共查询到20条相似文献，搜索用时 693 毫秒
1.
H. W. Nesbitt A. G. Schaufuss G. M. Bancroft R. Szargan 《Physics and Chemistry of Minerals》2002,29(1):72-77
Synchrotron excited X-ray photoelectron spectra (SXPS) of hexagonal pyrrhotite reveal three distinct Fe 3d-derived photopeaks within its outer valence band. The t
2gα band (majority spin) is centered at about 2.5 eV, the e
g
α band at about 1.0 eV and the t
2gβ (minority spin) contribution at about 0.25 eV. From these data the ligand field splitting energy is 1.5 (±0.2) eV and the
majority spin pairing energy is 2.25 (±0.2) eV. These are the first such XPS measurements for this mineral. S 3p-derived bonding and non-bonding bands are identified, with the former centred at about 6.5 eV and the latter near 4.5 eV.
The XPS results are remarkably consistent with SCF-Xα scattered wave molecular orbital calculations. Although the calculations
and the collected spectra are consistent, they differ from a recent interpretation of the pyrrhotite valence band. An explanation
for the discrepant results is provided. Auger resonant enhancement of Fe 3d photopeaks at 60 eV photon energy results in the
t
2gα emission (at 2.5 eV) being strongly enhanced and broader than the t
2gβ emission (0.25 eV). The explanation of these observations requires the presence of weak Fe–Fe π and π* crystal (molecular) orbitals located near 2.5 eV, and separated by no more than about 0.5 eV. The π-bonded crystal orbitals
are derived from weak mixing of adjacent Fe t
2g atomic orbitals along the c crystallographic axis.
Received: 15 June 2000 / Accepted: 11 June 2001 相似文献
2.
Raman spectra of diopside were collected from atmospheric pressure to 71 GPa. The pressure dependences of 22 modes were determined.
Changes occurred in the spectra at three different pressures. First, at approximately 10 GPa, the two Raman modes at 356 and
875 cm−1 disappeared, while the mode at 324 cm−1 split into two modes, diverging at this pressure with significantly different pressure shifts; second, at approximately 15
GPa, a small (1 to 2 cm−1) drop in several of the frequencies was observed accompanied by changes in the pressure dependency of some of the modes;
and third, above 55 GPa, the modes characteristic of chains of tetrahedrally coordinated silicon disappeared, while those
for octahedrally coordinated silicon appeared. The first change at 10 GPa appears to be a C2/c to C2/c transition involving a change in the Ca coordination. The third change above 55 GPa appears to be a change in the silicon
coordination. At 15 GPa, it is suggested that a change in compressional mechanism takes place.
Received: 14 November 2000 / Accepted: 9 January 2002 相似文献
3.
M. Zhang G. J. Redhammer E. K. H. Salje M. Mookherjee 《Physics and Chemistry of Minerals》2002,29(9):609-616
Synthetic aegirine LiFeSi2O6 and NaFeSi2O6 were characterized using infrared spectroscopy in the frequency range 50–2000 cm−1, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi2O6, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi2O6 (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P21/c structural phase transition of LiFeSi2O6 is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit
cell at the transition point. The appearance of an extra band near 688 cm−1 in the monoclinic P21/c phase, which is due to the Si–O–Si vibration in the Si2O6 chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral
changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and
330 cm−1 show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi2O6 measured at low temperatures, the change in LiFeSi2O6 is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze
the order parameter of the C2/c–P21/c phase transition, and the results suggest that the transition is close to tricritical.
Received: 21 January 2002 / Accepted: 22 July 2002 相似文献
4.
Torsional forced-oscillation and microcreep methods have been employed in a study of the viscoelastic behaviour of fine-grained
polycrystalline olivine at high temperatures (to 1300 °C), seismic frequencies and low strain amplitudes. The Fo90 specimens are of low porosity and low dislocation density. They vary in mean grain size from 8 to 150 μm and contain only
trace amounts (≪0.1 vol%) of quenched melt glass. For T ≤ 900 °C, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal from single-crystal elasticity data – confirming the suppression
of thermal microcracking in␣this study. At higher temperatures, pronounced absorption-band dissipation and associated dispersion␣of
the shear modulus provide evidence of linear viscoelastic behaviour. Both recoverable (anelastic) and permanent (viscous)
strains are involved and the proportion of the latter increases with increasing temperature and decreasing frequency. Comparison
of the results for the three specimens provides a clear indication that the viscoelastic behaviour, attributed to diffusional
processes, is grain-size-sensitive with the dissipation and associated dispersion increasing with decreasing grain size. Both
elastically accommodated and diffusionally accommodated grain-boundary sliding appear to be implicated.
