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1.
K.-D. Grevel A. Navrotsky W. A. Kahl D. W. Fasshauer J. Majzlan 《Physics and Chemistry of Minerals》2001,28(7):475-487
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.
Enthalpies of drop solution (ΔH
drop-sol) of CaGeO3, Ca(Si0.1Ge0.9)O3, Ca(Si0.2Ge0.8)O3, Ca(Si0.3Ge0.7)O3 perovskite solid solutions and CaSiO3 wollastonite were measured by high-temperature calorimetry using molten 2PbO · B2O3 solvent at 974 K. The obtained values were extrapolated linearly to the CaSiO3 end member to give ΔH
drop-sol of CaSiO3 perovskite of 0.2 ± 4.4 kJ mol−1. The difference in ΔH
drop-sol between CaSiO3, wollastonite, and perovskite gives a transformation enthalpy (wo → pv) of 104.4 ± 4.4 kJ mol−1. The formation enthalpy of CaSiO3 perovskite was determined as 14.8 ± 4.4 kJ mol−1 from lime + quartz or −22.2 ± 4.5 kJ mol−1 from lime + stishovite. A comparison of lattice energies among A2+B4+O3 perovskites suggests that amorphization during decompression may be due to the destabilizing effect on CaSiO3 perovskite from a large nonelectrostatic energy (repulsion energy) at atmospheric pressure. By using the formation enthalpy
for CaSiO3 perovskite, phase boundaries between β-Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where ΔG(P,T )=0] as the measured phase boundary. The calculations suggest that the phase equilibrium boundary occurs between 11.5 and
12.5 GPa around 1500 K. Its slope is still not well constrained.
Received: 20 September 2000 / Accepted: 17 January 2001 相似文献
3.
Alison Pawley 《Contributions to Mineralogy and Petrology》2000,138(3):284-291
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
4.
V. M. Gurevich M. A. Ryumin A. V. Tyurin L. N. Komissarova 《Geochemistry International》2012,50(8):702-710
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
5.
K. S. Gavrichev M. A. Ryumin A. V. Tyurin V. M. Gurevich L. N. Komissarova 《Geochemistry International》2010,48(4):390-397
The heat capacity of synthetic pretulite ScPO4(c) was measured by adiabatic calorimetry within a temperature range of 12.13–345.31 K, and the temperature dependence of
the pretulite heat capacity at 0–1600 K was derived from experimental and literature data on H
0(T)-H
0(298.15 K) for Sc orthophosphate. This dependence was used to calculate the values of the following thermodynamic functions:
entropy, enthalpy change, and reduced Gibbs energy. They have the following values at 298.15 K: C
p
0 (298.15 K) = 97.45 ± 0.06 J K−1 mol−1, S
0(298.15 K) = 84.82 ± 0.18 J K−1 mol−1, H
0(298.15 K)-H
0(0) = 14.934 ± 0.016 kJ mol−1, and Φ
0(298.15 K) = 34.73 ± 0.19 J K−1mol−1. The enthalpy of formation Δ
f
H
0(ScPO4, 298.15 K) = − 1893.6 ± 8.4 kJ mol−1. 相似文献
6.
K. S. Gavrichev M. A. Ryumin A. V. Tyurin V. M. Gurevich L. N. Komissarova 《Geochemistry International》2010,48(9):932-939
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
7.
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
8.
Ali Reza Fazeli J. A. K. Tareen B. Basavalingu G. T. Bhandage 《Journal of Earth System Science》1991,100(1):37-39
Hydrothermal equilibrium decomposition curve for MnCO3⇌MnO + CO2 in the total CO2 pressure range of 100–1700 bars and temperature range of 500–800°C was studied. The standard thermodynamic data obtained
are: ΔH0
f= − 894.382 ± 0.74 kj/mol and ΔG0
f
= − 822.170 ± 0.74 kj/mol. These values are more negative than the reported calorimetric data. 相似文献
9.
J. L. Mosenfelder 《Physics and Chemistry of Minerals》2000,27(9):610-617
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
10.
