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1.
We present an X-ray absorption near-edge structure study performed at the Yb LI- and LIII-edges on synthetic pyrope (Mg3Al2Si3O12) and grossular (Ca3Al2Si3O12) garnets containing about 1% wt of Yb. For the first time Yb L-edge XANES spectra are analyzed by full multiple scattering theory using clusters of different sizes and different final-state potentials. A comparison between experimental spectra and model calculations indicates that Yb3+ enters the dodecahedral X-site in both pyrope and grossular, in agreement with the results of an EXAFS study. Based on the present results, the charge balancing substitution mechanism required by the replacement of divalent Mg and Ca cations with trivalent Yb3+ is discussed in terms of vacancies in dodecahedral sites surrounding the central Yb3+ absorber. Received: 7 December 1998 / Revised, accepted: 7 May 1999  相似文献   

2.
Partition coefficients for the rare earth elements (REE) Ce, Sm and Tm between coexisting garnets and hydrous liquids have been determined at high pressure and temperatures (30 kbar and 1300 and 1500°C). Two synthetic systems were studied, Mg3Al2Si3O12-H2O and Ca3Al2Si3O12-H2O, in addition to a natural pyrope-bearing system.Deviations from Henry's Law behaviour occur at geologically relevant REE concentrations. At concentrations < 3 ppm Ce, < 12 ppm Sm, < 80 ppm Tm in pyrope and < 100 ppm Ce, < 250 ppm Sm, < 1000 ppm Tm in grossular (at 30 kbar and 1300°C), Dgarnet liquidREE increases as the REE concentration in the garnet decreases. At higher concentrations, DREE is constant. Dgrossular liquidREE also constant when the garnet contains less than about 2 ppm Sm or Tm. The REE concentration at which DREE becomes constant increases with increasing temperature, decreasing REE ionic radius and increasing Ca content of the garnet.Partitioning behaviour of Ce, Sm and Tm between a natural pyrope-rich garnet and hydrous liquid is analogous to that in the synthetic systems and substantiates the substitution model proposed by Harrison and Wood (1980).Values of DREEgarnet/liquid for which Henry's Law is obeyed are systematically higher for grossular than for pyrope (Dpyrope/liquid = 0.067(Ce), 0.108(Sm), 0.155(Tm) and Dgrossular/Liquid = 0.65(Ce), 0.75(Sm), 4.55(Tm).The implications of non-Henry's Law partitioning of REE for models of basalt petrogenesis involving garnet are far-ranging. Deviations from Henry's Law permit refinements to be made to calculated REE abundances once basic model parameters have been defined.  相似文献   

3.
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) DREE 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.  相似文献   

4.
We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg3Al2Si3O12 (pyrope) — Mg4Si4O12 (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO3 garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state 27Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.  相似文献   

5.
 We have performed atomistic computer simulations on trace element incorporation into the divalent dodecahedral X-sites of pyrope (Py — Mg3Al2Si3O12) – grossular (Gr — Ca3Al2Si3O12) solid solutions. An ionic model and the Mott–Littleton two-region approach to defect energies were used to calculate the energetics of substitution by a range of divalent trace-elements and of charge-balanced substitution by trivalent ions in the static limit. Results are compared with experimental high-temperature, high-pressure garnet-melt trace element partitioning data obtained for the same garnet solid solution to refine our understanding of the factors controlling element partitioning into solid solutions. Defect energies (U def,f), relaxation (lattice strain) energies (U rel), and solution energies (U sol) were derived using two different approaches. One approach assumes the presence of one type of hybrid X-site with properties intermediate between pure Mg and Ca sites, and the other assumes discrete Mg and Ca X-sites, and thus two distinct cation sublattices. The hybrid model is shown to be inadequate, since it averages out local distortions in the garnet structure. The discrete model results suggest trace elements are more soluble in Py50Gy50 than in either end-member compound. Physically this is due to small changes in size of the X-sites and the removal of unfavourable interactions between third nearest neighbours of the same size. Surprisingly, depending on the local order, large trace element cations may substitute for Mg2+ and small trace elements for Ca2+ in Py50Gr50. These solubilities provide an explanation for the anomalous trace-element partitioning behaviour along the pyrope–grossular join observed experimentally. Received: 27 January 2000 / Accepted: 14 February 2003  相似文献   