Received: 29 September 2000 / Accepted: 7 May 2001 相似文献
5.
J. M. Léger J. Haines C. Chateau G. Bocquillon M. W. Schmidt S. Hull F. Gorelli A. Lesauze R. Marchand 《Physics and Chemistry of Minerals》2001,28(6):388-398
The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction
data. It is tetragonal, space group I&4macr;2d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I&4macr;2d or I&4macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined
as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under
truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According
to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained
by quenching experiments, and the P–T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs
of PON and SiO2 is discussed.
Received: 7 August 2000 / Accepted: 21 January 2001 相似文献
6.
The structural behavior of synthetic gahnite (ZnAl2O4) has been investigated by X-ray powder diffraction at high pressure (0–43 GPa) and room temperature, on the ID9 beamline
at ESRF. The equation of state of gahnite has been derived using the models of Birch–Murnaghan, Vinet and Poirier–Tarantola,
and the results have been mutually compared (the elastic bulk modulus and its derivatives versus P determined by the third-order Birch–Murnaghan equation of state are K
0=201.7(±0.9) GPa, K
′
0=7.62(±0.09) and K
″
0=−0.1022 GPa−1 (implied value). The compressibilities of the tetrahedral and octahedral bond lengths [0.00188(8) and 0.00142(5) GPa−1 at P=0, respectively], and the␣polyhedral volume compressibilities of the four-␣and␣sixfold coordination sites [0.0057(2) and
0.0041(2) GPa−1 at P=0, respectively] are discussed.
Received: 15 January 2001 / Accepted: 23 April 2001 相似文献
7.
High(C2/c)-low(P21/c) phase transition in clinoenstatite and pigeonite was successfully observed in situ at high temperatures for the first time under a transmission electron microscope. The phase transition was revealed to possess the characteristics of a first-order transition, due to the coexistence of both phases separated by the sharp interfaces and the nucleation-growth process. The diffusionless and time-independent reaction suggests that the transition occurs athermal-martensitically. Furthermore, the small or even negative thermal hysteresis and the interface motion suggest that the transition is not a typical type but a thermoelastic type of the martensitic transformation. This type of the transformation, studied extensively in metallurgy in relation to shape memory effect, is first recognized in rock-forming minerals. 相似文献
8.
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the
(SO4)−4 group that lie between 400 and 1150 cm−1, hydroxyl-stretching vibrations between 3200 and 3600 cm−1, and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations
have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule
thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the
morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced
phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure.
The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum:
the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group
undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the
symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05,
in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within
gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit
near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.
Received: 31 July 2000 / Accepted: 5 April 2001 相似文献
9.
In order to elucidate high-pressure transformations of high-P clinopyroxene (C2/c) at kinetically low temperature where atoms are not thermally activated, the transformation processes of FeGeO3 clinopyroxene (C2/c) have been investigated at pressures up to 20 GPa and 365 °C by powder X-ray diffraction using a synchrotron radiation source
and TEM observation. With increasing pressure up to 20 GPa at room temperature, FeGeO3 high-P clinopyroxene (C2/c) reversibly transforms into a new high-pressure phase, FeGeO3(II). On increasing the temperature up to 365 °C, this phase rapidly transforms into FeGeO3 ilmenite within about 2 h. Intensity analysis of the X-ray diffraction pattern reveals that the high-pressure phase of FeGeO3(II) has an intermediate structure between clinopyroxene and ilmenite: the cation arrangement is similar to that of clinopyroxene
and the oxygen arrangement is similar to that of ilmenite. The comparison of the crystal structures of these polymorphs suggests
that clinopyroxene to FeGeO3(II) and FeGeO3(II) to ilmenite transformations are performed by the slight deformation of the oxygen packing and the short-range movement
of cations, respectively. It is shown that this high-P clinopyroxene transforms into ilmenite through a low-activation energy
path under the low-temperature condition.
Received: 30 August 2000 / Accepted: 10 February 2001 相似文献
10.
