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1.
A method of determining the number of Al-O-Al bonds per unit cell from 29Si nuclear magnetic resonance (NMR) data of synthetic cordierites with increasing Si, Al order is described. The number of Al-O-Al bonds is found to vary linearly with the logarithm of the annealing time. This may be correlated with previously published heat of solution data on similar samples (Carpenter et al. 1983) to determine the enthalpy change Δh, associated with a single Al?Si interchange in cordierite. Δh is found to be 8.1 kcal/mole. The NMR data show that the short range Al, Si order cannot be described in terms of twin domains of ordered orthorhombic cordierite. An ordering model derived from group theoretical constraints on possible Al, Si distributions within the hexagonal symmetry of the cordierite is found to provide a better fit to the NMR data.  相似文献   

2.
Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole “e” structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il? structure →- Cl? high albite structure: ~0.7-1.9 kcal/mole Il? structure equilibrated at low temperature → Il? structure equilibrated at high temperature: ~ 1.8?0.8 kcal/mole.ΔHsoln data for the high series overlap with the data of Newtonet al. (1980) for synthetic high structural state plagioclases except in the composition range ~An90–An100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl? symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il? and “e” structures.Values of the enthalpy change due to disordering (ΔHord) also show a number of systematic trends. Firstly, the values for e → Cl? are larger than for Il? → Cl? in the composition range where both e and Il? structures are observed (~An65-An72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔHord, which occurs at ~An50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il? anorthites and bytownites to a Cl? structure have been estimated by assuming that the Cl? solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.  相似文献   

3.
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   

4.
We report relative enthalpy measurements on quartz, cristobalite and amorphous SiO2 between 1000 and 1800 K. We have observed a glass transition around 1480 K for amorphous SiO2. From our results and available Cp, relative enthalpy, and enthalpy of solution data we have derived a consistent set of thermodynamic data for these phases. Our calculated enthalpies of fusion are 8.9 ± 1.0 kJ mole?1 for cristobalite at 1999 K and 9.4 ± 1.0 kJ mole?1 at 1700 K for quartz.  相似文献   

5.
Potassic cordierites with the chemical composition K x Mg2Al4+x Si5–xO18 (x = 0.00, 0.10, 0.20, and 0.25) were synthesized by annealing glasses at 1290° C for different lengths of time. The procedure resulted in cordierites with different states of Al,Si-order for the tetrahedral sites in the structure. The dependence between the potassium-content and the state of order on one side and between annealing time and the state of order on the other side was then studied using 29Si MAS nuclear magnetic resonance (NMR) spectroscopy. The spectra show that the state of order is a continuous function of annealing time for all compositions considered, but the rate of ordering decreases with increasing K-content. Since the substitution K+Al Si leads to higher Al/Si-ratios; the lower rate of ordering is discussed as a consequence of changed statistics for Al, Si site exchanges. The Al atoms replacing silicon in the structure to balance the charge of potassium cations are not located close to the potassium ion but at a maximum distance from it. This is shown to be a consequence of an improvement in coordination of all oxygen atoms in the cordierite framework.  相似文献   

6.
The cluster variation method, in the single prism approximation, is used to model phase relations in the system, Fe2O3-FeTiO3. Ordering in FeTiO3 is analyzed, and it is shown that the stabilization of FeTiO3 (relative to mechanical mixing of Fe2O3 and Ti2O3) includes: (1) a contribution from the redox reaction, Fe3++Ti3+→Fe2++Ti4+ (ΔH redox～?70kJ mole?1); and (2) a contribution from ordering (ΔH OD～?8kJ mole?1). A theoretical phase diagram is presented and compared with available experimental data. Semiquantitative agreement between theory and experiment (on the location of phase boundaries) is achieved; but, owing to the paucity of experimental data on coexisting phases, these results may be fortuitous.  相似文献   

7.
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   

8.
Reversal experiments at 1,150–1,300°C on the reaction forsterite+cordierite=aluminous orthopyroxene+spinel in the system MgO-Al2O3-SiO2 show the equilibrium to have a negativedT/dP. The slope andT-P location of this equilibrium have been modelled using available heat capacity data and various structural models which explore the configurational entropy contributions to the totalΔS. The experimental data are consistent with the aluminous orthopyroxene model of Ganguly and Ghose (1979) where limited Al disorder occurs between theM1 andM2 sites, Al-Si mixing occurs on the tetrahedralB site with the ‘aluminum avoidance’ principle maintained, and Mg-Al disorder occurs in spinel with an interchange enthalpy of 9–12 kcal mol?1. Additionally, Al-Si disordering which occurs in the indialite structure of cordierite is inconsistent with the experimental data and all pyroxene and spinel mixing models; consequently, Si and Al in anhydrous cordierites to 1,300°C in the system MgO-Al2O3-SiO2 must be largely ordered.  相似文献   

