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

2.
Natural humite minerals and synthetic fluoro-humites, ideally described as nMg2SiO4 · Mg(OH,F)2 (1<n<4), were investigated by using electron diffraction and imaging techniques. In clinohumite (n=4) and humite (n=3) irregularities in lattice spacings were observed parallel to (001). These were identified, using high resolution lattice images, as upper member lamellae in which n was always even (i.e., n=6, 8, 10 etc.). In fluoroclinohumite the higher members were sometimes perfectly ordered, giving rise to superstructures with relatively large c axes. These supercells always consisted of several consecutive unit cells of clinohumite (n=4) followed by a unit cell of the n=6 member; the largest observed had a d (001) spacing of 169 Å. It was also found that coherent intergrowths of massive portions of humite (n=3) and chondrodite (n=2) often occurred.  相似文献   

3.
Thermodynamic parameters for the reaction: $$\begin{gathered} Mg_2 Si_2 O_6 = Mg_2 Si_2 O_6 \hfill \\ enstatite clinopyroxene \hfill \\ \end{gathered} $$ in the system CaO-MgO-SiO2 have been deduced from phase equilibrium and enthalpy of solution data. From the regular solution theory, the seventeen currently available reversed experimental compositions of coexisting enstatite and clinopyroxene, presumed to be ordered diopside, lead, by a statistical regression, to the following best fit parameters: ΔH o=6.80 kJ ΔS o=2.75 J/K W H Cpx =24.47 kJ (regular solution enthalpy parameter) W V Cpx =0.105 J/bar (regular solution volume parameter). The derived parameters are not significantly affected by the (necessary) choice of W Opx in the range 20–50 kJ. The above values are in very good agreement with deductions from the solution calorimetry on synthetic CaMgSi2O6-Mg2Si2O6 clinopyroxenes of Newton et al. (1979), which also places bounds on possible departures from the optimal values of these parameters. The calorimetric data may also be interpreted in terms of a Bragg-Williams cooperative-disordering model (Navrotsky and Loucks, 1977), in which diopside-structure clinopyroxene and a ‘relaxed’ low-Ca clinopyroxene (‘Fe-free pigeonite’) approach each other in composition, structural state, and stability with increasing temperature. The ΔH o parameter deduced from the regular solution theory is reinterpreted as the enthalpy change of enstatite to Mg2Si2O6 pigeonite; the ΔH o of the transformation of enstatite to the diopside structure would, in this case, be considerably larger than 6.8 kJ. The curvature of the enthalpy of solution data, explained by the regular solution theory in terms of M2-site energetics (involving W H cpx ), is reinterpreted as due to disordering and ‘relaxation’ in the Navrotsky-Loucks model. Although the regular solution theory with the best-fit parameters accounts for all of the reversed enstatite and diopside compositions to within 18 ° C, and is a convenient representation of the phase equilibria for purposes of geothermometry, the disordering model is, at the present level of knowledge, equally valid and allows for a region of stability of two coexisting clinopyroxenes.  相似文献   

4.
The solubility of Tio2 in phlogopites has been experimentally determined in the system K2Mg6Al2Si6O20(OH)4-K2Mg4TiAl2Si6O20(OH)4-K2Mg5TiAl4Si4O20(OH)4 between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K2Mg6Al2Si6O20(OH)4-K2Mg4.5TiAl3Si5O20(OH)4 which represents a combination of the Mg[VI]2Si[IV] = Ti[VI]2Al[VI] and 2Mg[VI] = Ti[VI]□[VI] substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio2 content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K2O dissolved in H2O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K2Mg6Al2Si6O20(OH)4], eastonite [K2Mg5Al4Si5O20(OH)4], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K2(Mg4Ti□)Al2Si6)O20(OH)4, and Ti-eastonite [K2Mg5TiAl4Si4O20(OH)4]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H2O + 2 K2O. Lack of knowledge of H2O and K2O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.  相似文献   

5.
Calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions. Experiments were performed on the stability of tremolite plus forsterite in the system H2O-CaO-MgO-SiO2 from 5 to 20 kbar. A univariant curve was fitted to the experimental brackets using volume, water fugacity, and heat capacity data. The results indicate that the maximum stability of tremolite in the presence of forsterite is about 825° C at 5 kbar. Addition of Al2O3 to this system increases the stability of tremolitic amphibole by only 20°–40° C and induces solubility of 5–7 wt.% Al2O3 in the amphibole, as determined from quantitative SEM analyses of individual amphibole crystals. Thus substitution of the tschermakite component (Ca2(Mg3Al2) (Si6Al2) O22(OH)2) alone cannot lead to the greatly enhanced Al2O3 contents or thermal stability of natural calcic amphiboles. Comparison of the results from this study with experimental results from other studies on synthetic calcic amphiboles indicates that the high thermal stability of natural amphiboles is strongly linked with the substitution of alkalies (Na in particular) in the form of the component Na-Ca2(Mg4Al) (Si6Al2)O22(OH)2 (pargasite). Accordingly, experimental data from studies on pargasite have been combined with the appropriate univariant curves to obtain a phase diagram for amphibole-bearing ultramafic rocks modelled by the system H2O-Na2O-CaO-MgO-Al2O3-SiO2.  相似文献   