6.
We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425–1,750°C experiments on a high-Fe mantle melt (97SB68) from the Paraná-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (∼Py63Al25Gr12) and clinopyroxene (∼Ca0.2Mg0.6Fe0.2Si2O6) show a large variation with temperature but are less dependent on pressure. At 3 GPa, D cpx/liq values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts (∼Ca0.3–0.5Mg0.3–0.6Fe0.07–0.2Si2O6) but higher than those for Ca-poor pyroxenes from peridotites (∼Ca0.2Mg0.7Fe0.1Si2O6). D grt/liq values for light and heavy rare-earth elements are ≤0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents (Py70–88All7–20Gr8–14). 97SB68 D LREEgrt/liq values are higher and D HREEgrt/liq values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents (Py20–70Al10–50Gr10–55). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of co-existing melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models.  相似文献   

7.
Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO2, 8 wt.% Cr2O3 and 4 wt.% ZrO2, representing a solid solution of andradite (Ca3Fe3+ 2Si3O12 ≈46 mol%), uvarovite (Ca3Cr2Si3O12 ≈23 mol%), grossular (Ca3Al2Si3O12 ≈10 mol%), schorlomite (Ca3Ti2[Si,(Fe3+,Al3+)2]O12 ≈15 mol%), and kimzeyite (Ca3Zr2[Si,Al2]3O12 ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al3+, Fe3+ and Ti4+. Cr3+ and Zr4+ are found at the octahedral site along with Fe3+, Al3+ and Ti4+. The measured H2O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca2.99Mg0.03)Σ=3.02(Fe3+ 0.67Cr0.54Al0.33Ti0.29Zr0.15)Σ=1.98(Si2.42Ti0.24Fe0.18Al0.14)Σ=2.98O12OH0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.  相似文献   

8.
Manganocummingtonite occurs with spessartine, quartz and pyrolusite in the Chikmara area, Sausar fold belt, central India. Its composition is [Ca0.3–0.35(Mg3.3–3.5Mn1.6–1.8Fe2+ 1.4–1.5)(Si7.931–7.997Aliv 0.003–0.069)O22(OH1.5–2.0F0.0–0.5)] 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 .  相似文献   

9.
Thermodynamic properties of almandine-grossular garnet solid solutions   总被引：1，自引：0，他引：1  
The mixing properties of Fe3Al2Si3O12-Ca3Al2Si3O12 garnet solid solutions have been studied in the temperature range 850–1100° C. The experimental method involves measuring the composition of garnet in equilibrium with an assemblage in which the activity of the Ca3Al2Si3O12 component is fixed. Experiments on the assemblage garnet solid solution, anorthite, Al2SiO5 polymorph and quartz at known pressure and temperature fix the activity of the Ca3Al2Si3O12 component through the equilibrium: 1 $$\begin{gathered} {\text{3CaAl}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftarrows {\text{Ca}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} \hfill \\ {\text{Anorthite garnet}} \hfill \\ {\text{ + 2Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + SiO}}_{\text{2}} \hfill \\ {\text{ sillimanite/kyanite quartz}}{\text{.}} \hfill \\ \end{gathered}$$ This equilibrium, with either sillimanite or kyanite as the aluminosilicate mineral, was used to control \({\text{a}}_{{\text{Ca}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} }^{{\text{gt}}} \) . The compositions of the garnet solutions produced were determined by measurement of their unit cell edges. At 1 bar Fe3Al2Si3O12-Ca3Al2Si3O12 garnets exhibit negative deviations from ideality at the Fe-rich end of the series and positive deviations at the calcium end. With increasing pressure the activity coefficients for the Ca3Al2Si3O12 component increase because the partial molar volume of this component is greater than the molar volume of pure grossular. Previous studies indicate that the activity coefficients for the Ca3Al2Si3O12 component also increase with increasing (Mg/Mg+Fe) ratio of the garnet. The region of negative deviation from ideality implies a tendency towards formation of a stable Fe-Ca garnet component. Evidence in support of this conclusion has been found in a natural Fe-rich garnet which was found to contain two different garnet phases of distinctly different compositions.  相似文献   