9.
The published 29Si NMR data on synthetic Mg-cordierites have been used to estimate the changes in configurational Al-Si entropy of the samples due to metastable disorder. The results show that with the increase of the time of annealing in the range of 2 min-2000 h at 1185 °C the entropy of disorder in cordierite decreases from 17.1 to 6.4 J/mol K, while at 1400 °C in the range of 2 min–88 h the entropy changes from 15.4 to 8.8 J/mol K. The decrease in entropy is followed by the appearance and increase of long-range ordering which is reflected in changes of T1 and T2 site occupancies, decrease in the number of Si-O-Si and Al-O-Al groupings around O1 oxygens and in a decrease in the number of Al-O-Si-O-Al contacts among hexagonal 6T2-rings. The derived values of entropy effects together with published calorimetric data suggest that the enthalpy of metastable disordering strongly depends on the temperature of annealing.  相似文献   

10.
The high-grade assemblage Cd-Ga-Si-Qz can be thermodynamically modelled from available calorimetric data on the metastable reaction: (I) $$3 MgCd \rightleftarrows 2 Py + 4 Si + 5 Qz$$ naturalK D Fe-Mg between garnet and cordierite and experimental results on cordierite hydration. In the system SiO2-Al2O3-MgO-H2O, reaction (I) becomes (II) $$3 MgCd \cdot nH_2 O \rightleftarrows 2 Py + 4 Si + 5 Qz + 3 nH_2 O$$ . However, hydrous cordierite is neither a hydrate nor a solid solution between water and anhydrous cordierite and when nH2O (number of moles of H2O in Cd) is plotted against \(P_{H_2 O} \) , the resulting isotherms are similar to adsorption isotherms characteristic of zeolitic minerals. Reaction (II) can thus be considered as a combination of reaction (I) with a physical equilibrium of the type nH2O (in Cd)?nH2O (in vapor phase). Starting from a point on equilibrium (I), introduction of H2O into anhydrous Mg-cordierite lowers the chemical potential of MgCd and hence stabilizes this mineral to higher pressure relative to the right-hand assemblage in reaction (I). The pressure increment of stabilization,ΔP, above the pressure limit of anhydrous cordierite stability at constantT, has been calculated using the standard thermodynamics of adsorption isotherms. Cordierite is regarded as a mixture of Mg2Al4Si5O18 and H2O. The activity of H2O in the cordierite is evaluated relative to an hypothetical standard state extrapolated from infinite H2O dilution, by using measured hydration data. The activity of Mg2Al4Si5O18 in the cordierite is then obtained by integration of the Gibbs-Duhem equation, and the pressure stabilization increment,ΔP, computed by means of the relation: $$\Delta V_s \Delta P \cong - RT\ln a_{MgCd}^{MgCd \cdot nH2O} \left( {\Delta V indepentdent of P and T} \right)$$ . Thus, one may contour theP-T plane in isopleths of nH2O=constant within the area of Mg-cordierite stability allowed by the hydration data for \(P_{H_2 O} = P_{total} \) . The present model indicates greater stabilization of cordierite by H2O than the model of Newton and Wood (1979) and the calculated curve for metastable breakdown of hydrous MgCd is consistent with experimental data on cordierite breakdown at \(P_{H_2 O} = P_{total} \) . Mg/(Mg+Fe) isopleths have been derived for cordierites of varying nH2O in the SiO2-Al2O3-MgO-FeO-H2O system using the additional assumptions that (Fe, Mg) cordierite and (Fe, Mg) garnet behave as ideal solutions, and that typical values of the distribution coefficient of Fe and Mg between coexisting garnet and cordierite observed in natural parageneses can be applied to the calculations. The calculated stable breakdown curve of Fe-cordierite under conditions of \(P_{H_2 O} = P_{total} \) to almandine, sillimanite, quartz and vapor has a positive slope (dP/dT) apparently in contrast to the experimental negative slope. This may be explained by hydration kinetics, which could have allowed systematic breakdown of cordierites of metastable hydration states in the experiments. The bivariant Cd-Ga fields calibrated from the present model are, potentially, good petrogenetic indicators, as, given the iron-magnesium ratio of garnet and cordierite and the hydration number of cordierite, the temperature, pressure and water fugacity are uniquely determined. As this geothermobarometer is in part based on the water content of cordierite, it can be used only if there is some assurance that the actual hydration is inherited from high-grade metamorphic conditions. Such conditions could be realised if the slope of unloading-cooling retrograde metamorphism is more or less parallel to a cordierite isohydron.  相似文献   

