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1.
We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550?°C, and fO2 of FMQ ?3.2 to FMQ ?2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln(K 0) = ?21.79?±?0.04, ΔV 0?=?32.91?±?0.65 cm3mol?1, ΔH 0?=?107?±?21 kJ mol?1, and dissolution of CO2 as CO3 2?: ln(K 0 ) = ?21.38?±?0.08, ΔV 0?=?30.66?±?1.33 cm3 mol?1, ΔH 0?=?42?±?37 kJ mol?1, where K 0 is the equilibrium constant at some reference pressure and temperature, ΔV 0 is the volume change of reaction, and ΔH 0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.  相似文献   

2.
Two natural CO2-rich cordierite samples (1.00 wt% CO2, 0.38 wt% H2O, and 1.65 wt% CO2, 0.15 wt% H2O, respectively) were investigated by means of Raman spectroscopy and single-crystal X-ray diffraction at ambient and high pressures. The effect of heavy-ion irradiation (Au 2.2 GeV, fluence of 1 × 1012 ions cm?2) on the crystal structure was investigated to characterize the structural alterations complementary to results reported on hydrous cordierite. The linear CO2 molecules sustained irradiation-induced breakdown with small CO2-to-CO conversion rates in contrast to the distinct loss of channel H2O. The maximum CO2 depletion rate corresponds to ~12 ± 5 % (i.e. ~0.87 and ~1.49 wt% CO2 according to the two samples, respectively). The elastic properties of CO2-rich cordierite reveal stiffening due to the CO2 molecules (non-irradiated: isothermal bulk modulus K 0 = 120.3 ± 3.7 GPa, irradiated: K 0 = 109.7 ± 3.7 GPa), but show the equivalent effect of hydrous cordierite to get softer when irradiated. The degree of anisotropy of axial compressibilities and the anomalous elastic softening at increasing pressure agrees with those reported for hydrous cordierite. Nevertheless, the experimental high-pressure measurements using ethanol–methanol reveal a small hysteresis between compression and decompression, together with the noticeable effect of pressure-induced over-hydration at pressures between 4 and 5 GPa.  相似文献   

3.
The thermodynamic properties of monohydrocalcite, CaCO3 · H2O, have been obtained using a well-characterized natural specimen. Equilibration of the solid with water at 25°C under 0.97 atm CO2 led to an activity product [Ca2+][CO32?] = 10?7.60±0.03 and a free energy of formation ΔGfo = ?325,430 ± 270 calmol?. The enthalpy of solution of monohydrocalcite in 0.1 N HCl at 25°C led to a standard enthalpy of formation ΔHfo = ?358,100 ± 280 cal mol?1. Estimates of the variation of ΔGf with temperature and pressure showed monohydrocalcite to be metastable with respect to calcite and aragonite.  相似文献   

4.
Vaterite is shown to be unstable with respect to calcite at 25°C by measurements of the enthalpies of solution in 0·1 N HCl under 0·97 atm CO2 and the solubilities in water under 0·97 atm CO2 of the two polymorphs. For a pure, synthetic vaterite ΔH (tr) = ?1036 ±16 cal mol?1 and ΔG(tr) = ?790 ± 25 cal mol?1 for the transition to calcite. For other vaterites aged longer during preparation ΔH(tr) is smaller and shows a linear relationship with the X-ray line broadening which extrapolates to ΔH(tr) = ?545 ± 30 cal mo?1 for zero broadening. The use of X-ray line broadening as a measure of crystal imperfection and stability is discussed for various synthetic and natural vaterites.  相似文献   

