The miscibility gap between rhodonite and bustamite along the join MnSiO3-Ca0.60Mn0.40SiO3

The miscibility gap between rhodonite and bustamite has been experimentally determined at temperatures between 600° and 1,100° C. For temperatures below 700° C the resulting limbs have been extrapolated on T-X-diagram as at such low temperatures equilibrium could not be attained. According to microprobe analyses for the natural assemblages of Ravinella di Sotto (Ivrea zone, Italy) and Broken Hill (N.S.W., Australia) equilibrium temperatures are estimated to be at 500° to 550° C. However these assemblages are thought to have re-equilibrated during cooling and the compositions of equilibrium assemblages are also pressure dependent. According to experiments and to molar volume data the rhodonite structure is stabilized by high pressures whereas bustamite by high temperatures. Based on available experimental results and natural data an isobaric T-X _Ca diagram and two isotherm -X _Ca diagrams (for T=400° C and T=600° C) are given.     

The high-temperature P2/c<Subscript>1</Subscript>–C2/c phase transition in Fe-free Ca-rich P2<Subscript>1</Subscript>/c clinopyroxenes

An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca_0.40Mg_1.60Si₂O₆, Ca_0.52Mg_1.46Al_0.05Si_1.98O₆, Ca_0.59Mg_1.41Si₂O₆ and Ca_0.70Mg_1.30Si₂O₆, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ and Ca_0.59Mg_1.41Si₂O₆, evidence of for the P2₁/c-C2/c displacive phase transition was seen in changes in lattice parameters at T 550 and 300 °C respectively. Landau modelling of the phase transition behaviour for the sample with composition Ca_0.52Mg_1.46Al_0.05Si_1.98O₆ shows a tricritical behaviour [T _c =547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside–enstatite indicates that a decrease in T _{c ,} and a switch from first-order to tricritical behavior occurs with increasing Ca content. The change in the transition behaviour was related to an interaction with the antiphase domains at the nanoscale.     

Stability relations in the system CaSiO3-CaMnSi2O6-CaFeSi2O6

In the system CaSiO₃-CaMnSi₂O₆-CaFeSi₂O₆ extensive miscibility gaps between pyroxenoids and clinopyroxenes are observed. The miscibility gap between Mn-bustamite and Mn-wollastonite has been determined experimentally by a hydrothermal technique between 400° and 1200° C at P _f= 2 kbar. Further experiments have been performed at P _f=9 kbar, which revealed a shifting of the miscibility gap towards more Ca-rich compositions. The bustamite phase is stabilized by high pressures and the wollastonite structure is the stable phase at high temperatures.Similar phase relations as along the join CaSiO₃-CaMnSi₂O₆ exist along the join CaSiO₃-CaFeSi₂O₆ but with a more extensive two-phase field of bustamite-clinopyroxene.Possible phase relations along the joins CaSiO₃-CaMnSi₂O₆, CaSiO₃-CaFeSi₂O₆ and CaFeSi₂O₆-CaMnSi₂O₆ are given in temperature-composition diagrams for low pressures, based on natural and experimental data.     

Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).     

Contact metamorphism and metasomatism at a dolerite-limestone contact in the Gebel Yelleq area, Northern Sinai, Egypt

Summary At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7m wide upper than to the c. 25m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):(i) clinopyroxene + garnet ± wollastonite + calcite(ii) garnet ± wollastonite + calcite;(iii) wollastonite + calcite.In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs_56–86Adr_14–42Sch_0–2Sps_0–0.2Prp₀ in the lower, and Grs_29–94Adr_5–64Sch_0–12Sps_0–0.2Prp_0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X _Mg = Mg/(Mg + Fe²⁺) = 0.97–1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X _Mg-values of 0.49 and 0.53, respectively.The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO₂) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO₂) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.     

Occurrence of wide-chain Ca-pyriboles as primary crystals in the Salton Sea Geothermal Field,California, USA

