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1.
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this is consistent with the results of unpublished atomistic simulation studies.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
2.
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
3.
Diffusion of dissolved SiO2 in H2O at 1 GPa, with implications for mass transport in the crust and upper mantle 总被引:4,自引:0,他引:4
Cation diffusion rates at 690 ± 30 °C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles
in 40 garnets from two kyanite-bearing metapelite samples from the High Himalayan Crystalline Series, Zanskar, northwest India.
Knowledge of the initial growth profile of Mn in garnet is a pre-requisite for this technique. Following previous workers
we model Mn partitioning into growing garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mngarnet:whole rock partition coefficient is 60–100. Three-dimensional zonation profiles were obtained by successively grinding and polishing
∼1 cm slabs of each sample at 0.1–0.2 mm intervals and analysing the garnets at each stage, thus ensuring that core sections
were measured. The diffusion model assumes that garnet has a spherical geometry and behaves as a closed system, and simulates
diffusive modification of the hypothetical Mn Rayleigh growth profile for each garnet. The derived measure of the time-integrated
diffusion history for each garnet is then combined with radiometric and field-relation constraints for the duration of the
Himalayan metamorphic event to calculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets
in the two samples examined is (6 ± 3.2) × 10−23 m2s−1. This interdiffusion rate pertains to a temperature of 690 ± 30 °C, which is 0.97 × T
PEAK, the peak temperature conditions experienced by the samples estimated using standard thermobarometric techniques. Garnet
compositions are Py2–17Alm65–77Gro6–16Sp1–17. These new diffusion data are consistent with, and more precise than, existing high-temperature (>1000 °C) experimentally
determined diffusion data, although some uncertainties remain difficult to constrain. Qualitative comparison between diffusively
modified Mn growth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of
metamorphism affecting the Dalradian garnets was 10–20 times longer than that endured by the Himalayan garnets.
Received: 5 June 1996 / Accepted: 29 January 1997 相似文献
4.
S. A. T. Redfern G. Artioli R. Rinaldi C. M. B. Henderson K. S. Knight B. J. Wood 《Physics and Chemistry of Minerals》2000,27(9):630-637
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
5.
Craig S. Schwandt Randall T. Cygan Henry R. Westrich 《Contributions to Mineralogy and Petrology》1998,130(3-4):390-396
Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic
directions in natural orthoenstatite (En88Fs12). Experiments were conducted at 1 atm in CO-CO2 gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of 25Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched 25MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling.
The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E
a
, and frequency factors, D
o, where D = D
o exp (−E
a
/RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E
a
= 360 ± 52 kJ/mole and D
o = 1.10 × 10−4 m2/s; b-axis, E
a
= 339 ± 77 kJ/mole and D
o = 6.93 × 10−6 m2/s and c-axis, E
a
= 265 ± 66 kJ/mole and D
o = 4.34 × 10−9 m2/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for
diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite
provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to
previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/106 years) for the metal portions.
Received: 22 January 1997 / Accepted: 2 October 1997 相似文献
6.
Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have been determined in the temperature interval of 27–1050 °C
by X-ray powder diffraction technique. Expansion rates vary discontinuously with temperature with a break at 412 °C. Below
this temperature, the linear expansions (α) for a, b and c axis lengths are 3.74 × 10−5 K−1, 1.09 × 10−5 K−1, and 1.19 × 10−5 K−1, respectively, and above that they are 0.86 × 10−5 K−1, 0.80 × 10−5 K−1, and 1.93 × 10−5 K−1. The volume thermal expansion coefficients are 6.26 × 10−5 K−1 and 3.71 × 10−5 K−1 for low-temperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with
temperature could be due to the different mode of structural changes. Thermogravimetric analysis of the sample indicates the
oxidation of iron in the temperature range of 500–600 °C and dehydroxylation as well as decomposition of phlogopite in the
temperature range of 900–1200 °C.
Received: 8 September 1998 / Accepted: 28 February 2000 相似文献
7.
Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates 总被引:3,自引:0,他引:3
Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa
in a stream of CO2. Diffusion coefficients (D) were determined from 42Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol
and pre-exponential factor (D0)=0.13 m2/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural
(Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone
was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium
microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and
the effective grain boundary diffusion width (δ) were determined from 42Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between
D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed
Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa
is described by the Arrhenius parameters D′
0δ=1.5×10−9 m3/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest
volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological
processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks.
Received: 19 December 1994/Accepted: 30 June 1995 相似文献
8.
Angelika Kalt Fernando Corfu Jan R. Wijbrans 《Contributions to Mineralogy and Petrology》2000,138(2):143-163
A temperature–time path was constructed for high-temperature low-pressure (HT–LP) migmatites of the Bayerische Wald, internal
zone of the Variscan belt, Germany. The migmatites are characterised by prograde biotite dehydration melting, peak metamorphic
conditions of approximately 850 °C and 0.5–0.7 GPa and retrograde melt crystallisation at 800 °C. The time-calibration of
the pressure–temperature path is based on U–Pb dating of single zircon and monazite grains and titanite separates, on 40Ar/39Ar ages obtained by incremental heating experiments on hornblende separates, single grains of biotite and K-feldspar, and
on 40Ar/39Ar spot fusion ages of biotite determined in situ from sample sections. Additionally, crude estimates of the duration of peak
metamorphism were derived from garnet zoning patterns, suggesting that peak temperatures of 850 °C cannot have prevailed much
longer than 2.5 Ma. The temperature–time paths obtained for two areas approximately 30 km apart do not differ from each other
considerably. U–Pb zircon ages reflect crystallisation from melt at 850–800 °C at 323 Ma (southeastern area) and 326 Ma (northwestern
area). The U–Pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance.
The preservation of inherited monazite and the presence of excess 206Pb resulting from the incorporation of excess 230Th in monazite formed during HT–LP metamorphism suggest that monazite ages in the migmatites of the Bayerische Wald reflect
crystallisation from melt at 850–800 °C and persistence of older grains at these temperatures during a comparatively short
thermal peak. The U–Pb ages of titanite (321 Ma) and 40Ar/39Ar ages of hornblende (322–316 Ma) and biotite (313–309 Ma) reflect cooling through the respective closure temperatures of
approximately 700, 570–500 and 345–310 °C published in the literature. Most of the feldspars' ages (305–296 Ma) probably record
cooling below 150–300 °C, while two grains most likely have higher closure temperatures. The temperature–time paths are characterised
by a short thermal peak, by moderate average cooling rates and by a decrease in cooling rates from 100 °C/my at temperatures
between 850–800 and 700 °C to 11–16 °C/my at temperatures down to 345–310 °C. Further cooling to feldspar closure for Ar was
probably even slower. The lack of decompressional features, the moderate average cooling rates and the decline of cooling
rates with time are not easily reconciled with a model of asthenospheric heating, rapid uplift and extension due to lithospheric
delamination as proposed elsewhere. Instead, the high peak temperatures at comparatively shallow crustal levels along with
the short thermal peak require external advective heating by hot mafic or ultramafic material.
Received: 7 July 1999 / Accepted: 28 October 1999 相似文献
9.
Diffusion of tetravalent cations in zircon 总被引:18,自引:1,他引:18
D. J. Cherniak J. M. Hanchar E. B. Watson 《Contributions to Mineralogy and Petrology》1997,127(4):383-390
Diffusion rates for the three tetravalent cations U, Th and Hf have been measured in synthetic zircon. Diffusant sources
included oxide powders and ground pre-synthesized silicates. Rutherford backscattering spectrometry (RBS) was used to measure
depth profiles. Over the temperature range 1400–1650 °C, the following Arrhenius relations were obtained (diffusion coefficients
in m2sec−1):
log D
Th = (1.936 ± 0.9820) + (− 792 ± 34 kJ mol−1 /2.303 RT)
log D
U = (0.212 ± 2.440) + (− 726 ± 83 kJ mol−1 /2.303 RT)
log D
Hf = (3.206 ± 1.592) + (− 812 ± 54 kJ mol−1 /2.303 RT)
The data show a systematic increase in diffusivity with decreasing ionic radius (i.e., faster diffusion rates for Hf than
for U or Th), a trend also observed in our earlier study of rare earth diffusion in zircon. Diffusive fractionation may be
a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations when compared with the trivalent
rare earths. The very slow diffusion rates measured for these tetravalent cations suggest that they are essentially immobile
under most geologic conditions, permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited
cores.
