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1.
 Calorimetric and P–V–T data for the high-pressure phase Mg5Al5Si6O21(OH)7 (Mg-sursassite) have been obtained. The enthalpy of drop solution of three different samples was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr University Bochum, Germany) using lead borate (2PbO·B2O3) at T=700 ∘C as solvent. The resulting values were used to calculate the enthalpy of formation from different thermodynamic datasets; they range from −221.1 to −259.4 kJ mol−1 (formation from the oxides) respectively −13892.2 to −13927.9 kJ mol−1 (formation from the elements). The heat capacity of Mg5Al5Si6O21(OH)7 has been measured from T=50 ∘C to T=500 ∘C by differential scanning calorimetry in step-scanning mode. A Berman and Brown (1985)-type four-term equation represents the heat capacity over the entire temperature range to within the experimental uncertainty: C P (Mg-sursassite) =(1571.104 −10560.89×T −0.5−26217890.0 ×T −2+1798861000.0×T −3) J K−1 mol−1 (T in K). The P V T behaviour of Mg-sursassite has been determined under high pressures and high temperatures up to 8 GPa and 800 ∘C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with Vaseline to ensure hydrostatic pressure-transmitting conditions, NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus was determined as 116.0±1.3 GPa, (K ′=4), V T,0 =446.49 3 exp[∫(0.33±0.05) × 10−4 + (0.65±0.85)×10−8 T dT], (K T/T) P  = −0.011± 0.004 GPa K−1. The thermodynamic data obtained for Mg-sursassite are consistent with phase equilibrium data reported recently (Fockenberg 1998); the best agreement was obtained with Δf H 0 298 (Mg-sursassite) = −13901.33 kJ mol−1, and S 0 298 (Mg-sursassite) = 614.61 J K−1 mol−1. Received: 21 September 2000 / Accepted: 26 February 2001  相似文献   

2.
Low-temperature heat capacity measurements for MgCr2O4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K−1 mol−1) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr3+ ions in MgCr2O4 occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr2O4 has been missed. We have determined the position of the near-univariant reaction MgCr2O4+SiO2=MgSiO3+Cr2O3. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H2O, BaO-B2O3 or K2O-B2O3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr2O3, MgSiO3 and SiO2, give the entropy and enthalpy of formation of MgCr2O4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ f H ∘ 298=−1759.2±1.5 kJ mol−1 and S ∘ 298=122.1±1.0 J K−1 mol−1 for MgCr2O4. This entropy is some 16 J K−1 mol−1 more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr2O4 consistent with an effective spin quantum number (S') for Cr3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997  相似文献   

3.
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (fHOxides=-168.7Dž.4 kJ mol-1, or (fH0298=-4,872.5dž.0 kJ mol-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al2O3-SiO2-H2O (CASH). The following data for lawsonite and zoisite were obtained: (fH0298 (lawsonite)=-4,865.68 kJ mol-1 , S0298 (lawsonite)=229.27 J K-1 mol-1 , (fH0298 (zoisite)=-6,888.99 kJ mol-1 , S0298 (zoisite)=297.71 J K-1 mol-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.  相似文献   

4.
Dense isotropic polycrystalline specimens of majorite-rich garnets (Py100, Py62Mj38, Py50Mj50, Py21Mj79 and Mj100) along the pyrope (Mg3Al2Si3O12 = Py100)-majorite (MgSiO3 = Mj100) join were fabricated in a 2000-ton uniaxial split-sphere anvil apparatus (USSA-2000) at pressures from 10 to 18.5 GPa and temperatures from 1200 to 1850 °C, within their stability fields in runs of 2–4-h duration, using hot-pressing techniques developed by Gwanmesia et al. (1993). These specimens are single-phased, fine-grained (≤5 mm), free of microcracks, and have bulk densities greater than 99% of the corresponding single-crystal X-ray density. Elastic compressional (P) and shear (S) wave velocities were determined at room pressure and temperature for these polycrystalline garnet specimens by phase comparison ultrasonic interferometry. For Mj100, the P and S wave velocities are within 1% of the Hashin-Shtrikman averages calculated from the single crystal elastic moduli measured by Brillouin spectroscopy. Both the elastic bulk modulus (K) and the shear modulus (G) decrease continuously with increasing majorite content from pyrope garnet (Py100) to pure majorite garnet (Mj100). The compositional dependence of K and G are given by K = 172.3 (40) − 0.085X, and G = 91.6 (10) − 0.038X, where X = mol% majorite), respectively, indicating that substitution of Si for Mg and Al decreases both K and G by about 5% along the solid solution series. Received: 25 March 1999 / Accepted: 12 July 1999  相似文献   