6.
Stoichiometric anorthite, CaAl2Si2O8, Pˉ1, with sharp a, b, c, and d diffractions was grown, using a CaV2O6 solvent, by cooling at 2 ° C per hour from 1450 to 750 ° C in air. Euhedral crystals up to 5 × 3× 0.5 mm, with prominent {010} and well-developed {110} and {001}, were obtained by spontaneous nucleation. Nonstoichiometric anorthite with excess SiO2 (CaAl2Si2O8+Si2Si2O8) was grown on the join CaAl2Si2O8-SiO2. Chemical analysis of the synthetic anorthite, having the highest SiO2 content, with maximum vacancies on M-site gave □0.110 Na0.006Ca0.884Al1.80Si2.20O8, and X-ray diffraction showed a trend from stoichiometric Pˉ1 through diffuse Pˉ1 to body-centered Iˉ1 probably reflecting increasing disorder caused by a higher Si/Al ratio in the T-sites and the positional disorder accompanied by M-site vancancies. Annealing of the nonstoichiometric diffuse Iˉ1 anorthite in air at 1050 ° C for 14 days resulted in exsolution of minute SiO2 inclusions due to probable ordering in the T-sites and filling of M-sites by Ca. Stoichiometric Pˉ1 anorthite was not obtained by annealing at 1050 ° due to appreciable solubility of SiO2 in CaAl2Si2O8. Metastable hexagonal CaAl2Si2O8 was found to be a twinned monoclinic crystal with cell dimensions: a = 10.24 (2), b = 17.74 (3), c= 14.99 (5) ?, β = 92.05 (5) °, space group C2.  相似文献   

7.
The thermodynamic properties of 25 substances (elements, compounds, modifications) are calculated on the basis of an extrapolation of their caloric values and compressibilities into the region of pressures up to 2mbar and temperatures up to 4,000K. The extrapolation methods are described. The ratio of molar volumes is used to predict the thermodynamic properties of the high pressure modifications. It is inferred that water vapour and oxides of Mg, Fe, and Si ought to be stable in the entire mantle. In the lower mantle garnet should be more stable than the perovskite-type phase of MgSiO3 (in presence of Al2O3 or Fe2O3). ‘Perovskite’ phase plus MgO are more stable here than forsterite, Mg2SiO4. Pyrrhotite, FeS, reveals astonishing stability in the entire mantle and in the outer core as well. Carbon dioxide, CO2, may exist only in the upper mantle, whereas methane, CH4, remains stable in the entire mantle.  相似文献   

8.
The effect of Cr on the silicate system has been studied in air at 1 atm by adding a small amount of MgCr2O4 (0.2–0.5 wt.%) to the join Mg2SiO4 (forsterite) — CaAl2Si2O8 (anorthite) — CaMgSi2O6 (diopside), which has been considered to form a thermal divide in the system CaO-MgO-Al2O3-SiO2. The spinel primary field is enlarged compared with that in the Cr-free join at the expense of the anorthite primary field. The piercing points forsterite+anorthite+diopside+liquid and forsterite+anorthite+spinel+liquid approach each other with increasing MgCr2O4, meet at the join with 0.25 wt.% MgCr2O4 (0.20 wt.% Cr2O3) to form the ‘isobaric quaternary invariant point’ forsterite+anorthite+diopside+spinel+liquid, and then separate again as new ‘piercing points’ of diopside+spinel+anorthite+liquid and forsterite+diopside+ spinel+liquid. This process indicates that the join Mg2SiO4-CaAl2Si2O8-CaMgSi2O6 containing more than 0.2 wt.% Cr2O3 cannot be a thermal divide in the basalt tetrahedron. The results of the present study show that the presence of a minor amount of Cr causes a significant effect on the phase relations and therefore, the role of Cr must be taken into account in the formulation of a petrologic model.  相似文献   

9.
The crystal structure and chemical composition of a crystal of (Mg14?x Cr x )(Si5?x Cr x )O24 (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å3 and Z = 2. The structure was refined to R 1 = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.  相似文献   