Received: 12 July 1996 / Accepted: 2 December 1996 相似文献
10.
The thermal behaviour of ripidolite, an iron-rich chlorite, has been studied in situ by infrared emission spectroscopy up
to 800 °C. The more di,trioctahedral nature due to significant amounts of Fe3+ is reflected, in addition to the two bands around 3420 and 3560 cm−1, by an extra band around 3345 cm−1. This extra band is absent in pure dioctahedral chlorites without Fe3+. These bands have been assigned to (AlAl)O-OH, (SiAl)O-OH and (SiSi)O-OH stretching modes with increasing frequencies. The
bands disappear upon dehydroxylation around 650 °C. A similar behaviour is observed for the corresponding libration modes
around 716, 759 and 802 cm−1. The stretching and bending modes of the inner-OH of the octahedral sheet in the 2:1 clay-like layer are observed around
3645, 943 and 904 cm−1. Although the bands decrease in intensity, they remain present up to 800 °C as dehydroxylation of the octahedral sheet is
not yet complete at this temperature. The presence of two bending modes is explained as being due to a differentiation between
Mg-OH and Fe-OH modes. At 650 °C a new sharp band is observed around 502 cm−1 assigned to a (Fe,Mg)-O-Al bending mode caused by the formation of a spinel-like interlayer phase after dehydroxylation.
Received: 4 June 1999 / Accepted: 6 August 1999 相似文献
11.
H. A. Gilg A. Lima R. Somma H. E. Belkin B. De Vivo R. A. Ayuso 《Mineralogy and Petrology》2001,73(1-3):145-176
Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich
ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically
zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor
phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite
with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals.
Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a)
primary silicate melt inclusions (THOM = 1000–1050 °C), b) CO2 ± H2S-rich fluid inclusions (THOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (THOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing
saline-melt inclusions (THOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich
liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below
700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable 206Pb/204Pb (19.047–19.202), 207Pb/204Pb (15.655–15.670), and 208Pb/204Pb (38.915–39.069) and relatively low 143Nd/144Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ13CV-PDB = −5.4 to −1.1‰; δ18OV-SMOW = 11.7 to 16.4‰) indicate formation from a 18O- and 13C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules
suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution
of CO2-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns.
Received March 1, 2000; revised version accepted November 2, 2000 相似文献
12.
Hydrogen diffusion in natural and synthetic orthopyroxene 总被引:1,自引:0,他引:1
Hydrogen diffusion coefficients in natural orthopyroxenes and synthetic enstatite were determined by dehydration and hydration
experiments at 700 and 900 °C. In natural Opx (approximately En90Fs10) small but significant differences in diffusivities along the three crystallographic axes were observed, [001] being the
fastest direction, followed by [100] and [010]. Hydrogen diffusion in pure enstatite proved to be about 2 orders of magnitude
slower and isotropic. The activation energy for hydrogen diffusion in pure enstatite was determined to be −295 (±55) kJmol−1, and −213 (±47) kJmol−1 for orthopyroxene from Kilbourne Hole. Long-term hydration experiments did not lead to saturation in hydrogen. Instead, after
an initial increase in hydrogen concentration, a slow but continuing decrease could be observed in all cases. It is suggested
that the investigated samples lose their ability to store hydrogen even when heated in a hydrogen atmosphere. This loss in
storage ability can itself be described by a diffusion equation, its diffusion coefficients being more than 1 order of magnitude
slower than the diffusion of hydrogen.
