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

Wilhelm Johannes 《Contributions to Mineralogy and Petrology》1980,74(1):29-34

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

Intergrowths of nepheline-potassium feldspar and kalsilite-potassium feldspar: A re-examination of the ‘pseudo-leucite problem’     

J. Gittins  J. J. Fawcett  C. K. Brooks  J. C. Rucklidge 《Contributions to Mineralogy and Petrology》1980,73(2):119-126

The alkalic ultramafic Batbjerg intrusion of East Greenland contains rocks in which nepheline and leucite are important constituents. In addition, there are vermicular, finger print intergrowths of nepheline with potassium feldspar, and patchy to micrographic intergrowths of kalsilite with potassium feldspar. The history of the pseudoleucite problem is reviewed, and it is suggested that the term pseudoleucite be restricted to intergrowths of nepheline with alkali feldspar that appear to be pseudomorphs with the crystal morphology of leucite. It is further suggested that flame-like or feather-like finger print intergrowths of nepheline with alkali feldspar, that are either interstitial to the other minerals of the rock or have grown perpendicularly on relative large and often euhedral nepheline grains are an entirely different problem and are best explained by late-stage magmatic crystallization within the system NaAlSiO₄-KAlSiO₄-SiO₂-H₂O.In the Batbjerg intrusion the early crystallization of nepheline was followed by the co-crystallization of nepheline with leucite, or in some cases by nepheline and a silica-rich leucite. Although the magma was essentially dry, as indicated by the dominantly pyroxenitic character of the rocks, water pressure rose toward the late stages of crystallization as indicated by the presence of phlogopite and occasionally both amphibole and zeolite. Shrinkage of the leucite stability field attendant upon this rise in left the liquid that was crystallizing nepheline and leucite stranded on the nepheline-alkali feldspar cotectic. Shrinkage occurred too rapidly for the liquid to remain at the reaction point of the system, and leucite, therefore, was not resorbed. The remaining liquid crystallized rapidly as flames of vermicular intergrowth of nepheline with potassium feldspar (composition Ne 24.0, Ks 45.9, Qz 30.1), a texture that might be attributable to supercooling. Silica-rich leucite compositions (Ks 68.8, Qz 31.2) decomposed to intergrowths of kalsilite with potassium feldspar but reaction kinetics, or possibly variations in throughout the intrusion, prevented the breakdown of leucite.  相似文献   

Structure of mineral glasses—I. The feldspar glasses NaAlSi₃O₈, KAlSi₃O₈, CaAl₂Si₂O₈     

Mark Taylor  Gordon E. Brown 《Geochimica et cosmochimica acta》1979,43(1):61-75

The short range distribution of interatomic distances in three feldspar glasses has been determined by X-ray radial distribution analysis. The resulting radial distribution functions (RDF's) are interpreted by comparison with RDF's calculated for various quasi-crystalline models of the glass structure.The experimental RDF's of the alkali feldspar glasses were found to be inconsistent with the four-membered rings of tetrahedra associated with crystalline feldspars; the structures of these glasses are probably based on interconnected six-membered rings of the type found in tridymite, nepheline, or kalsilite. In contrast, the RDF of calcic feldspar glass is consistent with a four-membered ring structure of the type found in crystalline anorthite. T-O bond lengths (T = Si,Al) increase from 1.60 Å in SiO₂ glass [J. H. Konnert and J. Karle (1973) Acta Cryst.A29, 702–710] to 1.63 Å in the alkali feldspar glasses to 1.66 Å in the calcic feldspar glass due to the substitution of Al for Si in the tetrahedra] sites. The T-O-T bond angles inferred from the RDF peak positions are 151° in SiO₂ glass (see reference above), 146° in the alkali feldspar glasses, and 143° in the calcic feldspar glass. Detail in the RDF at distances greater than 5 Å suggests that the alkali feldspar glasses have a higher degree of long range order than the calcic feldspar glasses.Assuming that the structural details of our feldspar glasses are similar to those of the melts, the observed structural differences between the alkali feldspar and calcic feldspar glasses helps explain the differences in crystallization kinetics of anhydrous feldspar composition melts. Structural interpretations of some thermodynamic and rheologic phenomena associated with feldspar melts are also presented based on these results.  相似文献   

