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1.
Summary ?A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal
chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal
free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains
and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae.
The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than
that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated.
The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples.
In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and
decrease the difference between the M2–O3 distances.
Received October 18, 2001; revised version accepted February 15, 2002 相似文献
2.
In order to explore possible quantitative relations between crystal field stabilization energy, CFSE, and partitioning behaviour
of the 3d 6-configured Fe2+ ion, a suite of 29 paragenetic rock-forming minerals from 12 high-grade metamorphic rock samples of the Ukrainian shield,
including the parageneses garnet/orthopyroxene/clinopyroxene (2x), orthopyroxene/clinopyroxene, garnet/clinopyroxene, garnet/orthopyroxene/biotite,
garnet/biotite, garnet/cordierite, garnet/cordierite/biotite, garnet/orthopyroxene/clinopyroxene/Ca-amphibole, Ca-amphibole/biotite
(retrograde), was studied by electron microprobe analysis to obtain the respective K
D
Fe2+
(Ph1/Ph2) values and by polarized single crystal electronic absorption spectroscopy to evaluate the respective CFSEFe2+ values. Other than in the case of Cr3+, a clear quantitative relation between K
D
(Ph1/Ph2) and the ΔCFSE(Ph1/Ph2) was only observed when geometrical factors, mainly the volume of crystallographic sites and ionic radii of ions competing
in the partitioning process, are similar in the respective two paragenetic phases to within 15–20%. In such cases, the ΔCFSEFe2+ contribution to K
D
(Ph1/Ph2) amounts to 0.1 to 0.2 log K
D
per 100 cm−1ΔCFSE. The conclusion is that ΔCFSEFe2+ plays only a secondary role after geometrical factors, in the partitioning behaviour of Fe2+. The reason for this is seen in the facts that, compared to the 3d
3-configured Cr3+ ion, CFSE of the 3d 6-configured Fe2+ amounts only to 20–25%, and that the former ion enters only octahedral sites with similar geometrical properties in the paragenetic
mineral phases.
Received: 17 November 1998 / Accepted: 28 June 1999 相似文献
3.
Summary Transmission M?ssbauer spectra of synthetic Ca-free P21/c Mg0.22Fe0.78SiO3 clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K.
The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature,
all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe2+ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the
Fe2+ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were
found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K,
the principal component V
zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of
the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings
ΔE
Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE
Q(T ), the energy gaps δ1 and δ2 of the first excited electronic states within the 5D orbital term were estimated to be 410 ± 50 cm−1 and 730 ± 50 cm−1 for M1, and δ1 = 1050 ± 75 cm−1 for M2.
Received May 29, 2000;/revised version accepted July 13, 2001 相似文献
4.
Pavese et al. (1999) examined cation partitioning vs. temperature in a synthetic spinel of composition (Mg0.70 Fe0.23
3+) Al1.97 O4 using structure data obtained from in situ neutron powder diffraction. After imposing assumptions on the site assignment
of vacancies and Fe3+, they assigned the remaining cations by applying least-squares minimization to chemical constraints on site-occupancy sums,
site-scattering, chemical composition, and thermal expansion of the octahedral site. Their proposed site assignments exhibit
a sharp discontinuity in occupancy fractions versus temperature, a necessary consequence of their assumptions on vacancy assignments.
In this paper we reexamine the cation partitioning of the same spinel using the constrained least-squares formulation of OccQP (Wright et al. 2000), which optimizes site occupancies without ad hoc assumptions. We obtain strikingly different results,
supporting the general view that spinel undergoes a lambda transition at ∼1000 K. For all observed parameters, the residuals
obtained with the OccQP assignments are lower than those obtained with the Pavese et al. assignments, in some cases by more than 1 order of magnitude.
Received: 05 April 2000 / Accepted: 19 October 2000 相似文献
5.
Experimental reconstruction of sodic dolomitic carbonatite melts from metasomatised lithosphere 总被引:3,自引:0,他引:3
Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite
in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor
phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between
sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of
the carbonatite melt, releasing CO2-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions
and reacting them with CO2 at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with
minor apatite (composition 71001*) and one without apatite (composition 70965*). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001*, and pargasite+dolomite lherzolite for runs using composition 70965*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue
(olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are
consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions.
