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1.
2.
A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling 总被引：9，自引：0，他引：9
Paolo Nimis 《Contributions to Mineralogy and Petrology》1995,121(2):115-125
The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure
simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring
direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable
governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted
construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward
geobarometric formulation in the absence of direct X-ray analysis is:
P(kbar)=698.443+4.985⋅AlT−26.826⋅Fe2+
M1−3.764⋅Fe3+
+53.989⋅AlM1+3.948⋅Ti+14.651⋅Cr
−700.431⋅Ca−666.629⋅Na−682.848⋅MgM2−691.138⋅Fe2+
M2−688.384⋅Mn−6.267⋅(MgM2)2−4.144⋅(Fe2+
M2
where:
(Fe2+
M1⋅MgM2)/(Fe2+
M2⋅MgM1)=e**(0.238⋅R3++0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R3+=AlM1+Fe3++Ti+Cr,
with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev.
≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved
cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively
to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data
also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene
at high pressure. Within the considered compositional space at given P-T, a
CaO and a
SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes
from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al2O3 (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer.
Received: 3 April 1994 / Accepted: 23 December 1995 相似文献
3.
Renate Schumacher 《Contributions to Mineralogy and Petrology》1991,108(1-2):196-211
Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe3+-rich epidote-group minerals suggests the presence of Fe3+ in most of these amphiboles. An empirical Fe3+ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) c =-1.479 Na(M4) c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) u and ΣFM(M4) u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) u +1.479 ΣFM u ). From the deficient oxygen the Fe3+ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe3+ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe2+/(Fe2++Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe2+/(Fe2++Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X Ab/X Ed, where X Ab=Na/(Na+Ca) in plagioclase and X Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi-1)Hbl+H2O. 相似文献
4.
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
5.
6.
A crystallographic and m?ssbauer spectroscopy study of Fe 总被引：1，自引：0，他引：1
The crystal chemistry of garnet solid solutions on the Fe
3
2+
Al2Si3O12-Fe
3
2+
Fe
2
3+
Si3O12 (almandine-“skiagite”) and Ca3Fe
2
3+
Si3O12-Fe
3
2+
Fe
2
3+
Si3O12 (andradite-“skiagite”) joins have been investigated by single-crystal X-ray structure refinements and M?ssbauer spectroscopy.
Together, these two solid solution series encompass the complete range in Fe3+/ΣFe from 0.0 to 1.0. All garnets are isotropic and were re0fined in the Ia
d space group.
Small excess volumes of mixing are observed in andradite-“skiagite” solid solutions (W
v
=1.0±0.2 cm3 mol-1) and along the almandine-“skiagite” join (W
v
=-0.77±0.17cm3 mol-1). The octahedral (Al, Fe3+)-O bond lengths show a much greater variation across the almandine-skiagite join compared to the andradite-skiagite garnets.
The dodecahedral (X)-O bond lengths show the opposite behaviour. In andradite-“skiagite” solid solutions, the octahedral site
passes from being flattened to elongated parallel to the 3 axis of symmetry with increasing “skiagite” content. A perfect
octahedron occurs in a composition of ≈35 mol% “skiagite”. The occupancy of the neighboring dodecahedral sites has the greatest
effect on octahedral distortion and vice versa. The M?ssbauer hyperfine parameters of Fe2+remain constant in both solid solutions. The hyperfine parameters of Fe3+ (at room temperature: centre shift=0.32–0.40 mm/sec, quadrupole splitting (QS)≈0.21–0.55 mm/ sec) indicate that all Fe3+ is in octahedral coordination. The Fe3+ parameters are nearly constant in almandine-“skiagite” solid solutions, but vary significantly across the andradite-“skiagite”
join. The structural unit that contributes to the electric field gradient of the octahedral site is different from that of
the coordinating oxygen polyhedron, probably involving the neighboring dodeca-hedral sites. 相似文献
7.
The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe3+/Fe2+ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe3+/Fe2+ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO2) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed. 相似文献
8.
This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical
parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power
and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and
unpublished data. Selected compositions include the following cations: Mg2+, Al3+, Si4+, Ti4+, V3+, Cr3+, Mn2+, Mn3+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe3+ or Ni2+ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral
edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications
to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and
for identifying inconsistencies and inaccuracies in experimental data.