10.
Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al2O3–Cr2O3–SiO2 system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg3Cr2Si3O12 included chromiumbearing enstatite MgSiO3 (up to 3.2 wt % Cr2O3) and eskolaite Cr2O3. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.  相似文献   

11.
 In order to clarify Al2O3 content and phase stability of aluminous CaSiO3-perovskite, high-pressure and high-temperature transformations of Ca3Al2Si3O12 garnet (grossular) were studied using a MA8-type high-pressure apparatus combined with synchrotron radiation. Recovered samples were examined by analytical transmission electron microscopy. At pressures of 23–25 GPa and temperatures of 1000–1600 K, grossular garnet decomposed into a mixture of aluminum-bearing Ca-perovskite and corundum, although a metastable perovskite with grossular composition was formed when the heating duration was not long enough at 1000 K. On release of pressure, this aluminum-bearing CaSiO3-perovskite transformed to the “LiNbO3-type phase” and/or amorphous phase depending on its Al2O3 content. The structure of this LiNbO3-type phase is very similar to that of LiNbO3 but is not identical. CaSiO3-perovskite with 8 to 25 mol% Al2O3 was quenched to alternating lamellae of amorphous layer and LiNbO3-type phase. On the other hand, a quenched product from CaSiO3-perovskite with less than 6 mol% consisted only of amorphous phase. Most of the inconsistencies amongst previous studies could be explained by the formation of perovskite with grossular composition, amorphous phase, and the LiNbO3-type phase. Received: 11 April 2001 / Accepted: 5 July 2002  相似文献   

12.
Infrared reflectance (IR) and Raman spectra were collected on small (ca. 500 micron) single crystals of 5 natural garnets with nearly end-member compositions: pyrope (98% Mg3Al2Si3O12), almandine (83% Fe3Al2Si3O12), spessartine (98% Mn3Al2Si3O12), grossular (97% Ca3Al2Si3O12), and andradite (99% Ca3Fe2Si3O12). Frequencies and symmetry assignments were determined for all 17 IR modes and all 25 Raman modes. By using factor group analysis and by correlating the bands by their intensities, bands were assigned to either one of the SiO4 internal motions, as a rotation, or to a type of translation. The assignments are supported by (1) the distinct trends of frequencies with cell size and cation masses for each of the different types of motion, (2) the similarity of garnet energies for each of the different types of motion to those of olivine with the same cation, and (3) the closeness of the T 1 u IR frequencies to the T 2 g Raman frequencies. Mode mixing appears to be weak. Correlations between frequencies and structural parameters suggests a direct dependence of force constants on lattice parameter. This relationship arises from bond lengths in the garnet structure being constrained by the size and compressibility of adjacent polyhedra through edge-sharing. Comparison of our endmember data with previous powder IR studies of intermediate garnets indicates that dodecahedral (X) and octahedral (Y) sites alone exhibit two-mode behavior for those solid solutions involving two ions with considerably different masses. However, for solid solutions involving cations of much different ionic radii, two-mode behavior is found for the translations of SiO4 groups. This is the first report of two-mode behavior that is unrelated to mass, and instead is due to significantly different force constants in the pyralspites compared to the ugrandites.Anomalies in mixing volumes are linked to two-mode behavior of the SiO4 translations, which leads to the suggestion that the mixing volume behavior is caused by the resistance of the Si-O bond to expansion and compression, as well as to changes in the dodecahedral site. Crystal-field effects may also play an important role within the ugrandite series. Deviation of molar volume dependence on composition from a linear to a asymmetric, non-linear (sometimes sigmoidal) dependence can be linked to solid solutions that possess slightly non-equivalent cation sites.  相似文献   