Phase relations in the system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-SiO2 at 1 kilobar water vapour pressure     

R. S. James  D. L. Hamilton 《Contributions to Mineralogy and Petrology》1969,21(2):111-141

Crystal-melt relations at a water vapour pressure of 1 kilobar have been determined for planes at 3, 5, 7.5, and 10 weight per cent anorthite in the system NaAlSi₃O₈KAlSi₃O₈-CaAl₂Si₂O₈-SiO₂. The ratio of the silicate components in the liquids which are in univariant equilibrium with plagioclase, alkali feldspar, quartz and gas are Ab₃₁Or₂₈Q₃₈An₃ (weight per cent) at 730°±5–10° C, Ab₂₁Or₃₄Q₄₀An₅ at 745°±5–10° C and Ab₁₀Or₃₉ Q_43.5An_7.5 at 780°±10° C. The univariant curve on which the above compositions lieoriginates on the H₂O-saturated Or-An-Q plane at a composition containing less than 10 weight per cent An and terminates within 1.5 weight per cent An of the H₂O-saturated Or-Ab-Q plane. Experimental data for the synthetic system have been used to illustrate a discussion on the partial melting of metasediments and the possible significance of such a process with respect to the genesis of granitic rocks. Data taken from the literature (Winkler and v. Platen, 1960, 1961a) have been used to illustrate that the normative salic composition of a sediment has a strong influence on the composition of any melt which form when such a rock is subjected to high temperatures and pressures.  相似文献   

Metastable melting in the granite system Qz-Or-Ab-An-H2O     

W. Johannes 《Contributions to Mineralogy and Petrology》1980,72(1):73-80

Time studies were performed in the quinary system Qz-Or-Ab-An-H₂O at kbars and T=665 ° and 660 ° C. Starting material was a mixture of quartz, alkali feldspar Or₈₀ and plagioclase An₃₁. The compositions of plagioclases of run products were determined and compared with the plagioclase of stable solidus conditions.The solidus of the granite system was fixed at P _HäO=5 kbars using various plagioclase — and appropriate alkali feldspar — compositions besides quartz in the starting mixture (Fig. 1).The results of time studies (Table 3 and Fig. 3) reveal metastable melting in the granite system Qz-Or-Ab-An-H₂O. Plagioclase melts almost stoichiometrically. The new plagioclase compositions formed during melting of cotectic compositions approach the theoretically expected stable plagioclase compositions only extremely slowly. An extrapolation of the data achieved in run times of 5–1,500 h indicates attainment of equilibrium after 10¹⁴ years. Metastable melting of granitic compositions is not only considered as an experimental problem but also as a rock forming process in nature.  相似文献   

Mantled feldspars from the Golden Horn batholith,Washington     

Robert J. Stull 《Lithos》1978,11(3):243-249

Mantled feldspars that formed by resorption, development of skeletal plagioclase crystals, and filling with alkali feldspar are common in the Golden Horn batholith, Washington. Subhedral plagioclase mantles have weak normal zoning from An₁₇ to An₁₀. Plagioclase zoning and twinning are crosscut by resorption channels. Resorption cavities and channels are coated with albite (An₁₀). Anhedral, perthitic orthoclase within the plagioclase is optically continuous with orthoclase in channels and on the mantle exterior.This texture resulted from resorption of calcic cores of plagioclase as pressure decreased when water-undersaturated granite magma intruded to a shallow crustal level. At shallow level, only alkali feldspar and quartz crystallized and were available to fill the skeletal plagioclase.  相似文献   

Nucleation,growth and stability of CaAl2Si2O8 polymorphs     

Toshiya Abe  Katsuo sukamoto  Ichiro Sunagawa 《Physics and Chemistry of Minerals》1991,17(6):473-484