5.
Calcium-free carbonate cancrinite with formula unit Na8.28[Al5.93Si6.07O24](CO3)0.93(OH)0.49·3.64H2O (CAN) has been synthesized under hydrothermal conditions. The product has been characterized by the methods of scanning electronic microscopy and energy dispersive X-ray analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis with FTIR of evolved gases (TGA–FTIR), and X-ray powder diffraction. The heat capacity of CAN has been measured from 6 to 259 K via low-temperature adiabatic calorimetry. A linear combination of Einstein functions has been used to approximate the obtained data on the heat capacity. The thermal contributions to the entropy and enthalpy of CAN in the temperature range 0–300 K have been calculated from these data. The heat capacity and third-law absolute entropy of CAN at 298.15 K are 1,047 ± 30 and 1,057 ± 35 J mol?1 K?1, respectively. High-temperature oxide-melt solution calorimetry has been used to determine the enthalpy of formation from elements of CAN at 298.15 K; the value equals ?14,684 ± 50 kJ mol?1. The Gibbs energy of formation from elements at 298.15 K has been calculated and totaled ?13,690 ± 51 kJ mol?1.  相似文献   

6.
Enthalpies and entropies of transition for the Mg2GeO4 olivine-spinel transformation have been determined from self-consistency analyses of Dachille and Roy's (1960), Hensen's (1977) and Shiota et al.'s (1981) phase boundary studies. When all three data sets are analyzed simultaneously,ΔH 973 andΔS 973 are constrained between ?14000 to ?15300 J mol?1 and ?13.0 to ?14.1·J mol?1 K?1, respectively. High-temperature solution calorimetric experiments completed on both polymorpha yield a value of ?14046±1366 J mol?1 forΔH 973. Kieffer-type lattice vibrational models of Mg2GeO4 olivine and spinel based on newly-measured infrared and Raman spectra predict a value of ?13.3±0.6 J mol?1 K?1 forΔS 1000. The excellent agreement between these three independent determinations ofΔH andΔS suggests that the synthesis runs of Shiota et al. (1981) at high pressures and temperatures bracket equilibrium conditions. In addition, no configurational disorder of Mg and Ge was needed to obtain the consistent parameters quoted. The Raman spectrum and X-ray diffractogram show that little disorder, if any, is present in Mg2GeO4 spinel synthesized at 0.2 GPa and 973–1048 K.  相似文献   

7.
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8.
The enthalpy of formation of petalite, LiAlSi4O10, has been measured using high-temperature solution calorimetry. The measurements were carried out in a Calvet-type twin micro calorimeter at 728?°C. A 2PbO?·?B2O3 melt was used as a solvent. Tabulated heats of formation of the components and tabulated heat capacities of the reactants and the product (Robie and Hemingway 1995) were used to calculate the standard heat of formation of petalite from the measured heats of solution. The calculations yielded a mean value of Δ f H pet 298.15=?4872±5.4 kJ mol?1. This value may be compared to the heat of formation of Δ f H pet 298.15= ?4886.5±6.3 kJ mol?1 determined by the HF solution calorimetry by Bennington et?al. (1980). Faßhauer et?al. (1998) combined thermodynamic data with phase-equilibrium results to obtain best-fit thermodynamic results using the Bayes method, in order to derive an internally consistent dataset for phases in the NaAlSiO4– LiAlSiO4–Al2O3–SiO2–H2O system. They determined ?4865.6?±?0.8?kJ?mol?1 as the enthalpy of formation of petalite, a value that is appreciably closer to the enthalpy found in this work.  相似文献   

9.
Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18 .x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000?°C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm?1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.  相似文献   