Amphiboles and pyroxenes occurring in the Salton Sea Geothermal Field were found to contain coherent intergrowths of chain silicates with other than double and single chain widths by using transmission and analytical electron microscopy. Both occur in the biotite zone at the temperature (depth) interval of 310° C (1,060 m) to 330° C (1,547m) which approximately corresponds to temperatures of the greenschist facies. The amphiboles occur as euhedral fibrous crystals occupying void space and are composed primarily of irregularly alternating (010) slabs of double or triple chains, with rare quadruple and quintuple chains. Primary crystallization from solution results in euhedral crystals. Clinopyroxenes formed mainly as a porefilling cement and subordinately as prismatic crystals coexisting with fibrous amphiboles. Fine lamellae of double and triple chains are irregularly intercalated with pyroxene. AEM analyses yield formulae (Ca_1.8Mg_2.9Fe_1.9Mn_0.1) Si₈O_21.8(OH)_1.8 (310° C) and (Ca_2.0Fe_2.5Mg_2.3) Si₈O_21.8 (OH)_2.0 (330° C) for amphiboles and (Ca_1.1Fe_0.6Mg_0.3) Si₂O₆ for clinopyroxene. Thermodynamic calculations at P_fluid=100 bar of equilibrium reactions of (1) 3 chlorite +10 calcite + 21 quartz = 3 actinolite + 2 clinozoisite + 8 H₂O + 10 CO₂ and (2) actinolite+ 3 calcite+ 2 quartz = 5 clinopyroxene + H₂O + 3 CO₂ using Mg-end member phases indicate that formation of amphibole and pyroxene require very water-rich conditions at temperatures below 330° C.Contribution No. 420 from the Mineralogical Laboratory, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan     

Miscibility and compatibility of braunite,Mn2+Mn 6 3+ O8/SiO4, in the system Mn-Si-O at 1 atm in air

Experimental investigations between 800 ° to 1,100 ° C yielded no evidence for extensive substitution of Mn²⁺+Si⁴⁺2Mn³⁺ in braunite, leading to a complete solid solution series between partridgeite (Mn₂O₃) and braunites with silica contents up to 40 wt. % as proposed by Muan (1959a, b). In the presence of excess manganese braunite of nearly ideal composition coexists at 800 ° C with partridgeite and at T1,000 ° C with hausmannite (Mn₃O₄). At 800 ° C and 1,000 ° C braunite coexists, in the presence of excess silica, with a SiO₂-polymorph and at 1,100 ° C with rhodonite (MnSiO₃). Quantitative analysis of the X-ray patterns of coexisting cristobalite and braunite confirms a maximum silica-excess in braunite of only about 2 wt.% over the ideal composition, Mn²⁺Mn ₆ ³⁺ SiO₁₂.     

Diopside-jadeite join at 16–22 GPa

Phase relations on the diopside-jadeite join were experimentally determined at 16–22 GPa pressures and temperatures in the vicinity of 1500 °C under hydrous and 2100 °C under anhydrous conditions, using a split-sphere anvil apparatus (USSA-2000). Starting compositions on the diopside-jadeite join produced assemblages containing CaSiO₃ perovskite. This assured that the coexisting garnet with compositions in the ternary system Mg₂Si₂O₆(En)-CaMgSi₂O₆(Di)-NaAlSi₂ O₆(Jd) had the maximum Ca content possible under the given conditions. Garnet reached its maximum Ca content at 17 GPa, and exsolved CaSiO₃ perovskite at higher pressures. The garnet composition closest to the join, En₅Di_47.5Jd_47.5 (mol%), was reached at 18–19 GPa and 2100 °C. The maximum Na content of garnet limited by the coexisting pyroxene did not exceed 51 mol% jadeite at 22 GPa and 2100 °C. At 22 GPa, pyroxene was replaced with NaAlSiO₄ (calcium ferrite structure) and stishovite under anhydrous conditions, while in the presence of H₂O a new hydrous Na-bearing phase with the ideal composition Na₇(Ca, Mg)₃AlSi₅O₉(OH)₁₈ was synthesized instead. Garnet coexisting with CaSiO₃ perovskite and MgSiO₃ ilmenite at 22 GPa and 1400 °C was En₅₁Di₉Jd₄₀, coincidentally identical to the first garnet forming in the ternary system at 13 GPa. The new data are applicable to the Earth's transition zone (400–670 km depths) and suggest that the transformation from eclogite to garnetite would occur primarily over a limited depth interval from 400 to 500 km. Gaps in the observed garnet compositions suggest immiscibility, which could potentially cause a sharp 400 km discontinuity in an eclogitic mantle.     

Hydrothermal synthesis of pyroxenoids in the system MnSiO3-CaSiO3 at Pf=2 kb

Pyroxenoids on the join MnSiO₃-CaSiO₃ were synthesized from (Mn, Ca)-carbonate solid solutions and SiO₂ in CO₂-H₂O mixtures at a total pressure of 2 kb. The type of structure found (pyroxmangite, rhodonite, bustamite or wollastonite) is mainly dependent on the Mn/Ca ratio, but also to a lesser extent on temperature. Johannsenite-type structures were encountered at low temperatures over a wide compositional range. It has been possible to convert the pyroxenoids pyroxmangite → rhodonite, rhodonite → bustamite, bustamite → wollastonite, and johannsenite → bustamite or wollastonite with increase of temperature, but not the reverse. The compositional ranges of the synthetic pyroxenoids are very similar to those found in natural pyroxenoids.     