5.
The low-pressure stability of clinohumite has been investigated in phase-equilibrium experiments on the reaction forsterite + brucite = clinohumite. The reaction was bracketed between 2.45 and 2.84 GPa at 650 °C, extending to between 1.37 and 1.57 GPa at 850 °C. At temperatures above the reaction brucite = periclase + vapour, the reaction clinohumite = forsterite + vapour was bracketed between 1.27 and 1.52 GPa at 900 °C, rising to between 1.90 and 2.00 GPa at 1000 °C. The position of the reaction forsterite + brucite = clinohumite is ∼0.5 GPa below the position determined in previous work, the difference arising either from pressure uncertainties in both studies, from enhanced reaction to clinohumite in this study due to the presence of excess brucite in the starting material, or from different concentrations of defects in the two samples. The brackets on the reaction were combined with other measured and estimated thermodynamic data for clinohumite to determine its enthalpy of formation and entropy, in a revised version of the thermodynamic dataset of Holland and Powell (1998). The values obtained were ΔH f =−9607.29±3.05 kJ mol−1, S=445 J mol−1 K−1. These data were used to calculate positions of other reactions involving clinohumite. The calculations suggest a larger stability field for clinohumite than implied by the results of previous experimental studies, indicating a need for more high-pressure phase-equilibrium studies to provide better thermodynamic data. Received: 30 April 1999 / Accepted: 8 November 1999  相似文献   

6.
The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water has been experimentally investigated in the chemical system H2O−CaO− MgO−Al2O3−SiO2 over the range of 0.4–0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite + quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz, and water were obtained at 600, 650, and 700°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5±0.08 at the highest temperature. Thermodynamic analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S 0 (1 bar, 298 K) of 575.4±1.6 J/K · mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several experiments in the range of 1.0–1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770°C. Received: 24 January 1997 / Accepted: 2 October 1997  相似文献   

7.
The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and P H2O<30 bars. The water content of clinoptilolite can be predicted as a function of water activity and temperature with the equation: a H2O = [exp[[−ΔH h ∘/nRT] + [ΔS h ∘/nR] − 1/nRT· [W1 X h + W2 X h 2]− ln(X a/X h)]]−1 where T is degrees in Kelvin, ΔH h ∘ is the standard molal enthalpy of hydration, ΔS h ∘ is the entropy of hydration, X h and X a are, respectively, the mole fractions of the hydrous and anhydrous components of the solid solution, W 1 and W 2 are interaction parameters, n is the maximum number of moles of H2O per formula unit (based on 12 oxygens), and R is the gas constant. This equation can be used to locate clinoptilolite-H2O isohydrons in a H2O-T space below the liquid-vapor equilibrium curve of water. The standard molal Gibbs free energy of hydration is −47.62 ± 5.52 kJ/mol H2O and −5.40 ± 2.71 kJ/mol H2O for the Na- and K-clinoptilolite, respectively. These standard-state thermodynamic properties of clinoptilolite hydration are in good agreement with previous data at low H2O pressures. The experiments indicate that clinoptilolite progressively dehydrates with increasing temperature at pressures along the liquid-vapor equilibrium curve. Kinetic data above 150 °C show that clinoptilolite dehydration and hydration reactions are fast and reversible and that steady-state hydration states are attained in minutes. Received: 19 June 1998 / Revision, accepted 14 December 1998  相似文献   

8.
 Cation tracer diffusion coefficients, DMe *, for Me=Fe, Mn, Co and Ti, were measured using radioactive isotopes in the spinel solid solution (Ti x Fe 1−x )3−δO4 as a function of the oxygen activity. Experiments were performed at different cationic compositions (x=0, 0.1, 0.2 and 0.3) at 1100, 1200, 1300 and 1400 °C. The oxygen activity dependence of all data for DMe * at constant temperature and cationic composition can be described by equations of the type DMe *=D∘ Me[V]. CV·a O2 2/3+DMe[I] ∘·a O2 −2/3·DMe[V] ∘ and DMe[I] ∘ are constants and CV is a factor of the order of unity which decreases with increasing δ. All log DMe * vs. loga O2 curves obtained for different values of x and for different temperatures go through a minimum due to a change in the type of point defects dominating the cation diffusion with oxygen activity. Cation vacancies prevail for the cation diffusion at high oxygen activities while cation interstitials become dominant at low oxygen activities. At constant values of x, DMe[V] ∘ decreases with increasing temperature while DMe[I] ∘ increases.  相似文献   