13.
New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO –MgO –Al2O3 –SiO2 (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X Al M1 +17.702 1n X Ca M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi2O6+Mg3Al2Si3O12=3Mg2Si2O6+Ca3Al2Si3O12 has a V o of 3.5 cm3. The total pressure dependency of this reaction is however closer to a theoretical V o of about 5 cm3 when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.Abbreviations Used in the Text CaTs Ca-tschermak's molecule, CaAl2SiO6 - cpx clinopyroxene - di diopside, CaMgSi2O6 - en enstatite, Mg2Si2O6 - gr grossular, Ca3Al2Si3O12 - gt garnet - MgTs Mg-Tschermak's molecule, MgAl2SiO6 - opx orthopyroxene - px pyroxene - py pyrope, Mg3Al2Si3O12 - a i j activity of component i in phase j - activity coefficient - G(I) molar Gibbs free energy difference of reaction (I) at standard state unless specified otherwise - H(I), (H I) molar enthalpy (difference) of phase (reaction) (I) at standard state unless specified otherwise - S (I), (S I) molar entropy (difference) of phase (reaction) (I) at standard state unless specified otherwise - V o, (V I o) molar volume (difference) of phase (reaction) (I) at standard state - X i j mole fraction of component i in phase j  相似文献   

14.
Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg2.84Fe0.10Ca0,06) Al2Si3O12], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg3(MgSi)Si3O12], calcium-bearing majorite [(Ca0.49Mg2.51)(MgSi)Si3012], sodium majorite [(Na1.88Mgp0.12)(Mg0.06Si1.94)Si3O12], and an intermediate composition [(Na0.37Mg2.48)(Mg0.13Al1.07 Si080) Si3O12]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.  相似文献   

15.
The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al2O3 content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm−1. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm−1. All H is present on one site, with interstitial H both coupled to Al3+ substitutional defects on adjacent octahedral (Si4+) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al3+ substitution for Si4+ charge-balanced by oxygen vacancies at low bulk Al2O3 contents, and coupled substitution of Al3+ onto octahedral (Si4+) and interstitial sites at high bulk Al2O3 contents. Trends in OH stretching frequencies as a function of Al2O3 content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).  相似文献   

16.
The results of an experimental study of the pyrope (Mg3Al2Si3O12)-jadeite (NaAlSi2O6) system at P = 7.0 and 8.5 GPa and T = 1300?C1800°C are summarized in this paper. The main phases that were obtained in the experiments are garnet, pyroxene, kyanite (in some cases corundum), and quenched melt. Garnets are characterized by a stable Na2O admixture (up to 0.6 wt % at 7.0 GPa and up to 0.8 wt % at 8.5 GPa) and the high silicon content (Si = 3.016?C3.166). The maximal sodium concentrations in garnet were found at the solidus of the system, which results from an increase of the coefficient of sodium partitioning between garnet and melt during a temperature decrease.  相似文献   

17.
Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)3Al2Si3O12, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C.  相似文献   

18.
Five distinct paragenetic, morphological and compositional types of grossular garnet (G1, G2, G3, G4, G5) were distinguished within the individual (sub)units of the zoned leucotonalitic pegmatite cutting serpentinized lherzolite with rodingite dikes at ??ár near Ruda nad Moravou, Staré Město Unit, Northern Moravia. Detailed study using Electron Microprobe Analysis, Laser Ablation Inductively Coupled Plasma Mass Spectrometry, Cathodoluminiscence and Infrared Spectroscopy revealed distinct compositional trends in major, minor and trace elements. The contents of Fe3+, Mn, Mg and Ti increase from early garnet (G1) in the outermost grossular subunit through the interstitial garnet (G2) in the leucocratic subunit to graphic intergrowths of quartz+garnet (G3) in the coarse-grained unit. Then these constituents decrease in inclusions of garnet (G4) from the blocky unit and large crystals of garnet (G5) from the quartz core. Some trace elements (V, Ni, Y) exhibit the same trends, only Be evidently increases in garnet from border zone to the centre. Fluorine has negative correlation with Fe3+ as well as some trace elements (Ta, Pb). Concentrations of H2O in garnets, up to 0.22 wt.% H2O, are comparable with spessartine-almandine garnets from the Rutherford No. 2 pegmatite, Virginia, and grossular garnets from high-temperature calc-silicate rocks (skarns). Water contents correlate positively with Fe3+, but inversely with F. The use of water contents in garnet to elucidate the fluctuations of activity of H2O during the pegmatite formation is only limited; the incorporation of hydrous defects seems to be controlled instead by crystal-structural constraints. However, the sum of all volatile components (H2O + F) increases about twice from the outermost subunit to the centre of the pegmatite body.  相似文献   