10.
The effect of ion beam irradiations on the elastic properties of hydrous cordierite was investigated by means of Raman and X-ray diffraction experiments. Oriented single crystals were exposed to swift heavy ions (Au, Bi) of various specific energies (10.0–11.1 MeV/u and 80 MeV/u), applying fluences up to 5 × 1013 ions/cm2. The determination of unit-cell constants yields a volume strain of 3.4 × 10?3 up to the maximum fluence, which corresponds to a compression of non-irradiated cordierite at ~480 ± 10 MPa. The unit-cell contraction is anisotropic (e 1 = 1.4 ± 0.1 × 10?3, e 2 = 1.5 ± 0.1 × 10?3, and e 3 = 7 ± 1 × 10?4) with the c-axis to shrink only half as much as the axes within the ab-plane. The lattice elasticity for irradiated cordierite (? = 1 × 1012 ions/cm2) was determined from single-crystal XRD measurements in the diamond anvil cell. The fitted third-order Birch–Murnaghan equation-of-state parameters of irradiated cordierite (V 0 = 1548.41 ± 0.16 Å3, K 0 = 117.1 ± 1.1 GPa, ?K/?P = ?0.6 ± 0.3) reveal a 10–11 % higher compressibility compared to non-irradiated cordierite. While the higher compressibility is attributed to the previously reported irradiation-induced loss of extra-framework H2O, the anomalous elasticity as expressed by elastic softening (β a ?1 , β b ?1 , β c ?1  = 397 ± 9, 395 ± 28, 308 ± 11 GPa, ?(β ?1)/?P = ?4.5 ± 2.7, ?6.6 ± 8.4, ?5.4 ± 3.0) appears to be related to the framework stability and to be independent of the water content in the channels and thus of the ion beam exposure.  相似文献   

11.
Hydrothermal volatile-solubility and partitioning experiments were conducted with fluid-saturated haplogranitic melt, H2O, CO2, and S in an internally heated pressure vessel at 900°C and 200?MPa; three additional experiments were conducted with iron-bearing melt. The run-product glasses were analyzed by electron microprobe, FTIR, and SIMS; and they contain ??0.12 wt% S, ??0.097 wt% CO2, and ??6.4 wt% H2O. Apparent values of log f O2 for the experiments at run conditions were computed from the [(S6+)/(S6++S2?)] ratio of the glasses, and they range from NNO ?0.4 to NNO?+?1.4. The C?CO?CH?CS fluid compositions at run conditions were computed by mass balance, and they contained 22?C99?mol% H2O, 0?C78?mol% CO2, 0?C12?mol% S, and <3 wt% alkalis. Eight S-free experiments were conducted to determine the H2O and CO2 concentrations of melt and fluid compositions and to compare them with prior experimental results for C?CO?CH fluid-saturated rhyolite melt, and the agreement is excellent. Sulfur partitions very strongly in favor of fluid in all experiments, and the presence of S modifies the fluid compositions, and hence, the CO2 solubilities in coexisting felsic melt. The square of the mole fraction of H2O in melt increases in a linear fashion, from 0.05 to 0.25, with the H2O concentration of the fluid. The mole fraction of CO2 in melt increases linearly, from 0.0003 to 0.0045, with the CO2 concentration of C?CO?CH?CS fluids. Interestingly, the CO2 concentration in melts, involving relatively reduced runs (log f O2????NNO?+?0.3) that contain 2.5?C7?mol% S in the fluid, decreases significantly with increasing S in the system. This response to the changing fluid composition causes the H2O and CO2 solubility curve for C?CO?CH?CS fluid-saturated haplogranitic melts at 200?MPa to shift to values near that modeled for C?CO?CH fluid-saturated, S-free rhyolite melt at 150?MPa. The concentration of S in haplogranitic melt increases in a linear fashion with increasing S in C?CO?CH?CS fluids, but these data show significant dispersion that likely reflects the strong influence of f O2 on S speciation in melt and fluid. Importantly, the partitioning of S between fluid and melt does not vary with the (H2O/H2O?+?CO2) ratio of the fluid. The fluid-melt partition coefficients for H2O, CO2, and S and the atomic (C/S) ratios of the run-product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the H, S, and C contents of pre-eruptive magmatic fluids and volcanic gases for subduction-related magmatic systems thus confirming our experiments are relevant to natural eruptive systems.  相似文献   

12.
Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H2O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH− and molecular H2O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H2O content (H2OT = OH− + H2Om) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm−1 band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H2O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H2OT band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H2O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H2O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm−1. We provide new FTIR absorptivity coefficients (ε3550) for basalt (62.80 ± 0.8 L mol−1 cm−1) and basanite (43.96 ± 0.6 L mol−1 cm−1). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε3550) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H2O in silicate glasses when the matrix effects are properly considered.  相似文献   