In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral,
sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO2, released by metasomatic reactions, which are locally very large at around 5 wt% CO2.
Received: 15 December 1995/Accepted: 14 February 1996 相似文献
6.
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
7.
Richard O. Sack 《Contributions to Mineralogy and Petrology》1980,71(3):257-269
Existing data on the temperature and composition dependence of the Fe2+-Mg2+ distribution between Fe-Mg olivine and orthopyroxene, the intra-crystalline distribution of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxene, and macroscopic activity-composition relations in olivine and orthopyroxene are shown to be inconsistent
with generally accepted thermodynamic formulations which assume that the non-configurational Gibbs energy of orthopyroxene
is independent of the degree of long-range ordering of Fe2+ and Mg+ between M1 and M2 sites. These data are interpreted in terms of the constraints they provide on the size of Bragg-Williams type energy, entropy,
and volume terms for olivine and orthopyroxene. The apparent equilibrium constant for Fe-Mg exchange between olivine and orthopyroxene
is shown to be a potentially useful ‘geothermometer’ for olivine-orthopyroxene assemblages with olivines with mole fraction
of Fe2SiO4 component less than 0.2 or greater than 0.6. A provisional calibration of this ‘geothermometer’ is presented. 相似文献
8.
The spinel solid solution was found to exist in the whole range between Fe3O4 and γ-Fe2SiO4 at over 10 GPa. The resistivity of Fe3−
x
Si
x
O4 (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe3+ and Fe2+ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy
of the electron hopping becomes larger with increasing γ-Fe2SiO4 component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe3+–Fe2+ electron hopping with increasing the total Si content. This is probably because a large number of Si4+ ions occupies the octahedral site and the adjacent Fe2+ keeping the local electric neutrality around Si4+ makes a cluster, which generates a local deformation by Si substitution.
The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases
from 124(±2) K at x=0 (Fe3O4) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si4+ substitution.
Received: 15 March 2000 / Accepted: 4 September 2000 相似文献
9.
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
10.
Water solubility in pyrope to 100 kbar 总被引:14,自引:0,他引:14
The solubility and incorporation mechanism of water in natural, almost pure pyrope from Dora Maira, Western Alps was investigated.
The infrared spectrum of the natural, untreated sample (58 ppm water) shows several exceptionally sharp bands in the OH-stretching
region, including a single band at 3601.9 cm−1 and a band system with main components at 3640.5, 3650.8 and 3660.6 cm−1. High-temperature and high-pressure infrared spectra suggest that the two absorption features arise from almost free OH groups
in sites with different compressibility and thermal expansivity, with the site causing the 3601.9 cm−1 band being much stiffer. Pyrope samples were annealed in a piston-cylinder or multi-anvil apparatus for several days in the
presence of excess water, excess SiO2 and excess Al2SiO5 to determine the equilibrium solubility of water in pyrope to 100 kbar. Total solubility increases with pressure, however,
this is exclusively due to the high-frequency band system, while the intensity of the low-frequency band decreases with pressure.
At 1000 °C and the oxygen fugacity of the Ni-NiO buffer, the bulk solubility can be described by the equation c
OH
=Af
H2O
0.5exp(−PΔV/RT) with A = 0.679 ppm/bar0.5 and ΔV = 5.71 cm3/mol. This equation implies the incorporation of water in the crystal as isolated OH groups. With increasing temperature,
solubility appears to decrease with ΔH = − 14 kJ/mol. At Fe-FeO buffer conditions, solubility is 30 to 50% lower than with the Ni-NiO buffer, suggesting that the
incorporation of OH is not coupled to the reduction of Fe3+. Possibly, the 3601.9 cm−1 band is associated with the tetrahedral OH B defect and the high-frequency system with the dodecahedral OH Li defect. Based
on the experimentally established solubility model, it is estimated that garnet in a hot subducted slab will transport 170 ppm
of water into the mantle beyond the breakdown limit of amphibole. In a cold slab, 470 ppm of water can be incorporated into
garnet at the breakdown limit of phengite. These numbers imply that a significant fraction of the total water in the hydrosphere
has been recycled into the mantle since the Proterozoic.