19.
Low‐T calorimetry is an experimental science that measures the thermodynamic function heat capacity, Cp(T), from which the standard third‐law entropy (298.15 K), S°, is calculated. The recent technological development of relaxation calorimetry allows both new experimental strategies and types of Cp investigations to be made, which were previously not possible. The Cp measurements are fast and automated and can be made on mg‐sized mineralogical samples between 2 and 400 K. These advantages, when careful measurement procedures are used, permit better determinations of Cp(T) behaviour. The Cp of synthetic single‐crystal MgO was measured between 5 and 302 K, and S° calculated using relaxation calorimetry to further investigate the method's precision and accuracy. A number of synthetic and natural end‐member or nearly end‐member compositions of silicate garnet were investigated in the past via adiabatic calorimetry, an old and established technique, and more recently and extensively with the relaxation method. First Cp(T) and S° results, using relaxation calorimetry, have been obtained on spessartine (Mn3Al2Si3O12) and knorringite (Mg3Cr2Si3O12). Furthermore, reinvestigations on pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), grossular (Ca3Al2Si3O12) and andradite (Ca3Al2Si3O12), often on multiple samples, have resolved uncertainties and certain problems with published thermodynamic data. S° can be affected by various low‐T physical phenomena, such as cooperative magnetic phase transitions or Schottky anomalies at temperatures of <15 K, which were not described fully in some older adiabatic calorimetric studies. New Cp results show that small differences in the thermodynamic behaviour between some natural and synthetic silicates may exist as demonstrated by extensive work on grossular. Important and “new” research questions on the thermodynamic behaviour of minerals are coming to light and are being investigated. The Cp behaviour and S° values for six silicate garnet end‐members are analysed and the latter are compared to the “best fit or optimized” S° values given in various internally consistent thermodynamic databases. Conclusions are drawn on what types and directions of calorimetric study are required in order to obtain better thermodynamic property determinations of minerals, as well as achieving a better understanding of the underlying microscopic physical behaviour that determines the macroscopic Cp and S° functions.  相似文献   

20.
An experimental study of Ca-(Fe,Mg) interdiffusion in silicate garnets   总被引：1，自引：0，他引：1  
Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of grossular (Ca2.74Mg0.15 Fe0.23Al1.76Cr0.04Si3.05O12) and almandine (Ca0.21Mg0.40 Fe2.23Mn0.13Al2.00Cr0.08Si2.99O12 or Ca0.43Mg0.36Fe2.11 Al1.95Si3.04O12), were undertaken at 900–1100 °C and 30 kbar, and pressures of 15.0–32.5 kbar at 1000 °C. Samples were buffered by Fe/FeO in most cases. Diffusion profiles were determined by electron microprobe. Across the experimental couples the interdiffusion coefficients (D˜) were almost independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered samples. The temperature dependence of the D˜ (Ca-Fe,Mg) interdiffusion coefficients may be described by
at 30 kbar and 900–1100 °C. This activation energy is marginally higher than previous experimental studies involving Ca-free garnets; the interdiffusion coefficients are higher than previous studies for Fe-Mg and Fe-Mn exchange in garnet. The pressure dependence of D˜ (Ca-Fe,Mg) at 1000 °C yielded an activation volume of 11.2 cm3 mol−1, which is higher than previous results from studies involving garnet and olivine. Comparison with simulation studies suggests a vacancy mechanism for divalent ion migration in garnet, with extrinsic processes being dominant up to very high temperatures. Received: 15 December 1996 / Accepted: 3 November 1998  相似文献   

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