Thermodynamic properties of high-pressure minerals that are not recoverable from synthesis experiments by conventional quenching
methods (“unquenchable” phases) usually are calculated from equation of state data and phase diagram topologies. The present
study shows that, with cryogenic methods of recovery and sample treatment, phases with a suitable decomposition rate can be
made accessible to direct thermodynamic measurements. A set of samples of Ca(OH)2-II has been synthesized in a multianvil device and subsequently recovered by cooling the high-pressure assembly with liquid
nitrogen. Upon heating from liquid nitrogen to room temperature, the material transformed back to Ca(OH)2-I. The heat effect of this backtransformation was measured by differential scanning calorimetry. A commercial differential
scanning calorimeter (Netzsch DSC 404), modified to allow sample loading at liquid nitrogen temperature was used to heat the
material from −150 to +200 °C at rates varying between 5 and 15 °C min−1. The transformation started around −50 °C very gradually, and peaked at about 0 °C. To obtain a baseline correction, each
sample was scanned under exactly the same conditions after the backtransformation was complete. Because of the relative sluggishness,
onset and offset temperatures were not well defined as compared to fast (e.g., melting) reactions. To aid in integration,
the resulting signals were successfully fitted using a generic asymmetric peak model. The enthalpy of backtransformation was
determined to be ΔH =−10.37 ± 0.50 kJ mol−1. From previous in situ X-ray diffraction experiments, the location of the direct transformation in P-T space has been constrained to 5.7 ± 0.4 GPa at 500 °C (Kunz et al. 1996). With the reaction volume known from the same study,
and assuming that ΔC
p
of the transformation remains negligible between the conditions of our measurements and 500 °C, our result gives an estimate
of the entropy of transition and the P-T slope of the reaction curve. To a first approximation, the values ΔS = −16.00 ± 0.65 J(mol · K)−1 and dP/dT = 0.0040 ± 0.0002 GPa/K have been determined. These results need to be refined by equation of state data for Ca(OH)2-II.
Received: 30 December 1999 / Accepted: 10 April 2000 相似文献
11.
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
12.
A. Pavese V. Diella V. Pischedda M. Merli R. Bocchio M. Mezouar 《Physics and Chemistry of Minerals》2001,28(4):242-248
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.
Phase A, Mg7Si2O8(OH)6, is a dense hydrous magnesium silicate whose importance as a host of H2O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments
on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using
mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between
3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (105 Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version
of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H2O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH
o
f
=−7126±8 kJ mol-1, S
o=351 J K-1 mol-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for
example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite.
The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental
brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H2O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO2–H2O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase
A could crystallise at pressures above approximately 6 GPa.
Received: 3 July 1995/Accepted: 14 December 1995 相似文献
14.
V. M. Gurevich O. L. Kuskov N. N. Smirnova K. S. Gavrichev A. V. Markin 《Geochemistry International》2009,47(12):1170-1179
The heat capacity of eskolaite Cr2O3(c) was determined by adiabatic vacuum calorimetry at 11.99–355.83 K and by differential calorimetry at 320–480 K. Experimental
data of the authors and data compiled from the literature were applied to calculate the heat capacity, entropy, and the enthalpy
change of Cr2O3 within the temperature range of 0–1800 K. These functions have the following values at 298.15 K: C
p
0 (298.15) = 121.5 ± 0.2 J K−1mol−1, S
0(298.15) = 80.95 ± 0.14 J K−1mol−1, and H
0(298.15)-H
0(0) = 15.30±0.02 kJ mol−1. Data were obtained on the transitions from the antiferromagnetic to paramagnetic states at 228–457 K; it was determined
that this transition has the following parameters: Neel temperature T
N
= 307 K, Δ
tr
S = 6.11 ± 0.12 J K−1mol−1 and δ
tr
H = 1.87 ± 0.04 kJ mol−1. 相似文献
15.
Jibamitra Ganguly Weiji Cheng Sumit Chakraborty 《Contributions to Mineralogy and Petrology》1998,131(2-3):171-180
Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules
under anhydrous conditions at 22–40 kbar, 1057–1400 °C in a piston-cylinder apparatus. The concentration profiles that developed
in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca.
Because of their usually low concentrations and lack of sufficient compositional change across the interface of the diffusion
couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe)
and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D=D
0exp{−[Q(1 bar)+PΔV
+]/RT}. The values of the activation energy (Q) and activation volume (ΔV
+) depend on whether f
O2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe:Q(1 bar)=65,532±10,111 cal/mol, D
0=3.50 (±2.30)×10−5 cm2/s, ΔV
+=5.6(±2.9) cm3/mol, and for Mg:Q(1 bar)=60,760±8,257 cal/mol, D
0=4.66 (±2.48)×10−5 cm2/s, ΔV
+=5.3(±3.0) cm3/mol. Here the ΔV
+ values have been taken from Chakraborty and Ganguly (1992). For the condition of constant f
O2, the Q values are ∼9 kcal lower and ΔV
+ values are ∼4.9 cm3/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement
with the Mg tracer diffusion data (D
*
Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750–900 °C, when all data are normalized to the same
pressure and to f
O2 defined by graphite in the system C-O. The D
*
Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature
extrapolation of the present data, when all data are normalized to the same pressure and to f
O2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with
those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe
and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively
small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional
effects on D(I) along with the effects of thermodynamic nonideality, f
O2, and pressure.