Received: 6 January 1997 / Accepted: 27 March 1997 相似文献
11.
The effect of Cr on the solubility of Al in orthopyroxene: experiments and thermodynamic modelling 总被引:1,自引:1,他引:0
The partitioning of chromium and aluminium between coexisting orthopyroxene and spinel in equilibrium with forsterite in
the system MgO–Al2O3–SiO2–Cr2O3 (MAS–Cr) has been experimentally determined as a function of temperature, pressure and Cr/(Cr + Al) ratio. Experiments were
conducted at temperatures between 1300 and 1500 °C and at pressures from 5 to 54 kbar. Previous experimental results on the
(Al, Cr)2O3 and Mg(Al, Cr)2O4 solid solutions have been combined with the present results plus relevant data from the CMAS system to derive a thermodynamic
model for Al–Cr-bearing orthopyroxenes, spinels and corundum–eskolaite solid solutions. The orthopyroxene solid solution can
be modelled within the accuracy of all experimental constraints as a ternary solid solution involving the components Mg2Si2O6 ( E), MgAl2SiO6 (M) and MgCr2SiO6 (C), in which the activities are related to composition through the equations:
The mole fractions are defined as
where n
Al and n
Cr are the number of Al and Cr cations per orthopyroxene formula unit of six oxygens. These expressions reduce to one-site mixing
for Mg2Si2O6–MgAl2SiO6 orthopyroxenes in the Cr-free system, but are equivalent to two-site mixing for the exchange of Al and Cr between orthopyroxene
and spinel, as required by the experimental data. We find W
opx
EM
=W
opx
EC
= 20 kJ mol−1 and W
opx
MC
=0.
Received: 9 August 1999 / Accepted: 18 February 2000 相似文献
12.
The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal
diffraction method. The crystal data are (Co0.377Ni0.396Zn0.227)2SiO4, M
r
= 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm3, Z = 4, D
x
= 4.864 g cm−3, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect
of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are
about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn
in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii
and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)M1/(A/B)M2] = −0.272 (IR
A
-IR
B
) + 3.65 (EN
A
−EN
B
), where IR (pm) and EN are ionic radius and electronegativity, respectively.
Received: 8 April 1999 / Revised, accepted: 7 September 1999 相似文献
13.
K. Putirka Marie Johnson Rosamond Kinzler John Longhi David Walker 《Contributions to Mineralogy and Petrology》1996,123(1):92-108
Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions
are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions
(from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature
and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi2O6)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si2O6) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made
to the calibrated equilibrium constant the resulting geothermometer is
(i) 104
T=6.73−0.26* ln [Jdpx*Caliq*FmliqDiHdpx*Naliq*Alliq] −0.86* ln [MgliqMgliq+Feliq]+0.52*ln [Caliq]
an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more
sensitive to pressure resulting in a thermobarometer
(ii) P=−54.3+299* T104+36.4* T104 ln [Jdpx[Siliq]2*Naliq*Alliq] +367*[Naliq*Alliq]
which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Naliq represent the cation fraction of the given oxide (NaO0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a
normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess AlVI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO6); Ca remaining after forming CaTs and CaTiAl2O6 is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions
of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures,
the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties
from regression coefficients in (ii) gives VJd
f of 23.4 ±1.3 cm3/mol, close to the value anticipated from bar molar volume data (23.5 cm3/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as
great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient
time to re-equilibrate at lower pressures.
Received: 16 May 1994/Accepted: 15 June 1995 相似文献
14.