Pyroxene thermometry in simple and complex systems

Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO₃-MgSiO₃-Al₂O₃. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.     

Fictive component model of pyroxenes and multicomponent phase equilibria

Pyroxenes are considered as ideal solid solutions of some real components (e.g. diopside or orthoenstatite) and some fictive or hypothetical components (e.g. orthodiopside or orthohedenbergite). Using the reversed experimental data in the CaO-MgO-SiO₂ system, the Gibbs free energy of formation of fictive orthodiopside and of fictive clinoenstatite have been determined in the temperature range of 1,000 to 1,600 °K. The data on free energies of components in the binary system can be used to extend the fictive component model to the ternary CaSiO₃-MgSiO₃-FeSiO₃ system. Using published phase diagrams on the pyroxene quadrilateral, Gibbs free energy of formation of fictive orthohedenbergite has been calculated. Application of the ideally mixing fictive component model to computation of phase equilibria leads to the determination of compositions of coexisting Fe-Mg-Ca pyroxenes at different temperatures.Abbreviations and symbols G _f ⁰ Gibbs free energy of formation from the elements at 1 bar and temperature - G ^Ex excess free energy of mixing in a solution - G molar Gibbs free energy - R gas constant - H enthalpy - S entropy - T absolute temperature - P pressure - KJ/M kilojoules per mole - j joules - Opx orthopyroxene - Cpx clinopyroxene - H hedenbergite - D diopside - E enstatite - F ferrosilite - X mole fraction - K equilibrium constant     

The experimental determination of the johannsenite-bustamite equilibrium inversion boundary

The CaMnSi₂O₆ clinopyroxene, johannsenite, inverts to bustamite at elevated temperatures. The inversion boundary has been reversibly determined at five pressures up to 22 kbar, using synthetic phases, and shown to have the equation; P(kbar)=0.0411T(°C)–10.7The determination allows the calculation of the enthalpies and free energies of formation for both clinopyroxene and bustamite of CaMnSi₂O₆ composition, and these are compared with data available for the CaFeSi₂O₆ composition which shows analagous inversion behaviour. Johannsenite appears to be of comparable stability to hedenbergite at STP, but at elevated temperatures hedenbergite may be stabilised by crystal field effects associated with the Fe²⁺ ion. Together with the stabilisation of the bustamite structure by manganese, this accounts for the low inversion temperature of johannsenite compared to that for hedenbergite.     

Manganiferous pyroxenes and pyroxenoids from three Pb-Zn-Cu skarn deposits

Samples from the Pb-Zn-Cu skarns of M. Ci-villina (Italy), Valle del Temperino (Italy), and Empire Mine (New Mexico, USA) have been analysed for their pyroxenes and pyroxenoids. The samples were collected immediately adjacent to the marble-skarn replacement front. All contain manganiferous pyroxenoids and manganeserich Ca-pyroxenes. The pyroxenes from each deposit form distinct groups of compositions within the diopside-hedenbergite-johannsenite triangle, with no apparent miscibility gap. Diopside contents usually are below 15 mole percent. Fibrous bustamite occurs as monomineralic zones in the Empire and in the Temperino deposit. Although rhodonite may be a primary phase in some samples from the Empire Mine, it is commonly of secondary origin in the Empire Mine and in the Civillina deposit. Its formation from manganiferous clinopyroxenes is either due to increasing Mn activity in the hydrothermal skarn solution or to higher X(CO₂) in the vapour phase. When rhodonite is formed within clinopyroxenes as submicroscopic lamellae that eventually replace the whole host crystal, resulting compositions lie in the miscibility gap between rhodonite and bustamite. Textural relations indicate the replacement reaction: johannsenite + CO₂ = rhodonite + calcite + quartz. Equilibrium temperatures for this reaction have been calculated by using estimated thermochemical data for johannsenite, giving a T(eq)=385° C for X(CO₂)=0.1 at P(tot)= 1 kbar. Taking into consideration the reduced activity of Mn in rhodonite and of Ca in calcite, both buffered by the johannsenite, the temperature is increased for about 15° C at X(CO₂)=0.01. At lower temperatures, where johannsenite is stable, the X(CO₂) is confined to values below 0.01. Despite the mineralogical similarities of the three deposits differences in the development of the manganiferous skarns can be depicted.     