Received: 8 May 1997 / Accepted: 2 October 1997 相似文献
16.
Summary The kinetics of phytoplankton frustule dissolution has generally been studied as the appearance of silicic acid in a batch
reactor. Unfortunately, this approach, though often illuminating, has not so far been successful because of the difficulty
of parameterising the full reaction curve. This current study shows how the initial rate approach to chemical kinetics offers
a way around this bottleneck, thereby allowing much chemical kinetics information about frustule dissolution to be collected.
The technique is shown to be flexible and suited to short reaction times which facilitate detailed quantitative kinetics investigation,
indeed, as would be expected in a solution phase, kinetics study. The technique is exemplified by a dissolution study of uncleaned
frustules of Cyclotella crypticaat 40 °C and above. The frustules were found to yield the same dissolution rate after 5 weeks dark storage, at 4 °C. Meanwhile,
log dissolution rate was found to vary linearly with pH, with gradient 0.38 ± 0.01 (r
2=0.990). Linearity was upheld even at pHs as high as 14. Finally, a robust Arrhenius plot was established between 40 and 90 °C
yielding an activation energy for dissolution of 84 ± 3 kJ mol
−1. Follow through with the Eyring equation yielded an activation enthalpy, ΔH
‡, and an activation entropy, ΔS
‡, of 81 and 85 J mol
−1 K
−1, respectively. The discussion brings salient aspects of existing knowledge about diatom frustule dissolution kinetics into
the wider context of silicate mineral dissolution. 相似文献
17.
The heat capacity of end-member titanite and (CaTiSiO5) glass has been measured in the range 328–938 K using differential scanning calorimetry. The data show a weak λ-shaped anomaly
at 483 ± 5 K, presumably associated with the well-known low-pressure P21/a ⇆ A2/a transition, in good agreement with previous studies. A value of 0.196 ± 0.007 kJ mol−1 for the enthalpy of the P21/a ⇆ A2/a transition was determined by integration of the area under the curve for a temperature interval of 438–528 K, bracketing
the anomaly. The heat capacity data for end-member titanite and (CaTiSiO5) glass can be reproduced within <1% using the derived empirical equations (temperature in K, pressure in bars):
The available enthalpy of vitrification (80.78 ± 3.59 kJ mol−1), and the new heat capacity equations for solid and glass can be used to estimate (1) the enthalpy of fusion of end-member
titanite (122.24 ± 0.2 kJ mol−1), (2) the entropy of fusion of end-member titanite (73.85 ± 0.1 J/mol K−1), and (3) a theoretical glass transition temperature of 1130 ± 55 K. The latter is in considerable disagreement with the
experimentally determined glass transition temperature of 1013 ± 3 K. This discrepancy vanishes when either the adopted enthalpy
of vitrification or the liquid heat content, or both, are adjusted.
Calculations using Eq. (2), new P−V−T data for titanite, different but also internally consistent thermodynamic data for anorthite,
rutile, and kyanite, and experimental data for the reaction: anorthite + rutile = titanite + kyanite strongly suggest: (1)
the practice to adjust the enthalpy of formation of titanite to fit phase equilibrium data may be erroneous, and (2) it is
probably the currently accepted entropy of 129.2 ± 0.8 J/mol K−1 that may need revision to a smaller value.
Received: 30 December 1999 / Accepted: 23 June 2000 相似文献
18.
Iron tracer diffusion experiments in diopside have been performed using natural and synthetic single crystals of diopside,
and stable iron tracers enriched in 54Fe, at temperatures in the range 950–1100 °C, total pressure 1 atm, for times up to 29 days. Iron isotope diffusion profiles
were determined with an ion microprobe. For experiments performed at log pO2 = −13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe ∼ 1.8 at %) diopsides, the data obey
a single Arrhenius relationship of the form D = 6.22−5.9
+49.6×10−15 exp(−161.5 ± 35.0 kJ mol−1/RT) m2 s−1. A single datum for iron diffusion in iron-free, single-crystal diopside at 1050 °C, is approximately 1 order of magnitude
slower than in the natural crystals. The pO2 dependence of iron diffusion in natural crystals at 1050 °C (power exponent = 0.229 ± 0.036) indicates a vacancy mechanism;
this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotropy for iron
diffusion in diopside.