Structural parameters and cation ordering are determined for four compositions in the synthetic MgGa2O4-Mg2GeO4 spinel solid solution (0, 8, 15 and 23 mol% Mg2GeO4; 1400 °C, 1 bar) and for spinelloid β-Mg3Ga2GeO8 (1350 °C, 1 bar), by Rietveld refinement of room-temperature neutron diffraction data. Sample chemistry is determined by
XRF and EPMA. Addition of Mg2GeO4 causes the cation distribution of the MgGa2O4 component to change from a disordered inverse distribution in end member MgGa2O4, [4]Ga = x = 0.88(3), through the random distribution, toward a normal cation distribution, x = 0.37(3), at 23 mol% Mg2GeO4. An increase in ao with increasing Mg2GeO4 component is correlated with an increase in the amount of Mg on the tetrahedral site, through substitution of 2 Ga3+⇄ Mg2++Ge4+. The spinel exhibits high configurational entropy, reaching 20.2 J mol−1 (four oxygen basis) near the compositional upper limit of the solid solution. This stabilizes the spinel in spite of positive
enthalpy of disordering over the solid solution, where ΔH
D
= αx + βx
2, α = 22(3), β = −21(3) kJ mol−1. This model for the cation distribution across the join suggests that the empirically determined limit of the spinel solid
solution is correlated with the limit of tetrahedral ordering of Mg, after which local charge-balanced substitution is no
longer maintained.
Spinelloid β-Mg3Ga2GeO8 has cation distribution M1[Mg0.50(2)Ga0.50(2)] M2[Mg0.96(2)Ga0.04(2)] M3[Mg0.77(2) Ga0.23(2)]2 (Ge0.5Ga0.5)2O8 (tetrahedral site occupancies are assumed). Octahedral site size is correlated to Mg distribution, where site volume, site
distortion, and Mg content follow the relation M1<M3<M2. The disordered cation distribution provides local electrical neutrality
in the structure, and stabilization through increased configurational entropy (27.6 J mol−1; eight oxygen basis). Comparison of the crystal structures of Mg1+
N
Ga2−2
N
Ge
N
O4 spinel, β-Mg3Ga2GeO8, and Mg2GeO4 olivine reveals β-Mg3Ga2GeO8 to be a true structural intermediate. Phase transitions across the pseudobinary are necessary to accommodate an increasing
divergence of cation size and valence, with addition of Mg2GeO4 component. Octahedral volume increases while tetrahedral volume decreases from spinel to β-Mg3Ga2GeO8 to olivine, with addition of Mg and Ge, respectively. Furthermore, M-M distances increase regularly across the join, suggesting
that changes in topology reduce cation-cation repulsion.
Received: 9 November 1998 / Revised, accepted: 3 August 1999 相似文献
15.
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
16.
In order to develop a model for simulating naturally occurring chromian spinel compositions, we have processed published
experimental data on chromian spinel-melt equilibrium. Out of 259 co-existing spinel-melt experiments reported in the literature,
we have selected 118 compositions on the basis of run time, melt composition and experimental technique. These data cover
a range of temperatures 1150–1500° C, oxygen fugacities of −13<log f
O2< −0.7, and bulk compositions ranging from basalt and norite, to komatiite. Six major spinel components with Cr3+, Al3+, Ti4+, Mg2+, Fe3+ and Fe2+-bearing end-members were considered for the purpose of describing chromite saturation as a function of melt composition,
temperature and oxygen fugacity at 1 atmosphere pressure (0.101 MPa). The empirically calibrated mineral-melt expression based
on multiple linear regressions is:
K
Sp
i
=A/T(K)+B log f
O2+C ln (Fe3+/Fe2+)L+D ln R
L
+E,
where K
Sp
i
is an equilibrium constant and R
L
is a melt structure-chemical parameter (MSCP). Twenty-eight forms of equilibrium constants were considered, including single distribution coefficients, exchange equilibrium
constants, formation constants for AB2O4 components, as well as simple “spinel cation ratios”. For each form of the equilibrium constants, a set of 16 combinations
of the MSCPs have been investigated. The MSCP is present in the form of composite ratios [e.g., Si/O, NBO/T,(Al+Si)/Si, or (Na+K)/Al] or as simple cation ratios (e.g.,
Mg/Fe2+). For the calculation of Fe3+ and Fe2+ species in silicate melts, we used existing equations, whereas the Fe3+/Fe2+ ratio of spinels was calculated from the spinel stoichiometry. The regression parameters that best repoduce the experimental
data were for the following constants: (Fe3+/Fe2+)
Sp
, (Mg/Fe2+)
Sp
/(Mg/Fe2+)
L
, (Cr/Al)
Sp
/ (Cr/Al)
L
, K
FeCr2O4, and Ti
Sp
/Ti
L
. These expressions have been combined into a single program called SPINMELT, which calculates chromite crystallization temperature
and composition at a given f
O2 with an average accuracy of ∼10° C and 1–2 mol%. An example of the use of SPINMELT is presented for a magma parental to the
Bushveld Complex.