11.
Transposed temperature drop calorimetry at 1000 °C was performed on natural zircons (ZrSiO4) from Sri Lanka that were partially to completely metamict due to α-decay event damage (0.06 to 11.7×1015 α-decay events/mg). The enthalpy of annealing at room temperature (ΔHanneal) varies sigmoidally as a function of radiation dose. ΔHanneal reaches a saturation plateau at radiation doses greater than 5×1015 α-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, ?59±3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of Angstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO2-rich and ZrO2-rich regions in the metamict state.  相似文献   

12.
We report a FTIR (Fourier transform infrared) study of a set of cordierite samples from different occurrence and with different H2O/CO2 content. The specimens were fully characterized by a combination of techniques including optical microscopy, single-crystal X-ray diffraction, EMPA (electron microprobe analysis), SIMS (secondary ion mass spectrometry), and FTIR spectroscopy. All cordierites are orthorhombic Ccmm. According to the EMPA data, the Si/Al ratio is always close to 5:4; X Mg ranges from 76.31 to 96.63, and additional octahedral constituents occur in very small amounts. Extraframework K and Ca are negligible, while Na reaches the values up to 0.84 apfu. SIMS shows H2O up to 1.52 and CO2 up to 1.11 wt%. Optically transparent single crystals were oriented using the spindle stage and examined by FTIR micro-spectroscopy under polarized light. On the basis of the polarizing behaviour, the observed bands were assigned to water molecules in two different orientations and to CO2 molecules in the structural channels. The IR spectra also show the presence of small amounts of CO in the samples. Refined integrated molar absorption coefficients were calibrated for the quantitative microanalysis of both H2O and CO2 in cordierite based on single-crystal polarized-light FTIR spectroscopy. For H2O the integrated molar coefficients for type I and type II water molecules (ν3 modes) were calculated separately and are [I]ε?=?5,200?±?700?l?mol?1?cm?2 and [II]ε?=?13,000?±?3,000?l?mol?1?cm?2, respectively. For CO2 the integrated coefficient is $ \varepsilon_{{{\text{CO}}_{ 2} }} $ ?=?19,000?±?2,000?l?mol?1?cm?2.  相似文献   

13.
Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 ? Al:Si=0.80:0.20, T2?Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.  相似文献   

14.
In this work, we have reviewed a large compositional dataset (571 analyses) for natural and experimental glasses to understand the physico-chemical and compositional conditions of magmatic cordierite crystallization. Cordierite crystallizes in peraluminous liquids (A/CNK ≥1) at temperatures ≥750 °C, pressures ≤700 MPa, variable H2O activity (0.1–1.0) and relatively low fO2 conditions (≤NNO ? 0.5). In addition to A/CNK ratio ≥1, a required condition for cordierite crystallization is a Si + Al cation value of the rhyolite liquid of 4 p8O (i.e. calculated on the 8 oxygen anhydrous basis), which is consistent with low Fe3+ contents and the absence or low content of non-bridging oxygens (NBO). This geochemical condition is strongly supported by the rare, if not unique, structure of cordierite where the tetrahedral framework is composed almost exclusively of Si and Al cations the sum of which is equal to 4 p8O [i.e. (Mg,Fe)8/9Al16/9Si20/9O8], indicating that aluminium (and cordierite) saturation is limited by rhyolite liquids with Al = 4 ? Si. Indeed, synthetic or natural systems with Al > 4 ? Si always show metastable glass-in-glass separation or crystallization of refractory minerals such as corundum (Al16/3O8) and aluminosilicates (Al16/5Si8/5O8). Multivariate regression analyses of literature data for experimental glasses coexisting with magmatic cordierite produced two empirical equations to independently calculate the T (±13 °C; ME, maximum error = 29 °C) and P (±16 %; ME% = 27 %) conditions of cordierite saturation. The greatest influence on the two equations is exerted by H2Omelt and Al concentrations, respectively. Testing of these equations with other thermobarometric constraints (e.g. feldspar-liquid, GASP, Grt–Bt and Grt–Crd equilibria) and thermodynamic models (NCKFMASHTO and NCKFMASH systems) was successfully performed for Crd-bearing rhyolites and residual enclaves from San Vincenzo (Tuscany, Italy), Morococala Field (Bolivia) and El Hoyazo (Spain). The reliability of each calculated P–T pair was graphically evaluated using the minimum and maximum P–T–H2O relationships for peraluminous rhyolite liquids modified after the metaluminous relationships in this work. Both P–T calculations and checking can be easily performed with the attached user-friendly spreadsheet (i.e. Crd-sat_TB).  相似文献   