13.
Polarizedinfrared (IR) spectra of sanidine crystals from Volkesfeld, Eifel show the existence of two broad pleochroic absorption bands at 3,400 and 3,050 cm?1. Because overtones near 5,150 cm?1 were observed, the former bands are assigned to OH stretching frequencies of H2O molecules. On the basis of the pleochroic scheme of the bands it is proposed that H2O molecules occur as structural constituents entering theM site of the sanidine structure; the plane of the H2O molecules lies parallel to the symmetry plane.  相似文献   

14.
The following hardsphere modified Redlich-Kwong (HSMRK) equation of state was obtained by least squares fitting to available P-V-T data for methane (P in bars; T in Kelvins; v in cm3 mol?1; b = 60.00 cm3 mol?1; R = 83.14 cm3barmol?1K?1): PRT(1 + y + y2?y3v(1?y)3)-c(T) + d(T)v + e(T)v2/v(v + b)T12y = b4vc(T) = 13.403 × 106 + (9.28 × 104)T + 2.7 T2d(T) = 5.216 × 109 ? (6.8 × 106)T + (3.28 × 103)T2e(T) = (?2.3322 × 1011) + (6.738 × 108)T + (3.179 × 105)T2 For the P-T range of experimental data used in the fit (50 to 8600 bars and from 320 to 670 K), calculated volumes and fugacity coefficients for CH4 relative to experimentally determined volumes and fugacity coefficients have average percent deviations of 0.279 and 1.373, respectively. The HSMRK equation, which predicts linear isochores over a wide P-T range, should yield reasonable estimates of fugacity coefficients for CH4 to pressures and temperatures well outside the P-T range of available P-V-T data. Calculations for the system H2O-CO2-CH4, using the HSMRK equations for H2O and CO2 of Kerrick and Jacobs (1981) and the HSMRK equation for CH4 of this study, indicate that compared to the binary H2O-CO2 system, small amounts of CH4 in the ternary system H2O-CO2-CH4 slightly increases the activity of H2O, and significantly decreases the activity of CO2.  相似文献   

15.
The compression of cordierite (Mg, Fe)2Al4Si5O18·n (H2O, CO2; Na+, K+) has been studied up to 30 kbar (25° C) by volumetric measurements with a piston cylinder apparatus and by X-ray measurements with a diamond-anvil cell. Natural cordierite of intermediate Mg-Fe composition and synthetic Mg-cordierite served as samples. Two discontinuities at 2.2±0.3 and 9.0±0.6 kbar which are correlated with very small volume changes (0.3?0.05%) have been found. The X-ray data indicate, however, no symmetry change of the crystal structure. The two discontinuities are interpreted as phase transitions. The two discontinuities establish three pressure dependent phases referred to as low-pressure (LP)-, first high-pressure (HP1)- and second high-pressure (HP2)-phase. The gross compressibility of cordierite decreases from 1.1 Mbar?1 at low pressure to 0.7 Mbar?1 at 30 kbar for the intermediate Mg-Fe cordierite, and to 0.4 Mbar?1 for Mg-cordierite. Depending on the pressure transmitting medium used in the two different compression techniques, two kinds of compression behavior are observed for cordierite. The measurements with the piston cylinder apparatus where lead is used as quasihydrostatic pressure medium indicate normal compression properties. The X-ray data, however, obtained with the diamond anvil cell where a methanol-ethanol mixture provides hydrostatic pressure conditions yield, e.g. for the HP1-phase a dramatic decrease in compressibility to almost zero. IR-spectra from samples of augmenting experiments with methanol, deuteromethanol and D2O as pressure media indicate that pressure media of which the molecule size is comparable with the dimensions of the cordierite channels may be incorporated in the structure. This suggests that under such hydrostatic conditions the compression of cordierite is modified by a structure internal component which is acting via the channel system.  相似文献   