9.
The heat capacity of gadolinium orthophosphate (GdPO4) measured in the temperature range 11.15–344.11 K by adiabatic calorimetry and available literature data were used to calculate its thermodynamic functions at 0–1600 K. At 298.15 K, these functions are as follows: C p 0(298.15 K) = 101.85 ± 0.05 J K−1 mol−1, S 0(298.15 K) = 123.82 ± 0.18 J K−1 mol−1, H 0(298.15 K)–H 0(0) = 17.250 ± 0.012 kJ mol−1, and Φ 0(298.15 K) = 65.97 ± 0.18 J K−1 mol−1 The calculated Gibbs free energy of formation from the elements of GdPO4 is Δ f G 0 (298.15 K) = −1844.3 ± 4.7 kJ mol−1.  相似文献   

10.
 The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a series of several synthesis experiments is ΔH f = (−26.32 ± 4.27) kJ mol−1 for the formation from the component oxides, or ΔH f  = (−2482.81 ± 4.59) kJ mol−1 for the formation from the elements. The result is identical within experimental error to available estimates, although the previously predicted energy difference between the monoclinic and triclinic phases could not be verified. Received: 16 February 2000 / Accepted: 14 July 2000  相似文献   

11.
 The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (T cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type ΔG = −hQ + gTQ +  (T − T c)Q 2 +  Q 4, with a/h = 0.00406 K−1, b/h = 2.3, T c = 572 K and g/h = 0.00106 K−1. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases. Received: 12 August 1999 / Accepted: 25 April 2000  相似文献   

12.
 The thermoelastic parameters of natural andradite and grossular have been investigated by high-pressure and -temperature synchrotron X-ray powder diffraction, at ESRF, on the ID30 beamline. The P–V–T data have been fitted by Birch-Murnaghan-like EOSs, using both the approximated and the general form. We have obtained for andradite K 0=158.0(±1.5) GPa, (dK/dT )0=−0.020(3) GPa K−1 and α0=31.6(2) 10−6 K−1, and for grossular K 0=168.2(±1.7) GPa, (dK/dT)0=−0.016(3) GPa K−1 and α0=27.8(2) 10−6 K−1. Comparisons between the present issues and thermoelastic properties of garnets earlier determined are carried out. Received: 7 July 2000 / Accepted: 20 October 2000  相似文献   

13.
 The lattice constants of paragonite-2M1, NaAl2(AlSi3)O10(OH)2, were determined to 800 °C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25–600 °C, were: αa = 1.51(8) × 10−5, αb = 1.94(6) × 10−5, αc = 2.15(7) ×  10−5 °C−1, and αV = 5.9(2) × 10−5 °C−1. At T higher than 600 °C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements of natural paragonite, carried out at 25, 210, 450 and 600 °C, before dehydroxylation, showed that the larger thermal expansion along the c parameter was mainly due to interlayer thickness dilatation. In the 25–600 °C range, Si,Al tetrahedra remained quite unchanged, whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the interlayer cation Na became more regular with temperature. Tetrahedral rotation angle α changed from 16.2 to 12.9°. The structure of the new phase, nominally NaAl2 (AlSi3)O11, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room temperature and its expansion coefficients determined in the range 25–800 °C. The most significant structural difference from paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V 0 = 1 + 5.9(2) × 10−5 T(°C) − 0.00153(4) P(kbar). Received: 12 July 1999 / Revised, accepted: 7 December 1999  相似文献   

14.
The unit cell parameters, extracted from Rietveld analysis of neutron powder diffraction data collected between 4.2 K and 320 K, have been used to calculate the temperature evolution of the thermal expansion tensor for gypsum for 50 ≤ T ≤ 320 K. At 300 K the magnitudes of the principal axes are α 11  = 1.2(6) × 10−6 K−1, α 22  = 36.82(1) × 10−6 K−1 and α 33  = 25.1(5) × 10−6 K−1. The maximum axis, α 22 , is parallel to b, and using Institution of Radio Engineers (IRE) convention for the tensor orthonormal basis, the axes α 11 and α 33 have directions equal to (−0.979, 0, 0.201) and (0.201, 0, 0.979) respectively. The orientation and temperature dependent behaviour of the thermal expansion tensor is related to the crystal structure in the I2/a setting. Received 12 February 1998 / Revised, accepted 19 October 1998  相似文献   

