GPa;520～590℃,0．9～1．4GPa,相当于高压绿帘角闪岩相、高压角闪岩相与榴辉岩相之间的过渡。依据超高压榴辉岩区绿帘石黑云片麻岩中的石榴石环带特征推测,它们很可能经过超高压变质阶段。现有矿物组合记录了抬升阶段的条件。但是高压榴辉岩区的石榴石绿帘石二云片麻岩和宿松变质杂岩带中石榴石云母片岩中的石榴石环带特征表明,其现有矿物组合记录了变质峰期条件,不应该经历更高压变质阶段。在KFMASH体系的PT视剖面图上计算多硅白云母Si含量等值线表明,在多数矿物组合中白云母的Si含量可作为地质压力计,但Si等值线的斜率受到矿物组合的控制。     

Phase equilibria for graphitic metapelite including solution of CO2 in melt and cordierite: implications for dehydration,partial melting and graphite precipitation

When graphite is present, carbon‐bearing species dissolve in the C‐O‐H fluid and lower the activity of water (). Accordingly, metamorphic reactions that involve water, namely dehydration and partial melting reactions, adjust their P–T positions to accommodate the change of . In this modelling study, pseudosections are calculated for graphite‐bearing systems that are either closed or that progressively lose fluid and/or melt. The diagrams incorporate a new model of CO₂ solubility in felsic melts that we derived to be compatible with a recently published melt model. As the result of the lowered in the carbon‐bearing systems, the temperature displacements of the solidus can be as large as 50 °C at low pressures in cordierite‐bearing zones (<4 kbar), but are smaller than 15 °C at mid‐pressure P–T conditions (4–9 kbar). In the supersolidus region, the phase relations among silicate minerals + melt are very close to those in carbon‐free systems. The fluid CO₂ content increases as temperature increases in the supersolidus assemblages. The CO₂‐rich fluid can be stable in granulite facies conditions in an oxidized system. In graphitic systems, melt and/or cordierite dominate the CO₂ budget of high‐grade rocks. During cooling, the fluid that exsolves from such crystalizing melt is CO₂‐rich. In addition to the phase relations, the pseudosections presented in this study enable researchers to quantitatively investigate the evolution of phase modes, including graphite, along specific metamorphic P–T paths. At low pressures in the cordierite stability field, graphite is predicted to precipitate as the pressure increases or temperature decreases in the subsolidus assemblages, or temperature increases in the region of melt + fluid coexistence. On the other hand, the graphite abundance remains nearly constant along the mid‐pressure P–T series, but the graphite mode in the supersolidus region may increase due to residual enrichment if the melt is extracted. The modelling results show that metamorphic processes in closed systems lead to only small changes in graphite mode (a few tenths of a per cent). This strongly suggests that open‐system behaviours are required for large amounts of graphite deposition, including fluid infiltration and mixing or residual enrichment processes in high‐grade rocks. In addition to P–T pseudosections, P/T–X_O diagrams (X_O = O/(H + O) in the fluid) illustrate the thermodynamic features of internal buffering from another perspective, and explore the dependence of phase relations on the externally imposed redox state. If the system is equilibrated with CO₂ or CH₄‐rich infiltrating fluid, the temperature displacements of metamorphic reactions can be larger than 50 °C, compared with carbon‐free systems.     

大别山西段含蓝闪石-蓝晶石榴辉岩的相平衡研究

目前对于大别山西段超高压榴辉岩仍存在一些不清楚的问题和模糊的认识,如蓝闪石和蓝晶石组合的稳定范围,峰期温压条件和矿物组合,以及早期退变质过程的矿物演化和流体作用.本文对取自大别山西段新县高压-超高压榴辉岩单元内不同地点的超高压榴辉岩样品进行了详细的岩石学和矿物学研究,在此基础上使用相平衡定量分析方法的PT视剖面图对它们进行了正演模拟计算,结果表明含蓝闪石和蓝晶石榴辉岩处于相对低温或低压的蓝闪石榴辉岩和相对高温高压的蓝晶石榴辉岩的过渡区,其稳定的温压范围大致为温度590～700 ℃,压力1.7～3.3 GPa,而且压力大于2.5 GPa时温度范围很窄,为600～640 ℃.由石榴石边缘成分和PT视剖面图确定的榴辉岩峰期温压条件为压力2.85～2.95 GPa和温度625～630 ℃,峰期矿物组合为石榴石+绿辉石+蓝闪石+蓝晶石+硬柱石+柯石英±多硅白云母.峰期之后,榴辉岩经历了快速近等温降压(ITD)的早期高压退变质作用,这是一个非平衡过程,所发生的主要变化如下柯石英→石英,硬柱石→黝帘石+蓝晶石,在相对富镁岩石中出现滑石,当水含量较高时可以出现钠云母,蓝闪石在原来基础上有一定量的生长,并且绿辉石和多硅白云母很可能只部分地发生了成分变化,而石榴石几乎未发生改变.这样形成了目前观察到的矿物组成为石榴石+绿辉石+蓝闪石+蓝晶石+黝帘石/绿帘石+石英±多硅白云母±钠云母±滑石,它代表了UHP榴辉岩在早期高压退变质阶段结束时所具有的矿物组成,这一阶段结束时的温压条件大致为2.0～2.2 GPa和600～630 ℃;早期高压退变质阶段是脱水过程,流体是内部缓冲的.     