Received: 16 March 1999 / Accepted: 10 April 2000 相似文献
19.
The high-pressure stability of zoisite and phase relationships of zoisite-bearing assemblages 总被引:5,自引:0,他引:5
The fluid-absent reaction 12 zoisite = 3 lawsonite + 7 grossular + 8 kyanite + 1 coesite was experimentally reversed in the
model system CaO-Al2O3-SiO2-H2O (CASH) using a multi-anvil apparatus. The upper pressure stability limit for zoisite was found to extend to 5.0 GPa at 700 °C
and to 6.6 GPa at 950 °C. Additional experiments both in the H2O-SiO2-saturated and in the H2O-Al2O3-saturated portions of CASH provide further constraints on high pressure phase relationships of lawsonite, zoisite, grossular,
kyanite, coesite, and an aqueous fluid. Consistency of the present experiments with the H2O-saturated breakdown of lawsonite is demonstrated by thermodynamic analysis using linear programming techniques. Two sets
of data consistent with databases of Berman (1988) and Holland and Powell (1990) were retrieved combining experimental phase
relationships, calorimetric constraints, and recently measured elastic properties of solid phases. The best fits result in
G
f
,1,298
∘,zoisite=−6,499,400 J and S
1,298
∘,zoisite=302 J/K, and G
f
,1,298
∘,lawsonite=−4,514,600 J and S
1,298
∘,lawsonite=220 J/K for the dataset of Holland and Powell, and G
f
,1,298
∘,zoisite=−6,492,120 J and S
1,298
∘,zoisite=304 J/K, and G
f
,1,298
∘,lawsonite=−4,513,000 J and S
1,298
∘,lawsonite= 218 J/K for the dataset of Berman. Examples of the usage of zoisite as a geohygrometer and as a geobarometer in rocks metamorphosed
at eclogite facies conditions are worked, profiting from the thermodynamic properties retrieved here.
Received: 23 December 1996 / Accepted: 29 August 1997 相似文献
20.
Benita Putlitz Alan Matthews John W. Valley 《Contributions to Mineralogy and Petrology》2000,138(2):114-126
Oxygen and hydrogen stable isotope ratios of eclogite-facies metagabbros and metabasalts from the Cycladic archipelago (Greece)
document the scale and timing of fluid–rock interaction in subducted oceanic crust. Close similarities are found between the
isotopic compositions of the high-pressure rocks and their ocean-floor equivalents. High-pressure minerals in metagabbros
have low δ18O values: garnet 2.6 to 5.9‰, glaucophane 4.3 to 7.1‰; omphacite 3.5 to 6.2‰. Precursor actinolite that was formed during
the hydrothermal alteration of the oceanic crust by seawater analyses at 3.7 to 6.3‰. These compositions are in the range
of the δ18O values of unaltered igneous oceanic crust and high-temperature hydrothermally altered oceanic crust. In contrast, high-pressure
metabasalts are characterised by 18O-enriched isotopic compositions (garnet 9.2 to 11.5‰, glaucophane 10.6 to 12.5‰, omphacite 10.2 to 12.8‰), which are consistent
with the precursor basalts having undergone low-temperature alteration by seawater. D/H ratios of glaucophane and actinolite
are also consistent with alteration by seawater. Remarkably constant oxygen isotope fractionations, compatible with isotopic
equilibrium, are observed among high-pressure minerals, with Δglaucophane−garnet = 1.37 ± 0.24‰ and Δomphacite−garnet = 0.72 ± 0.24‰. For the estimated metamorphic temperature of 500 °C, these fractionations yield coefficients in the equation
Δ = A * 106/T
2 (in Kelvin) of Aglaucophane−garnet = 0.87 ± 0.15 and Aomphacite−garnet = 0.72 ± 0.24. A fractionation of Δglaucophane–actinolite = 0.94 ± 0.21‰ is measured in metagabbros, and indicates that isotopic equilibrium was established during the metamorphic
reaction in which glaucophane formed at the expense of actinolite. The preservation of the isotopic compositions of gabbroic
and basaltic oceanic crust and the equilibrium fractionations among minerals shows that high-pressure metamorphism occurred
at low water/rock ratios. The isotopic equilibrium is only observed at hand-specimen scale, at an outcrop scale isotopic compositional
differences occur among adjacent rocks. This heterogeneity reflects metre-scale compositional variations that developed during
hydrothermal alteration by seawater and were subsequently inherited by the high-pressure metamorphic rocks.
Received: 4 January 1999 / Accepted: 7 July 1999 相似文献