15.
The mineral chemistry of cordierites from three different sanidinite facies localities-1) volcanic xenoliths from the Eifel, Germany; 1) buchites of the Blaue Kuppe, Germany; 3) paralavas from the Bokaro coalfield, India-is characterized by unusually high potassium contents up to 1.71 wt%, equivalent to 0.22 K atoms per formula unit (p.f.u.) based on 18 oxygens. Significantly, these cordierites are either hexagonal highcordierites (indialites) with =0 or exhibit intermediate -values 0<<0.20 relative to well Al,Si-ordered orthorhombic low-cordierite. Based on microprobe analyses, the predominant substitutional mechanism for alkali incorporation is Alk[Channel]+Al[4] for +Si[4], thus leading to Al/Si-ratios deviating considerably from the value 4:5 in ideal cordierite M2[Al4Si5O18]. The most highly substituted cordierite from Blaue Kuppe is about (K0.22Na0.07)[Ch](Mg1.33Fe 0.66 2+ )[6][Al4.16Si4.79O18]. Bokaro cordierites are further characterized by obvious (Al+Si)-deficiencies against the ideal value of 9.0 p.f.u., a tendency of which is apparent in most Blaue Kuppe analyses as well. As the tetrahedral deficiencies are often equivalent to excess cations in the octahedra, we assume that ferric iron fills up the remaining tetrahedral sites, again linked with the introduction of potassium according to K+Fe3+ for +Si. In comparison with the available experimental data, these natural potassic cordierites are considered stable high-temperature phases regarding their compositions, but not their structural states. Although the substitution KAl for Si in Mg-cordierite is known to lower the maximum -value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher -values of the Blaue Kuppe cordierites might be caused by their topotactic origin from preexisting biotite. The complicated twin and domain patterns of the hexagonal Eifel and Bokaro cordierites as observed in thin section could perhaps be attributed to structural modulations as postulated recently for hexagonal cordierite shortly after its growth.  相似文献   

16.
Ferrocordierite with Fe+2/Fe+2+Mg=0,92 was synthesized as a single phase at 600° C and 1000 bars total pressure using cristobalite and natural monoclinic chloritoid as starting materials. Oxygen fugacities during synthesis were those given by the buffering power of the hydrothermal bombs. Pure ferrocordierite without Mg synthesized from various artificial starting materials was never obtained as a single phase, but coexisted with variable amounts of metastable hercynite+quartz. The ferrocordierites made in this study are orthorhombic with distortion indices Δ up to 0,26° as a function of pressure, temperature, and duration of the runs. The lattice constantsa 0 = 17,234 and b0 = 9,824 are considerably larger, c0 = 9,298 smaller than those of pure Mg-cordierite; mean indices of refraction vary within the range 1,569–1,573±0,002. In successful breakdown experiments natural Fe-rich cordierite intergrown with quartz from Mujinazawa, Japan, as well as the most Fe-rich natural cordierite so far discovered, from Dolni Bory, Moravia, were used. Ferrocordierite is stable only above relatively high temperatures (450°–600° C depending on pressure), which are within the limits of determination identical to the lower stability limits of Mg-cordierite. In contrast to the latter phase, however, ferrocordierite becomes unstable at a yet undefined pressure between 6000 and 10000 bars. The stable breakdown products at low temperatures are chloritoid+quartz or - possibly only at very low pressures - assemblages with 7 Å-chamosite, which have been synthesized (metastably ?) over a more extended PT-range. Runs seeded with chloritoid + cristobalite did not yield critical evidence as to the more stable ,assemblage. Except for occasional small amounts of pyrophyllite there is considerable uncertainty concerning the aluminous phase coexisting with chamosite; kaolinite with all critical peaks covered by chamosite orγ-Al203 amorphous to X-rays may be present. At high temperatures and at a pressure of 10000 bars ferrocordierite broke down to assemblages containing an orthoamphibole, probably ferrogedrite. It is uncertain, however, whether these assemblages represent stable equilibrium, or whether they are metastable substitutes for other parageneses, e.g. almandite silimanite + quartz. Given the requisite bulk compositions and geothermal gradients ferrocordierite remains stable under static conditions to crustal depths of at least 20 km. ThusChinner's mechanism of differential breakdown of Fe-bearing cordierites with increasing pressures may operate in deep zones of regional metamorphism within orogenic belts, but appears unlikely in shallow posttectonic contact metamorphism, unless this metamorphism is caused by hot basic magmas producing temperatures above the incongruent melting of ferrocordierite. In contrast to the relations found for Mg-cordierite the formation of feroocordierite is very sluggish, the more so the higher the temperature and pressure. Intermediate metastable assemblages of other phases appear in its place. It seems possible that these unfavorable reaction kinetics are - opposed to thermodynamic equilibrium - in part responsible for the rare occurrence of Fe-rich cordierites in nature. The overlap of the stability fields of ferrocordierite and chloritoid as demonstrated in this study allows the stable coexistence of these two phases within a limited temperature range.  相似文献   