16.
The dissolution of H2O and CO2 in structurally dense, nominally anhydrous and non-carbonate oxide matrices such as MgO and CaO is reviewed. H2O and CO2 are treated as gaseous oxide components which enter into solid solution with the refractory oxide hosts. They form anion complexes associated with cation vacancy sites. Evidence is presented that OH? pairs which derive from the dissolution of H2O are subject to a charge transfer (CT) conversion into peroxy moieties and molecular hydrogen, O 2 2? ... H2. Because the O 2 2? moiety is small (O?-O? distance ≈ 1.5 Å) high pressure probably favors the CT conversion. Mass spectroscopic studies show that molecular H2 may be lost from the solid which retains excess oxygen in the form of O 2 2? , leading to the release of atomic O. The dissociation of O 2 2? moieties into a vacancy-bound O? state and an unbound O? state can be followed by measuring the internal redox reactions involving transition metal impurities, the transient paramagnetism of the O? and their effect on the d.c. conductivity. Evidence is presented that CO2 molecules dissolve dissociatively in the structurally dense oxide matrix, as if they were first to dissociate into CO+O and then to form separate solute moieties CO 2 2? and O 2 2? , both associated with cation vacancy sites. In the CO 2 2? moiety (C-O? distance 1.2–1.3 Å, OCO angle ≈ 130°) the C atom probably sits off center. The transition of the C atom into interstitial sites is accompanied by dissociation of the CO 2 2? moiety into CO? and O?. This transition can be followed by infrared spectroscopy, using OH? as local probes. Further support derives from magnetic susceptibility, thermal expansion, low frequency dielectric loss and low temperature deformation measurements. The recently observed emission of O and Mg atoms besides a variety of molecules such as CO, CO2, CH4, HCN and other hydrocarbons during impact fracture of MgO single crystals is presented and discussed in the light of the other experimental data.  相似文献   

17.
The carbonato and hydrogencarbonato complexes of Mg2+ were investigated at 25 and 50° in solutions of the constant ClO4? molality (3 M) consisting preponderantly of NaClO4. The experimental data could be explained assuming the following equilibria: Mg2+ + CO2B + H2O ag MgHCO+3 + H+, log 1β1 = ?7.644 ± 0.017 (25°), ?7.462 ± 0.01 1 (50°), Mg2+ + 2 CO2g + 2 H2Oag Mg(HCO3)02 ± 2 H+, log 1β2 = ?15.00 ± 0.14 (25°), ?15.37 ± 0.39 (50°), Mg2+ + CO2g + H2Oag MgCO03 + 2 H+, log 1k1 = ?15.64 ± 0.06 (25°),?15.23 ± 0.02 (50°), with the assumption γMgCO30 = γMg(HCO3)02, ΔG0(I = 0) for the reaction MgCO03 + CO2g + H2O = Mg(HCO3)02 was estimated to be ?3.91 ± 0.86 and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.  相似文献   

18.
The speciation of CO2 in dacite, phonolite, basaltic andesite, and alkali silicate melt was studied by synchrotron infrared spectroscopy in diamond anvil cells to 1,000 °C and more than 200 kbar. Upon compression to 110 kbar at room temperature, a conversion of molecular CO2 into a metastable carbonate species was observed for dacite and phonolite glass. Upon heating under high pressure, molecular CO2 re-appeared. Infrared extinction coefficients of both carbonate and molecular CO2 decrease with temperature. This effect can be quantitatively modeled as the result of a reduced occupancy of the vibrational ground state. In alkali silicate (NBO/t = 0.98) and basaltic andesite (NBO/t = 0.42) melt, only carbonate was detected up to the highest temperatures studied. For dacite (NBO/t = 0.09) and phonolite melts (NBO/t = 0.14), the equilibrium CO2 + O2? = CO3 2? in the melt shifts toward CO2 with increasing temperature, with ln K = ?4.57 (±1.68) + 5.05 (±1.44) 103 T ?1 for dacite melt (ΔH = ?42 kJ mol?1) and ln K = ?6.13 (±2.41) + 7.82 (±2.41) 103 T ?1 for phonolite melt (ΔH = ?65 kJ mol?1), where K is the molar ratio of carbonate over molecular CO2 and T is temperature in Kelvin. Together with published data from annealing experiments, these results suggest that ΔS and ΔH are linear functions of NBO/t. Based on this relationship, a general model for CO2 speciation in silicate melts is developed, with ln K = a + b/T, where T is temperature in Kelvin and a = ?2.69 ? 21.38 (NBO/t), b = 1,480 + 38,810 (NBO/t). The model shows that at temperatures around 1,500 °C, even depolymerized melts such as basalt contain appreciable amounts of molecular CO2, and therefore, the diffusion coefficient of CO2 is only slightly dependent on composition at such high temperatures. However, at temperatures close to 1,000 °C, the model predicts a much stronger dependence of CO2 solubility and speciation on melt composition, in accordance with available solubility data.  相似文献   