15.
 Interactions between several silicate and metallic phases are studied by applying a self consistent thermodynamic approach and using recent thermodynamic data. We compute proportions and compositions of oxidized silicates and of reduced metallic phase in equilibrium at various temperatures and oxygen fugacities. The empirically observed activity-composition relationships for ternary metallic alloys are used and their applications to a general thermodynamic expression for a non-regular ternary system is explicitly discussed. We show that the stability limits of olivines and pyroxenes with respect to precipitation of metallic phases under reducing conditions are directly related to the presence of nickel impurities. We precisely evaluate the modifications of the stability limits as a function of nickel content. For typical mantle olivines [Fe/(Fe+Mg) = 0.1] the stability limits are given for values of x Ni= Ni/(Ni+Fe+Mg) ranging from 10 ppm to 1% by: ln f O2=−39.83+ 7.86 ln x Ni, ln f O2=−14.68+6.21 ln x Ni, at 900 K and 1600 K, respectively. Received: 17 November 1999 / Accepted: 14 May 2000  相似文献   

16.
The heat capacity of xenotime YPO4(c) was measured by adiabatic calorimetry at 4.78–348.07 K. Our experimental and literature data on H 0(T)-H 0(298.15 K) of Y orthophosphate were utilized to derive the C p 0(T) function of xenotime at 0–1600 K, which was then used to calculate the values of thermodynamic functions: entropy, enthalpy change, and reduced Gibbs energy. These functions assume the following values at 298.15 K: C p 0 (298.15 K) = 99.27 ± 0.02 J K−1 mol−1, S 0(298.15 K) = 93.86 ± 0.08 J K−1 mol−1, H 0(298.15 K) − H 0(0) = 15.944 ± 0.005 kJ mol−1, Φ0(298.15 K) = 40.38 ± 0.08 J K−1 mol−1. The value of the free energy of formation Δ f G 0(YPO4, 298.15 K) is −1867.9 ± 1.7 kJ mol−1.  相似文献   

17.
 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H2O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)2Al4Si5O18+H2O ⇄ (Fe,Mg)2Al4Si5O18.H2O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ° r =–37141±3520 J and ΔS °  r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n H2O=1/[1+1/(K ⋅ f H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H2O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996  相似文献   

18.
Activities of CoO in (Co,Mn)O solid solutions in contact with metallic Co have been determined on ten compositions ranging from 0.12 to 0.84 XCoO in order to calibrate the divariant equilibrium between (Co,Mn)O oxide solutions and Co metal as an oxygen fugacity sensor for application in experimental petrology. Experiments were conducted over the temperature range 900–1300 K at 1 bar, using an electrochemical technique with oxygen-specific calcia-stabilized zirconia (CSZ) electrolytes. Co + CoO or Fe + FeO was used as the reference electrode. Compositions of the (Co,Mn)O solid solutions were measured after each run by electron microprobe, and these were checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Activity–composition relations were fitted to the Redlich–Kister formalism. (Co,Mn)O solid solutions exhibit slight positive deviations from ideality, which are symmetrical (corresponding to a regular solution mixing model) across the entire composition range with A0 G = 3690(±47) Jmol−1. Excess entropies and enthalpies were also derived from the emf data and gave Sex=0.77(±0.08) JK−1 mol−1 and Hex=4558(±90) Jmol−1 respectively. The experimental data from this study have been used to formulate the (Co,Mn)O/Co oxygen fugacity sensor to give an expression: where μO2 CoCoO=−492,186 + 509.322 T − 53.284 T lnT + 0.02518 T2, taken from O'Neill and Pownceby (1993). Received: 10 September 1999 / Accepted: 4 April 2000  相似文献   

19.
 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na2O– CaO–MgO–Al2O3–SiO2–H2O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An50)+1.5 enstatite+H2O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An50)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An50). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled NaA-AlT1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG 0 f ,Pg=−12022.11±5.2 kJ mole-1, and S 0 Pg=591.7 ±7.9 JK-1 mole-1. Received: 31 July 1995/Accepted: 3 June 1996  相似文献   

20.
 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO2). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm−1, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/106 Si (at 5 GPa) to 1415 H/106 Si (at 10 GPa). Assuming a model of incorporation by (4H)Si defects, the data are fit well by the equation C OH=Af 2 H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/106 Si/GPa, and ΔV=20.6 × 10−6 m3 mol−1. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H2O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000  相似文献   

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