By means of in situ observation, the nucleation and growth of triclinic, pseudo-orthorhombic and pseudo-hexagonal CaAl₂Si₂O₈ were investigated and their relative stabilities established. Pseudo-hexagonal and pseudo-orthorhombic CaAl₂Si₂O₈ nucleate prior to anorthite and grow in a supercooled anorthite melt before the latter appears. They either dissolve or transform to anorthite once the latter nucleates and starts to grow. Corundum and mullite also nucleate metastably prior to anorthite in this melt. The liquidus temperatures of the four metastably nucleated phases were determined precisely by in situ measurements of the transformation temperatures from a polyhedral to a rounded morphology. On the basis of the supercooling thus determined, the growth rate versus supercooling relations were measured and analysed for the respective phases. The growth rate of the stable phase is one order of magnitude higher than those of any metastable phases. However, the growth rates of the three CaAl₂Si₂O₈ polymorphs normalized by the viscosities are nearly the same.  相似文献   

The system Mg2SiO4-Ca2SiO4-CaAl2O4-NaAlSiO4-SiO2: one atmosphere liquidus equilibria of analogs of alkaline mafic lavas     

Vivian Pan  John Longhi 《Contributions to Mineralogy and Petrology》1990,105(5):569-584

One atmosphere liquidus relationships in the system Mg₂SiO₄ (Fo)–Ca₂SiO₄ (La)–NaAlSiO₄ (Ne)–CaAl₂O₄ (CA)–SiO₂ (Sil) are presented as analogs for alkaline mafic lavas. Liquidus diagrams are constructed from electron microprobe analyses of quenched liquids and coexisting mineral phases produced in melting experiments and they are depicted in terms of sub-projections within the pseudo-quaternary system Fo–La–[Ne,CA]–Sil. The Ne and CA components are combined to create a normative feldspathoid component defined as Ne#=Ne/[Ne+CA]. Liquidus relations at Ne#=0.5 from this study are compared to relations at Ne#=0.0 and 1.0 from previous studies. In general, liquidus temperatures decrease and positions of liquidus boundaries involving feldspathic phases shift toward the [Ne, CA] component as Ne# increases. The pseudoinvariant points fo+di+pl+mel+l and fo+pl+mel+sp+l exist at Ne#=0.5. These equilibria between forsterite-plagioclase-melilite-liquid are not present in the system when Ne#=1.0 because a boundary curve (fo+di+ne+l) separates the plagioclase and melilite liquidus fields. The liquidus diagrams provide useful analogs for the crystallization sequences of natural primary alkali olivine basalts, basanitoids, basanites, olivine nephelinites, olivine-melilite nephelinites and olivine melilitites.  相似文献   

Ultrapotassic Rocks along Late Ductile Shear Zones from the Eastern Ghats Belt,India     

《Gondwana Research》2000,3(1):55-63

Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, Khyphen;feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midhyphen;crust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K₂O/Na₂O ratio of ∼12 (molar) and K₂O/Al₂O₃ ratio of ∼1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity. However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to Khyphen;feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt posthyphen;dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Panhyphen;African age. The extensive Khyphen;feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.  相似文献   

Activity/composition relations in the ternary feldspars   总被引：6，自引：0，他引：6  

Mark S. Ghiorso 《Contributions to Mineralogy and Petrology》1984,87(3):282-296

Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi₃O₈ CaAl₂Si₂O₈-KAlSi₃O₈. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of equilibrium tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi₃O₈ in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents.  相似文献   

Silicate liquid immiscibility in magmas and in the system K2O-FeO-AI2O3-SiO2: an example of serendipity     