19.
A well-characterized suite of vesuvianite samples from the volcanic ejecta (skarn or syenites) from Latium (Italy) was studied by single-crystal, polarized radiation, Fourier-transform infrared (FTIR) spectroscopy and secondary-ion mass-spectrometry (SIMS). OH-stretching FTIR spectra consist of a rather well-defined triplet of broad bands at higher-frequency (3,700–3,300 cm–1) and a very broad composite absorption below 3,300 cm–1. Measurements with E//c or Ec show that all bands are strongly polarized with maximum absorption for E//c. They are in agreement with previous band assignments (Groat et al. Can Mineral 33:609, 1995) to the two O(11)–H(1) and O(10)–H(2) groups in the structure. Pleochroic measurements with changing direction of the E vector of the incident radiation show that the orientation of the O(11)–H(1) dipole is OHc~35°, in excellent agreement with the neutron data of Lager et al. (Can Mineral 37:763, 1999). A SIMS-based calibration curve at ~10% rel. accuracy has been worked out and used as reference for the quantitative analysis of H2O in vesuvianite by FTIR. Based on previous SIMS results for silicate minerals (Ottolini and Hawthorne in J Anal At Spectrom 16:1266, 2001; Ottolini et al. in Am Mineral 87:1477, 2002) the SiO2 and FeO content of the matrix were assumed as the major factors to be considered at a first approximation in the selection of the standards for H. The lack of vesuvianite standards for quantitative SIMS analysis of H2O has been here overcome by selecting low-silica elbaite crystals (Ottolini et al. in Am Mineral 87:1477, 2002). The resulting integrated molar absorption FTIR coefficient for vesuvianite is i=100.000±2.000 l mol–1 cm–2. SIMS data for Li, B, F, Sr, Y, Be, Ba REE, U and Th are also provided in the paper.  相似文献   

20.
The enthalpy of Mg-Fe ordering in En50Fs50 orthopyroxene was measured using the transposed temperature drop calorimetric method. Heat effects associated with two consecutive drops were recorded. In the first drop, synthetic orthopyroxene samples equilibrated at 823?K, 0.1?MPa and a f?O2 of the WI buffer were dropped from 823?K into the calorimeter, which was held at 1173?K. The measured heat effect corresponds to the enthalpy change due to the heat capacity of the sample from 823 to 1173?K and to the enthalpy associated with the (dis)ordering of Mg and Fe2+. In the second drop, the samples, with an Fe-Mg order corresponding to 1173?K, were dropped again from 823 to 1173?K. From the difference of the heat effects measured in the two experiments, the enthalpy of disordering associated with the temperature change from 823 to 1173?K was calculated to be ?1.73±0.04 J mol?1. The observed enthalpy corresponds to a change in the mole fraction of iron on the M2 site, ΔX Fe,M2=?0.096 ± 0.001, which leads to of ΔH 0 exch of 18.0 ± 0.4 kJ mol?1 for the exchange reaction: The degree of Fe-Mg order was characterized by 57Fe Mössbauer resonance spectroscopy. In order to minimize the error due to the thickness of the absorber, the iron concentration of the absorber was reduced step by step from 5 to 1 mg?Fe?cm?2. The iron distribution extrapolated to zero thickness was used for the calculations of the enthalpy of exchange reaction.  相似文献   

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