10.
Run products from high pressure experiments at 800-1,200 °C and 5-14 GPa (corresponding to depths of 150 to 420 km) on a serpentine bulk composition [close to Mg3Si2O5(OH)4] were analysed by optical microscopy, micro-Raman spectroscopy and electron microprobe. All charges exhibit strong chemical zoning. Fluid, melt and hydrous solids were mostly concentrated at the top, bottom and along the wall of the capsules. The central part of the charge was devoid of H2O. Both fluid and hydrous magnesian phases exhibit a Mg/Si ratio higher than forsterite. In contrast, the centre of the capsule was enriched in SiO2. The observed zoning can neither be explained by gravitational settling nor by a thermal gradient alone. Most likely the fluid was separated from the solids by surface forces and thereby established the chemical gradient by preferentially dissolving MgO. If strong chemical zoning is taken into account, the occurrence of more phases than allowed by the phase rule can be explained by separating the bulk into several domains of different bulk compositions. Results indicate that small amounts of F increase the stability field of clinohumite, Mg9Si4O16(OH,F)2, compared to OH-clinohumite in pure MSH previously reported. Clinohumite coexists with enstatite up to 975 °C at 5 GPa, and up to 1,100 °C at 12 GPa. At 14 GPa (close to the !/#-Mg2SiO4 transition) phase E becomes the most important water carrier. The new results indicate that clinohumite could be an important mantle mineral for transporting water into the Earth's transition zone due to its high thermal stability compared to other important water carriers such as serpentine and phase A.  相似文献   

11.
An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca0.40Mg1.60Si2O6, Ca0.52Mg1.46Al0.05Si1.98O6, Ca0.59Mg1.41Si2O6 and Ca0.70Mg1.30Si2O6, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca0.52Mg1.46Al0.05Si1.98O6 and Ca0.59Mg1.41Si2O6, 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 Ca0.52Mg1.46Al0.05Si1.98O6 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.  相似文献   

12.
Subsolidus phase relations on the join CaMgSi2O6-CaFe3+ AlSiO6-CaTiAl2O6 were studied by the ordinary quenching method at \(f_{O_2 } = 10^{ - 11} \) atm and 1,100°C. Crystalline phases encountered are clinopyroxeness (ss:solid solution) (Cpxss), melilite (Mel), perovskite (Pv), spinelss (Spss), magnetitess (Mtss) and anorthite (An). There is no Cpxss single phase field, and the following assemblages were found; Cpxss+Mel, Cpxss+Mel+Spss, Cpxss+Mel+Pv, Cpxss+Mel+Spss+Pv, Cpxss+Pv+Spss+An, Spss+Pv+Mel+An+Cpxss, Mel+Mtss+An+Spss+Cpxss+liquid and Mel+Mtss+An+Spss+Cpxss+Pv. Mössbauer spectral study revealed that Cpxss contains both Fe2+ and Fe3+ in the octahedral site, and it was confirmed that the CaFe3+ AlSiO6 content in the Cpxss at low \(f_{O_2 } \) is considerably less than that in the Cpxss crystallized in air, whereas the CaFe2+Si2O6 component increases. The maximum solubility of CaTlAl2O6 component in the Cpxss at low \(f_{O_2 } \) is higher than that in air. The decrease of CaFe3+ AlSiO6 in the Cpxss at low \(f_{O_2 } \) may cause increase of CaTial2O6 in the Cpxss.  相似文献   

13.
Crystal chemistry of wadsleyite II and water in the Earth’s interior   总被引：1，自引：1，他引：0  
Wadsleyite II is a variably hydrous magnesium-iron silicate phase similar to spinelloid IV and a potential host for H in the Transition Zone of the Earths mantle. Two separate samples of wadsleyite II synthesized at 17.5 GPa and 1400°C and at 18 GPa and 1350°C have been characterized by electron microprobe, single-crystal X-ray diffraction, visible, IR, Raman, and Mössbauer spectroscopies, and transmission electron microscopy including electron energy-loss spectroscopy. The two samples have the following chemical formulae: Mg1.71Fe0.18Al0.01H0.33Si0.96O4 and Mg1.60Fe0.22Al0.01 H0.44Si0.97O4. Mössbauer spectroscopy and electron energy loss spectroscopy (EELS) indicate that about half of the iron present is ferric. Refinement of the structures shows them to be essentially the same as spinelloid IV. Calculated X-ray powder diffraction patterns show only subtle differences between wadsleyite and wadsleyite II. The hydration mechanism appears to be protonation of the non-silicate oxygen (O2) and possibly the oxygens surrounding the partially vacant tetrahedral site Si2, charge-balanced by cation vacancies in Si2, M5 and M6. The unit cell volume of this phase and its synthesis conditions indicate that it may be an intermediate phase occurring between the fields of wadsleyite and ringwoodite, if sufficient trivalent cations are available. The unit cell parameters have been refined at pressures up to 10.6 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The refined bulk modulus for the sample containing 2.8 wt% H2O is 145.6 ± 2.8 GPa with a K of 6.1 ± 0.7. Similar to wadsleyite and ringwoodite, hydration has a large effect on the bulk modulus. The presence of this phase in the mantle could serve to obscure the seismic expression of the phase boundary between wadsleyite and ringwoodite near 525 km. The large apparent effect of hydration on bulk modulus is consistent with hydration having a larger effect on seismic velocities than temperature in the Transition Zone.  相似文献   