Received: 30 May 1995/Accepted: 1 November 1995 相似文献
17.
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
18.
The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa
and 1200–1600 ∘C using both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence
on the capsule material used due to the variation in Fe3+ concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules,
where metallic Fe was also added to the starting materials, the difference in the interaction parameters for the two solid
solutions (W
FeMg
mw−W
FeMg
ring) is calculated to be 8.5±1 kJ mol−1. Similar experiments performed in Re metal capsules result in a value for W
FeMg
mw−W
FeMg
ring that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic
analyses, however, show Fe3+ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and
that Fe3+ partitions preferentially into magnesiowüstite, with K
D
Fe3+
ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, a least–squares fit to the
partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W
FeMg
ring) of 3.5±1 kJ mol−1. The equivalent regular interaction parameter for magnesiowüstite (W
FeMg
mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe3+ concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol−1 for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T−298)+18.9P with T in K, P in kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation
of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of
high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of
ringwoodite to form magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of
these results with previous studies shows that the presence of Fe3+ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined
thermodynamic properties.
Received: 13 September 2000 / Accepted: 25 March 2001 相似文献
19.
We have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border
group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve their closure temperatures
(T
c) of Fe–Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton, conductive cooling
dominated the high and low temperature regimes, which were separated by an intermediate temperature regime in which the cooling
was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states
of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼340–400° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from
the cation ordering states of orthopyroxene vary from ∼1 to 270 K/ka. These results agree well with a thermal model calculated
using an assumed average permeability of 10-12 cm2 for the pluton, but not completely with a model calculated on the basis of an average permeability of 10-13 cm2, although both values produced shifts of δ18O that are comparable to those observed in the pluton.
Received: 27 February 1995/Accepted: 20 July 1995 相似文献
20.
Mariko Nagashima Takahiro Ishida Masahide Akasaka 《Physics and Chemistry of Minerals》2006,33(3):178-191
Two pumpellyites with the general formula W
8
X
4
Y
8
Z
12O56-n
(OH)
n
were studied using 57Fe Mössbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe(X):Fe(Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe(X) and Fe(Y) represent Fe content at the X and Y sites, respectively. The Mössbauer spectra consisted of two Fe2+ and two Fe3+ doublets for the Mitsu pumpellyite, and one Fe2+ and two Fe3+ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mössbauer doublets and the Fe(X):Fe(Y) ratios determined by the Rietveld refinements, Fe2+(X):Fe3+(X):Fe3+(Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe2+:Fe3+-ratio determined by the Mössbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca8(Fe
0.88
2+
Mg0.68Fe
0.77
3+
Al1.66)Σ3.99(Al5.67Fe
2.34
3+
)Σ8.01Si12O42.41(OH)13.59 and Ca8(Mg1.24Fe
0.65
2+
Fe
0.46
3+
Al1.66)Σ4.01(Al6.71Fe
1.29
3+
)Σ8.00Si12O42.14(OH)13.86, respectively. Mean Y–O distances and volumes of the YO6 octahedra increase with increasing mean ionic radii, i.e., the Fe3+→Al substitution at the Y site. However, change of the sizes of XO6 octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO6 octahedra. Thus, the geometrical change of the YO6 octahedra with Fe3+→Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO6 octahedra by the ionic substitution of Fe3+ for Al causes gradual change of the octahedra to more symmetrical and regular forms. 相似文献