An olivine grain from a peridotite nodule 9206 (Udachnaya kimberlite, Siberia) was investigated by TEM methods including
AEM, HRTEM, SAED and EELS techniques. A previous study of the 9206 olivine sample revealed OH absorption bands in the IR spectrum
and abundant nanometer-sized OH-bearing inclusions, of hexagonal-like or lamellar shape. Inclusions, which are several hundred
nm in size, consist of 10 ? phase, talc and serpentine (chrysotile and lizardite). The lamellar (LI) and hexagon-like small
inclusions of several ten nm in size (SI) are the topic of the present paper. AEM investigations of the inclusions reveal
Mg, Fe and Si as cations only. The Mg/Si and Fe/Si atomic ratios are lower in the inclusions than in the host olivine. The
Si concentration in the olivine host and both lamellar inclusions and small inclusions is the same. A pre-peak at 528eV was
observed in EEL spectra of LI and SI, which is attributed to OH− or Fe3+. From these data it is concluded that there is a OH−- or Fe3+-bearing cation-deficient olivine-like phase present.
HRTEM lattice fringe images of LI and SI exhibit modulated band-like contrasts, which are superimposed onto the olivine lattice.
Diffraction patterns (Fourier-transforms) of the HREM images as well as SAED patterns show that the band-like contrasts in
HRTEM images of the inclusions are caused by periodic modulations of the olivine lattice. Three kinds of superperiodicity
in the olivine structure such as 2a, 3a and 3c, were observed in SAED patterns. The corresponding olivine supercells labelled
here as Hy-2a, Hy-3a and Hy-3c were derived. The M1-vacancies located in the (100) and (001) octahedral layers of the olivine
lattice are suggested to form ordered arrays of planar defects (PD), which cause the band-like contrasts in HRTEM patterns
as well as the superperiodicity in the SAED patterns.
The vacancy concentrations as well as the chemical composition of Hy-2a, Hy-3a and Hy-3c olivine supercells were calculated
using crystal chemical approaches, assuming either {(OH)<
O−V"
Me−(OH)<
O}↔, or {F
e
<
Fe
−H
Me
′}↔ or {2F
e
<
Fe
−V
Me
"}↔ point defect associates. The calculated theoretical compositions Mg1.615Fe+2
0.135v0.25SiO4H0.5 (Hy-2a) and Mg1.54Fe2+
0.12v0.33SiO4H0.66 (Hy-3a and Hy-3c) are in a good agreement with the AEM data on inclusions. Hy-2a, Hy-3a and Hy-3c are considered to be a
hydrous olivine with the extended chemical formula (Mg1-yFe2+
y)2−xvxSiO4H2x. The crystal structure of hydrous olivine is proposed to be a modular olivine structure with Mg-vacant modules. The crystal
chemical formula of hydrous olivines in terms of a modular structure can be written as [MgSiO4H2] · 3[Mg1.82Fe0.18SiO4] for Hy-2a, [MgSiO4H2] · 2[Mg1.82Fe0.18SiO4] for Hy-3a and Hy-3c.
Hydrous olivine is suggested to be exsolved from the olivine 9206, which has been initially saturated by OH-bearing point
defects. The olivine 9206 hydration as well as the following exsolution of hydrous olivine inclusions is suggested to occur
at high pressure-high temperature conditions of the upper mantle.
Received: 15 January 2001 / Accepted: 2 July 2001 相似文献
11.
Victor L. Vinograd Julian D. Gale Björn Winkler 《Physics and Chemistry of Minerals》2007,34(10):713-725
Static lattice energy calculations (SLEC), based on empirical interatomic potentials, have been performed for a set of 800
different structures in a 2 × 2 × 4 supercell of C2/c diopside with compositions between diopside and jadeite, and with different states of order of the exchangeable Na/Ca and
Mg/Al cations. Excess static energies of these structures have been cluster expanded in a basis set of 37 pair-interaction
parameters. These parameters have been used to constrain Monte Carlo simulations of temperature-dependent properties in the
range of 273–2,023 K and to calculate a temperature–composition phase diagram. The simulations predict the order–disorder
transition in omphacite at 1,150 ± 20°C in good agreement with the experimental data of Carpenter (Mineral Petrol 78:433–440,
1981). The stronger ordering of Mg/Al within the M1 site than of Ca/Na in the M2 site is attributed to the shorter M1–M1 nearest-neighbor
distance, and, consequently, the stronger ordering force. The comparison of the simulated relationship between the order parameters
corresponding to M1 and M2 sites with the X-ray refinement data on natural omphacites (Boffa Ballaran et al. in Am Mineral
83:419–433, 1998) suggests that the cation ordering becomes kinetically ineffective at about 600°C. 相似文献
12.