Received: 22 February 2002 / Accepted: 18 October 2002 相似文献
13.
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
14.
Zr diffusion in titanite 总被引:2,自引:0,他引:2
D. J. Cherniak 《Contributions to Mineralogy and Petrology》2006,152(5):639-647
Chemical diffusion of Zr under anhydrous, pO2-buffered conditions has been measured in natural titanite. The source of diffusant was either zircon powder or a ZrO2–Al2O3–titanite mixture. Experiments were run in sealed silica glass capsules with solid buffers (to buffer at NNO or QFM). Rutherford Backscattering Spectrometry (RBS) was used to measure diffusion profiles. The following Arrhenius parameters were obtained for Zr diffusion parallel to c over the temperature range 753–1,100°C under NNO-buffered conditions: D
Zr = 5.33 × 10−7 exp(−325 ± 30 kJ mol−1/RT) m2 s−1 Diffusivities are similar for experiments buffered at QFM. These data suggest that titanite should be moderately retentive of Zr chemical signatures, with diffusivities slower than those for O and Pb in titanite, but faster than those for Sr and the REE. When applied in evaluation of the relative robustness of the recently developed Zr-in-titanite geothermometer (Hayden and Watson, Abstract, 16th V.M. Goldschmidt Conference 2006), these findings suggest that Zr concentrations in titanite will be less likely to be affected by later thermal disturbance than the geothermometer based on Zr concentrations in rutile (Zack et al. in Contrib Mineral Petrol 148:471–488, 2004; Watson et al. in Contrib Mineral. Petrol, 2006), but much less resistant to diffusional alteration subsequent to crystallization than the Ti-in-Zircon geothermometer (Watson and Harrison in Science 308:841–844, 2005). 相似文献
15.
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
16.
Isothermal thermogravimetric experiments have been carried out to determine the reaction kinetics of the dehydration processes
in fuller's earth, a natural Ca-montmorillonite. Dehydration in swelling clays is a complex reaction, and analysis of the
thermogravimetric data using empirical rate equations and time-transformation analysis reveals that the nature of the rate
controlling mechanism is dependent upon both the temperature regime of the sample as well as the extent of reaction. For fuller's
earth, we find that the dehydration kinetics are dominated by a nucleation and growth mechanism at low temperatures and fractions
transformed (stage I), but above 90 °C the last stages of the reaction are diffusion controlled (stage II). The activation
energy for dehydration during stage I is around 35 kJ · mol−1, whereas the removal of water during stage II requires an activation energy of around 50 kJ · mol−1. These two stages of dehydration are associated with primary collapse of the interlayer (stage I) and movement of water that
is hydrated to cations within the interlayer (stage II).
Received: 28 August 1998 / Revised, accepted: 27 January 1999 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.
Low-temperature single-crystal Raman spectrum of pyrope 总被引:1,自引:1,他引:0
The single-crystal polarized Raman spectra of synthetic pyrope, Mg3Al2Si3O12, were measured at room temperature and 5 K, as were the room-temperature unpolarized spectra of two natural pyrope-rich crystals.
No major differences in the spectra between room temperature and 5 K are observed or are present between the synthetic and
the natural crystals. The spectra are consistent with the proposal that the Mg cation is dynamically disordered and not statically
distributed over subsites in the large triangular-dodecahedral E-site in pyrope. A low-energy band at about 135 cm−1 softens and shows a large decrease in its line width with decreasing temperature. The presence of a weak, broad band at about
280 cm−1 may be due to anharmonic effects, as could the one at 135 cm−1. The latter is assigned to the rattling motion of Mg in pyrope in the plane of the longer Mg-O(4) bonds (Kolesov and Geiger
1998). The successful modeling of the anisotropic motion of the Mg cation in pyrope, which has an anharmonic character, provides
a valuable test of the validity of empirical or semi-empirical lattice-dynamic calculations for silicates.
Received: 10 May 1999 / Accepted: 10 April 2000 相似文献
19.
Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust 总被引:13,自引:0,他引:13
The behaviour of niobium and tantalum in magmatic processes has been investigated by conducting MnNb2O6 and MnTa2O6 solubility experiments in nominally dry to water-saturated peralkaline (aluminium saturation index, A.S.I. 0.64) to peraluminous
(A.S.I. 1.22) granitic melts at 800 to 1035 °C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the
concurrence of the solubility products from dissolution, crystallization, Mn-doped and Nb- or Ta-doped experiments at the
same pressure and temperature. The solubility products of MnNb2O6 (Ksp
Nb) and MnTa2O6 (Ksp
Ta) at 800 °C and 2 kbar both increase dramatically with alkali contents in water-saturated peralkaline melts. They range from
1.2 × 10−4 and 2.6 × 10−4 mol2/kg2, respectively, in subaluminous melt (A.S.I. 1.02) to 202 × 10−4 and 255 × 10−4 mol2/kg2, respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explained by five
non-bridging oxygens being required for each excess atom of Nb5+ or Ta5+ that is dissolved into the melt. The Ksp
Nb and Ksp
Ta also increase weakly with Al content in peraluminous melts, ranging up to 1.7 × 10−4 and 4.6 × 10−4 mol2/kg2, respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subaluminous and peraluminous melts (A.S.I.
1.02 and 1.22) are strongly temperature dependent, increasing by a factor of 10 to 20 from 800 to 1035 °C. By contrast columbite-tantalite
solubility in the peralkaline composition (A.S.I. 0.64) is only weakly temperature dependent, increasing by a factor of less
than 3 over the same temperature range. Similarly, Ksp
Nb and Ksp
Ta increase by more than two orders of magnitude with the first 3 wt% H2O added to the A.S.I. 1.02 and 1.22 compositions, whereas there is no detectable change in solubility for the A.S.I. 0.64
composition over the same range of water contents. Solubilities are only slightly dependent on pressure over the range 800
to 5000 bars. The data for water-saturated sub- and peraluminous granites have been extrapolated to 600 °C, conditions at
which pegmatites and highly evolved granites may crystallize. Using a melt concentration of 0.05 wt% MnO, 70 to 100 ppm Nb
or 500 to 1400 ppm Ta are required for manganocolumbite and manganotantalite saturation, respectively. The solubility data
are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt partition coefficients
increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions. It is demonstrated that
the γNb2O5/γTa2O5 activity coefficient ratio in the melt phase depends on melt composition. This ratio is estimated to decrease by a factor
of 4 to 5 from andesitic to peraluminous granitic melt compositions. Accordingly, all the relevant accessory phases in subaluminous
to peraluminous granites are predicted to incorporate Nb preferentially over Ta. This explains the enrichment of Ta over Nb
observed in highly fractionated granitic rocks, and in the continental crust in general.
Received: 9 August 1996 / Accepted: 26 February 1997 相似文献
20.
Sample cylinders of two galena ore hand specimens from Braubach, Germany were axially shortened in the strain rate range
5 × 10−5 s−1–5 × 10−7 s−1 at a confining pressure of 200 (300) MPa and at temperatures of 20 °C–600 °C. Neutron diffraction analyses of the crystallographic
preferred orientation (texture) were carried out before and after experimental deformation on the same sample cylinder. Up
to a deformation temperature of 300 °C and a strain rate of 5 × 10−6 s−1 a more or less complete <110> fiber texture develops, the strength of the fiber texture only depending on strain and the
strength of the original preferred orientation. At slower strain rate and higher temperature, there is a distinct decrease
of the fiber texture development. Diffusional mass transfer starts to become a significant deformation mechanism. Deformation
at 500 °C changes the original texture only slightly, which indicates a rapid increase of importance of diffusional flow processes.
The alteration of the accompanying sulfosalts indicates that the temperature is high enough for the movement of atoms. The
microstructure only reveals remarkable deformation structures at higher strains and in areas of locally higher stresses.
Received: 10 June 1997 / Accepted: 14 May 1998 相似文献