Edwin Roedder 《Geochimica et cosmochimica acta》1978,42(11):1597-1617

The concept of silicate liquid immiscibility was invoked early in the history of petrology to explain certain pairs of compositionally divergent rocks, but. as a result of papers by Greig (Am. J. Sci.13, 1–44, 133–154) and Bowen (The Evolution of the Igneous Rocks), it fell into disfavor for many years. The discovery of immiscibility in geologically reasonable temperature ranges and compositions in experimental work on the system K₂O-FeO-Al₂O₃-SiO₂, and of evidence for immiscibility in a variety of lunar and terrestrial rocks, has reinstated the process.Phase equilibria in the high-silica corner of the tetrahedron representing the system K₂O- FeO-Al₂O₃-SiO₂ are presented, in the form of constant FeO sections through the tetrahedron, at 10% increments. Those sections, showing the tentative relationships of the primary phase volumes, are based on 5631 quenching runs on 519 compositions, made in metallic iron containers in pure nitrogen. Thirteen crystalline compounds are involved, of which at least six show two or more crystal modifica-tions. Two separate phase volumes, in each of which two immiscible liquids, one iron-rich and the other iron-poor, are present at the liquidus. One of these volumes is entirely within the quaternary system, astride the 1:1 K₂O:Al₂O₃ plane. No quaternary compounds as such have been found, but evidence does point toward at least partial quaternary solid solution, with rapidly lowering liquidus temperatures, from K₂O·Al₂O₃· 2SiO₂ (‘potash nepheline’, kalsilite. kaliophilite) to the isostructural compound K₂O·FeO·3SiO₂, and from K₂O·Al₂O₃·4SiO₂ (leucite) to the isostructural compound K₂O·FeO·5SiO₂, Both of these series apparently involve substitution, in tetrahedral coordination. of a ferrous iron and a silicon ion for two aluminum ions. Some of the ‘impurities’ found in analyses of the natural phases may reflect these substitutions.As a result of the geometry of the immiscibility volume located entirely within the quaternary system, compositions near it show a number of phase changes and large amounts of crystallization with small temperature changes, generally in the range 1100–1150 C. Similar low-temperature, high-alkali immiscibility was discovered in a few exploratory runs in the equivalent systems with Rb or Cs substituting for K. But not in those with Li or Na.A review of the compositions and general behavior of systems involving immiscibility, both stable and metastable, and of the evidence for natural immiscibility. indicates that it may be a much more common feature than generally thought. Several examples of natural immiscibility are detailed; most yield a felsic. alkali-aluminosilicate melt and a mafic melt. from a wide variety of generally basaltic parental magmas, both under- and over saturated. Unfortunately, the best line of evidence for immiscibility in terrestrial rocks, a sharply defined meniscus between two compositionally disparate glasses, is by its very nature self-destructing, since it is effectively eliminated by either crystallization or gravitative separation and coalescence into separate magmas. Verification of operation of the exosolutionor ‘splitting’ process on a large scale will probably require careful study of isotopic and trace element partitioning in both laboratory and field.  相似文献   

Stability of scapolite in the system Ab-An-NaCl-CaCO₃ at 4 kb and 750°C     

Philip M. Orville 《Geochimica et cosmochimica acta》1975,39(8):1091-1105

Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab₈₅An₁₅ to Ab₇₀An₃₀, although albite + halite is stable relative to marialite, Na₄Al₃Si₉O₂₄Cl, and anorthite + calcite is stable relative to meionite, Ca₄Al₆Si₆O₂₄CO₃. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa₃Al₅Si₇O₂₄CO₃ (Ab. 2An. CaCO₃). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab₆₀An₄₀) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab₆₀An₄₀-Ab₃₅An₆₅ and another three-phase invariant assemblage, calcic plagioclase (~Ab₁₅An₈₅) + scapolite + calcite is stable over the approximate range Ab₃₀An₇₀-An₁₅An₈₅.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO₃ ratio in the large anion site can vary as a function of NaCl and CaCO₃ activities.  相似文献   

Geochemistry of Santo Antônio da Barra Kamafugites, Goiás, Brazil     

Patricia Barbosa de Albuquerque Sgarbi  Jos Carlos Gaspar 《Journal of South American Earth Sciences》2002,14(8):236

This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO₂, high to moderate contents of Al₂O₃, alkalis, and P₂O₅, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO₂, K₂O, and Na₂O contents and in the frequent presence of normative nepheline and leucite. K₂O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents.  相似文献   

High-pressure phase transitions of potassium aluminosilicates with an emphasis on leucite     