The clinopyroxenes spodumene (LiAlSi2O6), LiScSi2O6 and ZnSiO3, all with space group C2/c at ambient conditions, were studied under high pressures by single-crystal X-ray diffraction in a diamond-anvil cell. Changes
in the evolution of the unit-cell parameters, optical properties and the appearance of h + k odd reflections characteristic of a primitive lattice, indicate that all three pyroxenes undergo phase transitions. The transitions
are mostly displacive in character, and are non-quenchable. Transition pressures are 3.19 GPa in spodumene, ∼0.6 GPa in LiScSi2O6 and 1.92 GPa in ZnSiO3. The space group of all three high-pressure phases was determined to be P21/c by structure refinement to single-crystal X-ray intensity data collected in the DAC. In the ZnSiO3 clinopyroxene the intermediate P21/c phase further transforms to a second C2/c phase (HP-C2/c) at 4.9 GPa (confirmed by structure refinement). The volume change at this transition is about 2.6%, three times larger than
in the first phase transition, and typical of the P21/c→ HP-C2/c phase transitions found previously in MgSiO3, FeSiO3, etc. These results therefore provide the first direct evidence that the HP-C2/c and the HT-C2/c structures of pyroxenes are distinct polymorphs with the same space group. The phase transition from C2/c to P21/c symmetry in spodumene and LiScSi2O6 therefore occurs because the polymorphs stable at ambient conditions are isotypic to the high-temperature C2/c phases of clinopyroxenes such as pigeonite and clinoenstatite.
Received: 22 December 1999 / Accepted: 7 June 2000 相似文献
13.
Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO2=NNO+2.3. The space group at room T is P21/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)∘, V=415.2 ?3]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi2O6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The
crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding M2(Li0.85Mg0.09Fe2+
0.06) M1(Fe3+
0.85Mg0.15)Si2O6. Li is ordered at the M2 site and Fe3+ is ordered at the M1 site, whereas Mg (and Fe2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200
°C) allowed characterization of a reversible displacive P21/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation.
For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition
temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate
radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian
P21/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and
thermodynamic behaviour in clinopyroxenes.
Received: 7 May 2002 / Accepted: 23 October 2002 相似文献
14.
S. Merkel A. P. Jephcoat J. Shu H.-K. Mao P. Gillet R. J. Hemley 《Physics and Chemistry of Minerals》2002,29(1):1-9
Physical properties including the equation of state, elasticity, and shear strength of pyrite have been measured by a series
of X-ray diffraction in diamond-anvil cells at pressures up to 50 GPa. A Birch–Murnaghan equation of state fit to the quasihydrostatic
pressure–volume data obtained from laboratory X-ray source/film techniques yields a quasihydrostatic bulk modulus K
0T
=133.5 (±5.2) GPa and bulk modulus first pressure derivative K
′
0T
=5.73 (±0.58). The apparent equation of state is found to be strongly dependent on the stress conditions in the sample. The
stress dependency of the high-pressure properties is examined with anisotropic elasticity theory from subsequent measurements
of energy-dispersive radial diffraction experiments in the diamond-anvil cell. The calculated values of K
0T
depend largely upon the angle ψ between the diffracting plane normal and the maximum stress axis. The uniaxial stress component
in the sample, t=σ3−σ1, varies with pressure as t=−3.11+0.43P between 10 and 30 GPa. The pressure derivatives of the elastic moduli dC
11/dP=5.76 (±0.15), dC
12/dP=1.41 (±0.11) and dC
44/dP=1.92 (±0.06) are obtained from the diffraction data assuming previously reported zero-pressure ultrasonic data (C
11=382 GPa, C
12=31 GPa, and C
44=109 GPa).
Received: 21 December 2000 / Accepted: 11 July 2001 相似文献
15.
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
16.
F. Farges 《Physics and Chemistry of Minerals》2001,28(9):619-629
Fe–K edge XAFS spectra (pre-edge, XANES and EXAFS) were collected for eight grandidierites from Madagascar and Zimbabwe,
as well as for Fe(II) and Fe(III) model compounds (staurolite, siderite, enstatite, berlinite, yoderite, acmite, and andradite).