Manganocummingtonite from the mesoproterozoic,Sausar fold belt,central India

Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca_0.3–0.35(Mg_3.3–3.5Mn_1.6–1.8Fe²⁺ _1.4–1.5)(Si_{7.931–7.997}Al^iv _{0.003–0.069})O₂₂(OH_1.5–2.0F_0.0–0.5)] being fairly rich in Ca, which is indicative of metamorphic temperature in the amphibolite facies. The garnet contains 77.5% spessartine, 13% almandine and minor andradite, grossular and pyrope components. Unusually, there is no carbonate, pyroxene, pyroxmangite, rhodonite, magnetite or hematite. The available Al in the rock stabilized garnet and this mineral incorporated minor Fe³⁺ present in the rock as andradite component. The manganocummingtonite-garnet pairs developed at ～600°C during amphibolite facies metamorphism in low $X_{CO_2 } $ system, stabilized with $X_{Mn/(Mn + Fe^{2 + } + Mg)} $ = 0.25 to 0.28 in the amphibole and 0.85 in the garnet and formed under unusually low fO ₂ conditions for the Sausar region, near channelized fluids which deposited quartz may have controlled the fO ₂ .     

An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria

The partitioning of samarium and thulium between garnets and melts in the systems Mg₃Al₂-Si₃O₁₂-H₂O and Ca₃Al₂Si₃O₁₂-H₂O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D _Sm (D _Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D _Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10^–13 cm² s^–1 at 1,300 ° C and 2 × 10^–12 cm² s^–1 at 1,500 ° C, and for grossular, 8.3 × 10^–12 cm² s^–1 at 1,200 ° C and 4.6 × 10^–11 cm² s^–1 at 1,300 ° C. The equilibrium values of D _Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D _Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm₃Al₅O₁₂ in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) D_REE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg₃Al₂Si₃O₁₂-H₂O at low Sm concentrations. These change D _Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.     

High pressure phase transformations in the joins Mg2SiO4-Ca2SiO4 and MgO-CaSiO3

High pressure phase transformations for all the mineral phases along the joins Mg₂SiO₄-Ca₂-SiO₄ and MgO-CaSiO₃ in the system MgO-CaO-SiO₂ were investigated in the pressure range between 100 and 300 kbar at about 1,000 °C, by means of the technique involving a diamond-anvil press coupled with laser heating. In addition to the four end-members, there are three stable intermediate mineral components in these two joins. Phase behaviour of all the end-member components at high pressure have been reported earlier and are reviewed here. Results of this study reveal that the three intermediate components are all unstable relative to the end-members at pressures greater than 200 kbar. Ultimately, monticellite (CaMgSiO₄) decomposes into CaSiO₃ (perovskite-type)+MgO; merwinite (Ca₃MgSi₂O₈) decomposes into Ca₂SiO₄(K₂NiF₄-type)+CaSiO₃ (perovskite-type)+MgO; and akermanite (Ca₂MgSi₂O₇) decomposes into CaSiO₃ (perovskite-type)+MgO. Note that the decomposition reactions of all phases studied here result in the formation of MgO. Intermediate Ca-Mg silicates transform to pure Ca-silicates plus MgO, while pure Mg₂SiO₄ transforms to MgSiO₃+MgO.     

Ca−Sr substitution in clinopyroxenes along the join CaMgSi2O6−SrMgSi2O6

Summary In order to define the limits of expansion of the M2 polyhedron in theC2/c clinopyroxenes of formulaX ^M2Mg^M1 [Si₂O₆] as the mean ionic radius in the M2 site increases, the join CaMgSi₂O₆–SrMgSi₂O₆ (Di–SrPx) has been investigated atP=1 atm and between 1090°C and 1350°C. The extent of the clinopyroxene solid solutions is limited to the compositional range Di₁₀₀–Di₇₀SrPx₃₀. Within this range the unit-cell parameters of the clinopyroxenes show a linear variation with the increase of Sr content. The comparison of the variations caused in the unit-cell dimensions by the increase of the mean ionic radius in the M2 site (Di–SrPx series) with those caused by the decrease of the mean ionic radius in M2 (Di–En series) displays a different trend ofb in the two series. This different trend ofb suggests a different mechanism of the structure deformation in the two solid solution series. The narrow extent of the Di–SrPx solid solutions atT=1200°C shows that the increase of the mean ionic radius in the M2 site is restricted to the range 1.12–1.16 Å.