14.
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   

15.
A possible structure for hydrous modified olivine (HyM-α) has been obtained by the subtraction of Mg3SiO5 from forsterite by crystallographic shear along a direction parallel to the [010] direction of olivine. The subtraction of Mg3SiO5 results in the subtraction of MgO from bulk chemistry (?Mg3SiO5=?Mg2SiO4?MgO). A possible structure for HyM-α thus obtained has the chemical formula Mg9Si5H2O20 (= 5 × Mg1.8SiH0.4O4) with monoclinic unit cell a=4.754 Å, b=10.19 Å, c=29.90 Å, ρ=3.126 g cm?3, and space group=Ac2m (no. 39). Since the X-ray powder diffraction pattern of HyM-α proposed in this study is very close to that of clinohumite, there is the possibility of this phase having been undiscovered. The humite group minerals and HyM-α proposed in this study make a homologous series as recombination structures: Mg(2 m + n )Si m H2 n O2(2 m + n ) for the humite group and Mg(2 m + n )Si( m + n )H2 n O4( m + n ) for HyM-α A characteristic feature is that Mg/Si > 2 for the humite group and Mg/Si < 2 for HyM-α. Forsterite specimens containing around 100 ppm H2O reported in mantle xenoliths might be the disordered case with n=1 and m=1200 of the humite group or HyM-α.  相似文献   

16.
The stability relations of lawsonite, CaAl2Si2O7(OH)2H2O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H2O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al2O3-SiO2-H2O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO3(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg3Al2Si3O12] show that at 10 GPa the reaction lawsonite=Gr-Pyss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr20Py80. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H2O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H2O to an even greater depth in the mantle.  相似文献   

17.
Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with XFe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, XFe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B2O3in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at aH2O=1 in the MgO-Al2O3-SiO2-H2Osystem, it might be stabilized at aH2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower aH2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.  相似文献   

18.
A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.  相似文献   

19.
Ab initio force constants calculated for Si-O stretch and Si...Si non-bonded interactions in H6Si2O7 are found comparable with experimental values derived from the lattice dynamics of α quartz. The bulk moduli of α quartz and α cristobalite are calculated using the molecular Si...Si force constant and assuming rigid regular SiO4 tetrahedra. In the case (α quartz) where data are available the calculation agrees well with experiment.  相似文献   

20.
Enthalpies of solution of synthetic clinopyroxenes on the join CaMgSi2O6-Mg2Si2O6 have been measured in a melt of composition Pb2B2O5 at 970 K. Most of the measurements were made on samples crystallized at 1600°–1700°C and 30 kbar pressure, which covered the range 0–78 mole per cent Mg2Si2O6, and whose X-ray patterns could be satisfactorily indexed on the diopside (C2/c) structure. For the reaction: Mg2Si2O6→-Mg2Si2O6 enstatite diopside the present data, in conjunction with previous and new measurements on Mg2Si2O6 enstatite, determine ΔH° ~ 2 kcal and WH (regular solution parameter) ~ 7 kcal. These values are in good agreement with those deduced by Saxena and Nehru (1975) from a study of high temperature, high pressure phase equilibrium data under the assumption that the excess entropy of mixing is small, but, in light of the recent theoretical treatment of Navrotsky and Loucks (1977, Phys. Chem. Min.1, 109–127), the meanings of these parameters may be ambiguous.Heat of solution measurements on Ca-rich binary diopsides made by annealing glasses at 1358°C in air gave slighter higher values than the higher temperature high pressure samples. This may be evidence for some (Ca, Mg) disorder of the sort postulated by Navrotsky and Loucks (1977, Phys. Chem. Min.1, 109–127), although no differences in heat of solution dependent on synthesis temperature in the range 1350°–1700°C could be found in stoichiometric CaMgSi2O6.  相似文献   

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