The pre-edge spectra for these samples are consistent with dominantly 5-coordinated ferrous iron. The analysis of the XANES
and EXAFS spectra confirms that Fe(II) substitutes for Mg(II) in grandidierite, with a slight expansion of the local structure
around Mg by ∼2%. In addition, ferric iron was also detected in some samples [5–10 mol% of the total Fe or 500–1100 ppm Fe(III)].
Based on theoretical calculations of the EXAFS region, Fe(III) appears to be located in the 5-coordinated sites of Mg(II)
or in the most distorted 6-coordinated sites of Al (depending on the sample studied). Special attention is therefore required
when using grandidierite as a model for ferrous iron in C3v geometry, because of the possible presence of an extra contribution related to Fe(III). This additional contribution enhances
significantly the Fe–K pre-edge integrated area [+40% for 1000 ppm Fe(III)]. Therefore, only a few grandidierite samples can
be used as a robust structural model for the study of the Fe(II) coordination in glasses and melts.
Received: 26 June 2000 / Accepted: 19 February 2001 相似文献
17.
We present H2O analyses of MgSiO3 pyroxene crystals quenched from hydrous conditions in the presence of olivine or wadsleyite at 8–13.4 GPa and 1,100–1,400°C.
Raman spectroscopy shows that all pyroxenes have low clinoenstatite structure, which we infer to indicate that the crystals
were high clinoenstatite (C2/c) during conditions of synthesis. H2O analyses were performed by secondary ion mass spectrometry and confirmed by unpolarized Fourier transform infrared spectroscopy
on randomly oriented crystals. Measured H2O concentrations increase with pressure and range from 0.08 wt.% H2O at 8 GPa and 1,300°C up to 0.67 wt.% at 13.4 GPa and 1,300°C. At fixed pressure, H2O storage capacity diminishes with increasing temperature and the magnitude of this effect increases with pressure. This trend,
which we attribute to diminishing activity of H2O in coexisting fluids as the proportion of dissolved silicate increases, is opposite to that observed previously at low pressure.
We observe clinoenstatite 1.4 GPa below the pressure stability of clinoenstatite under nominally dry conditions. This stabilization
of clinoenstatite relative to orthoenstatite under hydrous conditions is likely owing to preferential substitution of H2O into the high clinoenstatite polymorph. At 8–11 GPa and 1,200–1,400°C, observed H2O partitioning between olivine and clinoenstatite gives values of D
ol/CEn between 0.65 and 0.87. At 13 GPa and 1,300°C, partitioning between wadsleyite and clinoenstatite, D
wd/CEn, gives a value of 2.8 ± 0.4. 相似文献
18.
An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca0.8Mg1.2Si2O6 clinopyroxene (Di80En20) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic,
with βb ⋙ βc > βa > βasinβ for any pressure range and for both diopside and Di80En20. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di80En20 than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di80En20. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along
a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In
diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di80En20 the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a
given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour
is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan
equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3)
for Di80En20; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected
by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich
orthopyroxenes.
Received: 3 January 2000 / Accepted: 21 May 2000 相似文献
19.
S. Ono E. Ito T. Katsura A. Yoneda M. J. Walter S. Urakawa W. Utsumi K. Funakoshi 《Physics and Chemistry of Minerals》2000,27(9):618-622
In situ synchrotron X-ray experiments in the system SnO2 were made at pressures of 4–29 GPa and temperatures of 300–1400 K using sintered diamond anvils in a 6–8 type high-pressure
apparatus. Orthorhombic phase (α-PbO2 structure) underwent a transition to a cubic phase (Pa3ˉ structure) at 18 GPa. This transition was observed at significantly lower pressures in DAC experiments. We obtained the
isothermal bulk modulus of cubic phase K
0 = 252(28) GPa and its pressure derivative K
′=3.5(2.2). The thermal expansion coefficient of cubic phase at 25 GPa up to 1300 K was determined from interpolation of the
P-V-T data obtained, and is 1.7(±0.7) × 10−5 K−1 at 25 GPa.
Received: 7 December 1999 / Accepted: 27 April 2000 相似文献
20.
We used an in situ measurement method to investigate the phase transition of CaGeO3 polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with intense synchrotron
X-ray radiation was used. The transition boundary between a garnet and a perovskite phase at T = 900–1,650 K and P = 3–8 GPa was determined as occurring at P (GPa) = 9.0−0.0023 × T (K). The transition pressure determined in our study is in general agreement with that observed in previous high-pressure
experiments. The slope, dP/dT, of the transition determined in our study is consistent with that calculated from calorimetry data. 相似文献