Lin -gun Liu 《Contributions to Mineralogy and Petrology》1987,95(1):1-3

Phase behaviors of kalsilite (KAlSiO₄), leucite (KAlSi₂O₆) and sanidine (KAlSi₃O₈) at high pressures and temperatures have been reviewed. New experimental data for leucite composition were obtained in a diamond-anvil press coupled with laser heating. At about 1,000° C, the leucite composition breaks down into an assemblage of kalsilite +kyanite+K₂Si₄O₉ (wadeite-type) in the range 60 to 120 kbar, of kalsilite+KAlSi₃O₈ (hollandite-type) in the range 120 to 150 kbar, and of KAlSi₃O₈ (hollandite-type) + KAlO₂ (?) at 170 kbar or above. The suggestion that kalsilite is the most appropriate potassium silicate in the anhydrous upper mantle needs to be revised. It is suggested that the wadeite-type K₂Si₄O₉ is the most appropriate host for potassium in the upper mantle at depths greater than about 150 km.  相似文献   

Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2     

Art Boettcher  Robert W. Luth  Bradford S. White 《Contributions to Mineralogy and Petrology》1987,97(3):297-304

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO₂ vapor. The depression of the temperature of the solidus of albite by CO₂ decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO₂ in NaAlSi₃O₈ liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO₂ lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si₃O₈ composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO₂ and CaO and depleted in Al₂O₃ relative to CaAl₂Si₂O₈.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO₂ component in the liquid ( ). We predict that this increases the ratio of molecular CO₂/CO ₃ ^2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO₂ in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi₃O₈ liquids, which is in contrast to that in CaAl₂Si₂O₈ liquid. Also, reaction of CO₂ with NaAlSi₃O₈ liquid to form CO ₃ ^2– that is complexed with Na⁺ must be accompanied by a change in Al³⁺ from network-former to network-modifier, as Na⁺ is no longer abailable to charge-balance Al³⁺ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO ₃ ^2– with the excess Ca²⁺ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO₂ increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO₂-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO₂ produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO ₃ ^2– , resulting in a low CO₂/ CO ₃ ^2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744  相似文献   

Theoretical approach to evaluating plagioclase dissolution mechanisms     

Li Zhang 《Geochimica et cosmochimica acta》2009,73(10):2832-4139

The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution-reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite (An₀) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An₂₀ is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An₃₀. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content (An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An₈₀ and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, ΔG dependence, and formation of altered surface layers in the framework of the interfacial dissolution-reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution-reprecipitation mechanism governs the dissolution of plagioclase feldspars.  相似文献   

The System Na2O-Al2O3-Fc2O3-SiO2 at 1 Atmosphere, and the Petrogenesis of Alkaline Rocks     

BAILEY  D. K.; SCHAIRER  J. F. 《Journal of Petrology》1966,7(1):114-170

Equilibria involving acmite, albite, nepheline, quartz, anda liquid phase constitute the petrologically important partof the system Na₂O–Al₂O₃–Fe₂O₂–SiO₂, and theunivariant and invariant relations provide useful analogiesfor a wide variety of alkaline igneous rocks. These relationsare dominated by the incongruent melting behaviour of acmite,which does not appear on the liquidus of the join acmite-nepheline-silica;instead, a broad field of hematite is present and acmite crystallizesonly from liquids containing potential sodium silicate. Consequently,the oversaturated and undersaturated eutectics, correspondingto granitic and nepheline syenitic liquids, are rich in sodiumsilicate and distinct from those found in Petrogeny's Residuasystem: the temperatures of the eutectics are 7285C and 7155C, respectively. Survival of peralkaline granite in the aluminouscontinental crust can be explained by the strongly peralkalinecomposition of the oversaturated eutectic. Magma of this typemay be the primitive granite of the non-orogenic zones. Theubiquitous alkali metasomatism around alkaline complexes canalso be interpreted in terms of residual liquids enriched inalkali silicates. Transition from undersaturated to oversaturatedliquids is possible by fractionation of hematite and a new processfor achieving the reverse transition has been found. This dependson the substitution of Fe³ for Al³ in feldspar and suggestsa more important role for syenite in any scheme of petrogenesis. Each of the two eutectics is linked to a corresponding peritecticat which hematite reacts to give acmite. The liquid at the undersaturated,quaternary reaction point is of ijolitic type, providing thefirst intimation that ijolite may represent a low-melting fractionin nature. The system Na₂O–Al₂O₃–Fe₂O₃–SiO₂thus constitutes the peralkaline residua system and on thisbasis a coherent picture of stable continental magmatism canbe constructed. Ijolite is seen as the low-melting fractionfrom a range of peralkaline compositions and from rocks suchas melilite basalt, while the frequently associated carbonatiteis considered to be the volatile-rich, fugitive material fromthe mantle. Such a relationship is consistent with the dualassociation of carbonatite with either ijolite or kimberliteunder different tectonic conditions. The more common syenite,nepheline syenite, and alkaline granite of the non-orogenicregions are regarded as low-melting fractions from basalticmaterials in the deep crust. Most of this activity, involvingmagmas of residual type, could thus be explained in terms ofpartial melting in the deep crust and upper mantle. A possiblemechanism for this would be arching of the rigid continentalcrust, the consequent relief of lithostatic load giving riseto melting, and the concentration of fugitive constituents,in the underlying zones.  相似文献   