17.
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   

18.
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   

19.
The enthalpies of solution of synthetic Mg2SiO4-Fe2SiO4 olivine solid solutions have been measured in Pb2B2O5 melt at 970 K. The heat of solution of forsterite was found to be 15.62 ± 0.3 kcal mol?1 and that of fayalite 9.39 ± 0.14 kcal mol?1. Solid solutions between these end-members exhibit small positive deviations from mixing ideality, asymmetric towards the Fe end-member. In terms of the sub-regular solution model, excess enthalpies of intermediate olivine are adequately represented by the equation Hxs = 2(1000 + 1000XFe) XFeXMgThe enthalpies of solution at 970 K are consistent with high temperature phase equilibrium measurements of activity-composition relationships in the olivine series. Excess entropy terms are not needed to relate the phase equilibrium data to the calorimetric data presented here.The enthalpy of solution of FeSiO3 ferrosilite at 970 K was found to be 4.36 ± 0.10 kcal mol?1. This value, when taken together with calorimetric measurements on fayalite and quartz, is consistent with phase equilibrium investigations of the reaction: 2FeSiO3 = Fe2SiO4 + SiO2 Ferrosilite Fayalite QuartzThese provide a check on the internal consistency of the calorimetric data presented here.  相似文献   

20.
The enthalpies of solution of petrologically important phases in the system MgO-Al2O3-SiO 2 were measured in a melt of composition 2PbO · B2O3 at 970 ± 2K. The substances investigated included synthetic and natural (meteoritic) enstatite (MgSiO3), synthetic aluminous enstatite (MgSiO30.9Al2O30.1), synthetic and natural cordierite (Mg2Al4Si5O18), synthetic and natural sapphirine (approx. 7MgO·9Al2O3 · 3SiO2), synthetic spinel (MgAl2O4), natural sillimanite (Al2SiO5), synthetic forsterite (Mg2SiO4), synthetic pyrope (Mg3Al2Si3O12), natural quartz (SiO2), synthetic periclase (MgO) and corundum (Al2O3). Improvement in standardization of the calorimeter solvent made possible greater precision in this study than obtainable in former work in this laboratory on some of the same substances.The enthalpies of formation of enstatite, synthetic cordierite, forsterite and spinel are in reasonable agreement with values previously determined by solution calorimetry. The enthalpy of formation of enstatite is about 0.7 kcal less negative than the value for clinoenstatite resulting from the HF calorimetry of Torgesen and Sahama (J. Amer. Chem. Soc.70. 2156–2160, 1948), and is in accord with predictions based on analysis of published pyroxene equilibrium work. Aluminous enstatite with 10 wt.% Al2O3 shows an enthalpy of solution markedly lower than pure MgSiO3: the measurements lead to an estimate of the enthalpy of formation at 970 K for MgAl2SiO6 (Mg-Tschermak) orthopyroxene of + 9.4 ± 1.5 kcal/mole from MgSiO3 and Al2O3.Comparison of the enthalpies of formation of synthetic cordierite and anhydrous natural low-iron cordierite shows that they are energetically quite similar and that the synthetic cordierite is not likely to have large amounts of (Al, Si) tetrahedral disorder. Comparison of the enthalpies of formation of synthetic sapphirine and natural low-iron sapphirine shows, on the other hand, that the former is not a good stability model for the latter. The lower enthalpy of formation of the high-temperature synthetic sample is undoubtedly a consequence of cation disordering.The enthalpy of formation of natural sillimanite is considerably less negative than given by the tables of Robie andWaldbaum (U.S. Geol. Surv. Bull.1259 1968).The measured enthalpy of formation of synthetic pyrope is consistent with that deduced from published equilibrium diagrams in conjunction with the present measured enthalpy of formation of aluminous enstatite. Calculation of the entropy of synthetic pyrope from the present data yields surprisingly high values and suggests that synthetic pyrope is not a good stability model for natural pyrope-rich garnets. Hence, considerable doubt exists about the direct quantitative application of experimental diagrams involving pyropic garnet to discussions of the garnet stability field in the Earth's outer regions.  相似文献   

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