Phase equilibria in the hornblende-bearing basic gneisses of the Uvete area, central Kenya

Abstract The hornblende-bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio-clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A'(Al + Fe³⁺− (13/7)Na)-F(Fe²⁺)-M'(Mg)-C'(Ca-(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other.
The phase relations in the Uvete area are in contrast with those in the staurolite-kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low-variance mineral assemblages formed under different values of μ_H2O and μ_CO2. These assemblages define overlapping composition volumes in the A'-F'-M'-C'tetrahedron.
The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH₂O value. Such a fluid composition was externally controlled by the supply of H₂O-rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.     

Enthalpies of ordering in the plagioclase feldspar solid solution

Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole “e” structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il? structure →- Cl? high albite structure: ~0.7-1.9 kcal/mole Il? structure equilibrated at low temperature → Il? structure equilibrated at high temperature: ~ 1.8?0.8 kcal/mole.ΔH_soln data for the high series overlap with the data of Newtonet al. (1980) for synthetic high structural state plagioclases except in the composition range ~An₉₀–An₁₀₀. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl? symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il? and “e” structures.Values of the enthalpy change due to disordering (ΔH_ord) also show a number of systematic trends. Firstly, the values for e → Cl? are larger than for Il? → Cl? in the composition range where both e and Il? structures are observed (~An₆₅-An₇₂). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔH_ord, which occurs at ~An₅₀, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il? anorthites and bytownites to a Cl? structure have been estimated by assuming that the Cl? solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.     

Phase equilibria of phlogopite lamprophyres from western Mexico: biotite-liquid equilibria and P-T estimates for biotite-bearing igneous rocks

 Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900 to 1300° C, and then quenched rapidly, at oxygen fugacities controlled between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers. The liquidus of both samples is suppressed ∼100° C at water-saturated conditions and 1500 bar. Both lavas contained between 3 and 4 wt% water at the stage of phenocryst precipitation. The partitioning of ferric and ferrous iron between phlogopite and liquid has been determined on eight samples across 3 log f _O2 units; when these determinations are combined with previous studies, Fe₂O₃/(Σ FeO total) of Mg-rich biotite can be calculated knowing log f _O2, T, and X _Fe. Thermodynamic modelling of biotite-liquid equilibria results in two expressions for calculating activity coefficients (γ) for annite and phlogopite in natural biotites. Based on the partitioning of BaO and TiO₂ between biotite and liquid, we have formulated a thermometer and barometer. Over the range of 400° C, TiO₂ partitioning between phlogopite and liquid is a function of temperature (±50° C), and is insensitive to pressure and H₂O and O₂ activities. BaO partitioning between phlogopite and liquid is a function of both temperature and pressure (±4 kbar), the latter being most important. Applying the TiO₂ and BaO partitioning expressions to lamprophyre and lamproite suites shows that Mexican minettes equilibrated at low pressures (5 to 15 kbar;±4 kbar) and temperatures (1090 to 1160° C; ±50° C), while Australian lamproites equilibrated at higher P (up to 30 kbar; ±4 kbar) and T (1125 to 1400° C; ±50° C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO- and TiO₂- based pressure calculations indicate that felsic minettes from the Mexican suite of lavas can be generated by simple fractionation of a more mafic parent minette at mid to lower crustal pressures. Received: 1 August 1994/Accepted: 30 June 1995     

Hydrogen and carbon in solid solution in oxides and silicates

The dissolution of H₂O and CO₂ in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H₂O and CO₂ are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH^? pairs which derive from the dissolution of H₂O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O ₂ ^2? ... H₂. Because the O ₂ ^2? moiety is small (O^?-O^? distance ≈ 1.5 Å) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H₂ may be lost from the solid which retains excess oxygen in the form of O ₂ ^2? , leading to the release of atomic O. The dissociation of O ₂ ^2? moieties into a vacancy-bound O^? state and an unbound O^? state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O^? and their effect on the d.c. conductivity. Evidence is presented that CO₂ molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO ₂ ^2? and O ₂ ^2? , both associated with cation vacancy sites. In the CO ₂ ^2? moiety (C-O^? distance 1.2–1.3 Å, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO ₂ ^2? moiety into CO^? and O^?. This transition can be followed by infrared spectroscopy, using OH^? as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO₂, CH₄, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.     

Phase equilibria and compositions of Fe-Mg-Al minerals and melts in water-saturated peraluminous granitic systems

Phase assemblages and phase compositions were studied experimentally in water-saturated, biotite-bearing peraluminous granitic melts as a function of alumina excess and temperature. The runs were performed at 2 and 5 kbars under NNO buffer. Biotite was stable only in composition containing 5% of normative corundum; it coexisted with cordierite and hercynite at 2 kbars and with hercynite at 5 kbars. In composition containing 10% of normative corundum biotite was not observed; abundant cordierite and hercynite were the only Fe-Mg-Al minerals. These relationships show that, at constant pressure, the amount of cordierite increases with increasing excess of alumina. Simultaneously the stability of biotite decreases due to preferential partitioning of Mg into cordierite and Fe into biotite. Besides the distribution of Fe, Mg and Al among the coexisting solid phases, solubility of these elements in the melts is given. Below 900° C melts are poor in iron and magnesium and correspond, in terms of these elements, to leucogranites. It is suggested that the leucogranitic magmas, such as parental magmas of European Hercynian and Himalayan leucogranites, must have been formed through highly efficient separation of partial melt from restite, in which ferromagnesian components are concentrated. Peraluminous granites rich in ferromagnesian minerals originate supposedly from restite-bearing magmas.