Studies in the system KAlSiO4-BaAl2Si2O8-SiO2-H2O     

A. P. Mall  V. Rudert 《Contributions to Mineralogy and Petrology》1974,48(1):81-88

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⁺⁺.  相似文献   

Growth of plagioclase rims around metastable kyanite during decompression of high‐pressure felsic granulites (Bohemian Massif)     

R. ABART  G. NEUSSER  D. RHEDE 《Journal of Metamorphic Geology》2011,29(9):1003-1018

Samples of high‐pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high‐pressure) mineral assemblage of garnet – kyanite – plagioclase – K‐feldspar – quartz ± biotite show well‐developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K‐feldspar. Kyanite is regarded as a relict of the high‐pressure metamorphic assemblage that became metastable during transition to a low‐pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An_32–25 at the contact with kyanite to An_20–19 at the contact with the matrix or to the perthitic K‐feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim‐forming reaction to provide the Al₂O₃ component for the growth of plagioclase, whereas in other cases no Al₂O₃ from kyanite was necessary. In a majority of examples, the necessary Al₂O₃ was supplied with CaO and Na₂O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ～ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ～ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K‐feldspar grains.  相似文献   

Mafic Microgranular Enclaves and Interaction Between Felsic and Mafic Magmas in the Agacoren Intrusive Suite: Evidence from Petrographic Features and Mineral Chemistry     

《International Geology Review》2012,54(9):854-867

The Agacoren Intrusive Suite is exposed as a large intrusive body over ～500 km² east of Lake Tuz in central Anatolia and consists of the Cokumkaya gabbro, the Agacoren granitoid, and young dikes. The Agacoren granitoid is the predominant lithology of the Agacoren Intrusive Suite, and is differentiated into several subunits ranging in composition from monzonite, through granite, to alkali feldspar granite. The Cokumkaya gabbro occurs as stocks enclosed in the Agacoren granitoid; individual bodies range in size from 10 m × 20 m to 7 km × 3 km. Young dikes cut both the Cokumkaya gabbro and the Agacoren granitoid, and are particularly abundant in the central part of the intrusive body.

Centimeter- to meter-size mafic microgranular enclaves (MME) are enclosed in the Agacoren granitoid. The enclaves are diorite, quartz diorite, and monzodiorite in composition, and represent blobs of mafic magma injected into a felsic host magma. The MME have a mineral assemblage (plagioclase + amphibole + biotite ± quartz ± K-feldspar) almost identical to that of host granitoid, but with different mineral proportions. The characteristic petrographic features of the MME are the presence of acicular apatite, blade-shaped biotite, quartz ocelli, and K-feldspar poikilitically enclosing mafic minerals. Microprobe analyses performed on amphibole and plagioclase reveal similar mineral chemistries for both the MME and the host granitoid. The anorthite contents of the plagioclases show an increase from rim to core in both the MME and the host granitoid. The rims of the MME plagioclase have compositions ranging from An₅ to An₄₀, whereas those of the host granitoid vary from An₀ to An₄₂. The cores, on the other hand, range from An₃₀ to An₉₀ and An₂₀ to An₉₀ in the MME and the host, respectively. Amphiboles are essentially of ferro-hornblende composition in the MME, and of ferro- to magnesiohornblende composition in the host granitoid. The similarity in mineral compositions reflects chemical equilibrium attained through the magma-mixing process.  相似文献