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La substitution Ca–Sr dans les clinopyroxènes le long du joint CaMgSi₂O₆–SrMgSi₂O₆

Résumé Le joint CaMgSi₂O₆–SrMgSi₂O₆ (Di–SrPx) a été étudié entre 1090°C et 1350°C à 1 atm dans le but d'établir quelles sont les limites de l'expansibilité du polyhèdre M2 dans les clinopyroxènesX ^M2Mg^M1 [Si₂O₆] (group spatialC2/c) avec l'augmentation du rayon jonique moyen dans le site M2. La solution solide est limitée à l'intervalle de composition Di₁₀₀–Di₇₀ SrPx₃₀. Dans ce domaine les paramètres de la maille varient d'une façon linéaire avec la teneur croissante de Sr. Si on compare les variations de la maille, déterminées par le rayon jonique moyen croissant dans le site M2 (série Di–SrPx), avec celles causées par la diminution du rayon jonique moyen dans le site M2 (série Di–En), on observe une tendance différente du paramètreb dans les deux séries. Ceci indique un mécanisme différent de la déformation structuralle dans les deux séries de solutions solides. Puisque àT=1200°C le domaine des solutions solides Di–SrPx est étroit, l'augmentation du rayon ionique moyen dans le site M2 est bornée à l'intervalle 1.12–1.16 Å.

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With 5 Figures     

Microstructural evolution of the Seridó Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

The polyphase evolution of the Seridó Belt (NE-Brazil) includes D₁ crust formation at 2.3–2.1 Ga, D₂ thrust tectonics at 1.9 Ga and crustal reworking by D₃ strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D₂ and D₃ events were used to constrain the tectono-thermal evolution of the belt. D₂ shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz c-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from c-prism to positive and negative a-rhombs. During D₃, enhancement of ductility by dissipation of heat that came from syn-D₃ granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D₃ shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from a-prism to a-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths.     

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Sodic pyroxene (jadeite content X _jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X _Acm=0.26–0.50) than that in the epidote-free samples (X _Acm=0.45–0.47). X _Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X _Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X _Jd from core to rim, the other type by decreasing X _Jd from core to rim. Both types of zoned pyroxenes show an increase in X _{Fe 2+}[=Fe²⁺/(Fe²⁺+Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.     

Heterogene Gleichgewichte in der Wolframitgruppe

Zusammenfassung Im System Fe–Mn–W–O wurden die heterogenen Gleichgewichte bei 1000°C ausgehend von allen binären und ternären Randsystemen untersucht. Im System Fe–Mn–W wurde die intermetallische Verbindung Mn₅Fe_2,7W_2,3 gefunden. Im System Fe–Mn–O gibt es keine ternären Verbindungen, in den anderen Dreistoffsystemen nur FeWO₄, Fe₂WO₆ und MnWO₄. Mn₂WO₆ ließ sich bis pO₂=100 atm nicht darstellen. Quaternäre Verbindungen fehlen völlig. ZnWO₄ und NiWO₄ sind gegen FeO und MnO nicht stabil und reagieren zu FeWO₄ und MnWO₄ plus ZnO und NiO. Hydrothermal konnte bei pH₂O=2000 atm vollständige Mischbarkeit von FeWO₄ und MnWO₄ bis 160°C herab nachgewiesen werden. Die früher (Schröcke, 1960) durch Festkörperreaktionen festgestellte asymmetrische Mischungslücke im System FeWO₄–NiWO₄ konnte korrigiert werden,T _kj =525°C,x _kr =0.15 FeWO₄. FeWO₄–ZnWO₄ und MnWO₄–ZnWO₄ sind bis mindestens 414°C, MnWO₄ und NiWO₄ bis mindestens 454°C herab vollständig mischbar.

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Heterogeneous equilibria in the Wolframite group

Summary In the system Fe–Mn–W–O solid state equilibria at 1000°C were determined beginning with all binary and ternary bordering systems. In the system Fe–Mn–W the ternary phase Mn₅Fe_2.7W_2.3 was found. In the system Fe–Mn–O there does not exist any ternary phase. In the other systems only FeWO₄, Fe₂WO₆ and MnWO₄ exist. Mn₄WO₆ could not be synthesized up to 100 atm partial pressure of oxygen. Quaternary phases do not exist. ZnWO₄ and NiWO₄ are not stable in coexistence with FeO and MnO oxides. Reaction products are always FeWO₄ or MnWO₄ with ZnO or NiO. Hydrothermal studies at pH₂O=2000 atm showed complete solid solution in the system FeWO₄–MnWO₄ down to 160°C.Schröcke (1960) found an asymmetrical miscibility gap in the system FeWO₄–NiWO₄ by means of solid state reactions. Now this miscibility gap has been corrected: Critical temperature 525°C, critical composition 0,15 FeWO₄. Complete miscibility exists in the systems FeWO₄–ZnWO₄ and MnWO₄–ZnWO₄ down to at least 414°C, in the system MnWO₄–NiWO₄ down to at least 454°C.

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Mit 6 Abbildungen