Received: 19 February 2001 / Accepted: 1 June 2001 相似文献
9.
Mössbauer spectra for two riebeckite minerals were collected at temperatures in the range 4.2 to 500 K. The magnetic-ordering temperatures were found to be 33±1 and 31±1 K respectively. Fitting the paramagnetic spectra with a discrete number of doublets (three or four) did not lead to consistent results. Instead, a superposition of an Fe3+ (M2) doublet and one distributed ferrous component was found to produce adequate fits with reasonable parameter values. For both samples, a minor fraction of ferrous ions was observed to be present at the M4 sites and for one of the samples at the M2 sites as well. The temperature variations of the center shifts were well reproduced using the Debye model of the lattice vibrational spectrum to evaluate the second-order Doppler shift. The characteristic Mössbauer temperatures were calculated to be in the range 340–390 K for Fe2+, and 520 K for Fe3+. The temperature dependences of the various ferrous quadrupole splittings could not be explained in terms of the point-charge model and assuming a temperature-independent energy-level scheme for the 5D term. It is suggested that a gradual change with temperature of the orbital-level splittings takes place. All calculations yielded a positive sign for the principal component of the electric field gradient (EFG). The spectrum recorded at 4.2 K for one of the riebeckites was fitted with a superposition of an Fe3+ and a Fe2+ hyperfine-field distribution, the latter one primarily characterizing the Fe2+ (M1) cations. The following relevant hyperfine data were calculated: H hf=161 kOe, ΔE Q=3.11 mm/s, and V zz<0, all referring to the maximum-propability values. For the second riebeckite at 4.2 K, an additional distributed ferrous component could independently be resolved. The two maximum-probability hyperfine fields were found to be 189 and 98 kOe and the corresponding ΔE Q values 3.10 and 2.67 mm/s. Both components exhibit a negative V zz. The subspectra were attributed to M1 and M3 sites respectively. The Fe3+ hyperfine fields are 548+-2 kOe for both riebeckites. The different values found for the Fe3+ quadrupole shift 2?Q for the two samples is explained by a different angle between the hyperfine field and the EFG's principal axis. The magnetic spectra recorded at 15 K and higher, could not be reproduced adequately with reasonable parameter values. 相似文献
10.
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
11.
The temperature dependence of the absorption spectra of ilvaite, Ca(Fe2+,Fe3+)Fe2+Si2O8(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P21/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and
450 K in a frequency range between 600 and 23 000 cm−1. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg
theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling
with the Fe2+-Fe3+ ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH)
x
. 相似文献
12.
Geochemical modeling of coal mine drainage, Summit County, Ohio 总被引：4，自引：1，他引：4
A. Foos 《Environmental Geology》1997,31(3-4):205-210
Geochemical modeling was used to investigate downstream changes in coal mine drainage at Silver Creek Metro-park, Summit
County, Ohio. A simple mixing model identified the components that are undergoing conservative transport (Cl–, PO4
3–, Ca2+, K+, Mg2+ and Na+) and those undergoing reactive transport (DO, HCO3
–, SO4
2–, Fe2+, Mn2+ and Si). Fe2+ is removed by precipitation of amorphous iron-hydroxide. Mn2+ are removed along with Fe2+ by adsorption onto surfaces of iron-hydroxides. DO increases downstream due to absorption from the atmosphere. The HCO3
– concentration increases downstream as a result of oxidation of organic material. The rate of Fe2+ removal from the mine drainage was estimated from the linear relationship between Fe+2 concentration and downstream distance to be 0.126 mg/s. Results of this study can be used to improve the design of aerobic
wetlands used to treat acid mine drainage.
Received: 4 June 1996 · Accepted: 17 September 1996 相似文献
13.
A. Dal Negro S. Carbonin C. Domeneghetti G. M. Molin A. Cundari E. M. Piccirillo 《Contributions to Mineralogy and Petrology》1984,86(3):221-229
A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated
with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model
and/or crystallization processes prevailing at high pressure).
The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple
substitution AlVIFe
M1
2
Ti4+⇌Cr3+ Fe3+Mg
M1
2+
corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe2+ (mg) for the clinopyroxene. A relative Ca2+ increase in M2 ensures the necessary charge balance. However, Na+ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to Fe
M2
2+
depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron)
by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show
distinct intracrystalline variations, essentially reflecting lower AlVI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens).
The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger
degree of mantle melting at shallower depths relative to the Leura analogues. 相似文献
14.
Iron sulfide oxidation and the chemistry of acid generation 总被引：3，自引：0，他引：3
Acid mine drainage, produced from the oxidation of iron sulfides, often contains elevated levels of dissolved aluminum (AI),
iron (Fe), and sulfate (SO4) and low pH. Understanding the interactions of these elements associated with acid mine drainage is necessary for proper
solid waste management planning. Two eastern oil shales were leached using humidity cell methods. This study used a New Albany
Shale (4.6 percent pyrite) and a Chattanooga Shale (1.5 percent pyrite). The leachates from the humidity cells were filtered,
and the filtrates were analyzed for total concentrations of cations and anions. After correcting for significant solution
species and complexes, ion activities were calculated from total concentrations. The results show that the activities of Fe3+, Fe2+, Al3+, and SO4
2− increased due to the oxidation of pyrite. Furthermore, the oxidation of pyrite resulted in a decreased pH and an increased
pe+pH (redox-potential). The Fe3+ and Fe2+ activities appeared to be controlled by amorphous Fe(OH)3 solid phase above a pH of 6.0 and below pe+pH 11.0. The Fe3+, Fe2+, and SO4
2− activities reached saturation with respect to FeOHSO4 solid phase between pH 3.0 and 6.0 and below pe+pH 11.0 Below a pH of 3.0 and above a pe+pH of 11.0, Fe2+, Fe3+, and SO4
2− activities are supported by FeSO4·7H2O solid phase. Above a pH of 6.0, the Al3+ activity showed an equilibrium with amorphous Al(OH)3 solid phase. Below pH 6.0, Al3+ and SO4
2− activities are regulated by the AlOHSO4 solid phase, irrespective of pe+pH. The results of this study suggest that under oxidizing conditions with low to high leaching
potential, activities of Al and Fe can be predicted on the basis of secondary mineral formation over a wide range of pH and
redox. As a result, the long-term chemistry associated with disposal environments can be largely predicted (including trace
elements). 相似文献
15.
Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach 总被引：3，自引：1，他引：3
Mario Aigner-Torres Jon Blundy Peter Ulmer Thomas Pettke 《Contributions to Mineralogy and Petrology》2007,153(6):647-667
Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric
pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180°C), and at
three different oxygen fugacities (fO2 = IW, QFM, air) in order to assess the effect of fO2 on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS.
Most trace element Ds increase slightly as temperature decreases, except for D
Zr, D
Fe, D
Eu and D
Cr that vary systematically with fO2. Applying the Lattice Strain Model to our data suggests the presence of Fe2+ entirely in the octahedral site at highly to moderate reducing conditions, while Fe3+ was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework
for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO2and confirm the greater compatibility of Eu2+, which has an ionic radius similar to Sr, relative to Eu3+ in plagioclase and the higher Eu2+/ Eu3+ under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu2+–Sr and Fe–Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks. 相似文献
16.
The incorporation of hydrogen into ferrosilite, Fe-bearing enstatite and orthopyroxene containing different trivalent cations (Cr3+ and Al3+, Cr3+ and Fe3+) was investigated experimentally at 25 kbar. Hydrogen concentration was determined by FTIR-spectroscopy on oriented crystal sections and by secondary ion mass spectroscopy, whereas Mößbauer spectroscopy and optical spectroscopy were used to characterise the valence state of Fe in orthopyroxene. Results suggest that hydrogen incorporation in ferrosilite is achieved by a similar mechanism as in pure enstatite. In Cr-bearing samples, however, hydrogen incorporation is reduced by the presence of other trivalent cations by an increased tendency to form Tschermaks substitutions, e.g. Si T 4+ + Mg M1 2+ ? Al T 3+ + Cr M1 3+ . Thus, hydrogen solubility in natural orthopyroxenes from the Earth’s mantle, containing significant amounts of Cr3+, Al3+, and Fe3+, may be much more limited than expected from their trivalent cation content, as a large fraction of the trivalent cations does not participate in H-incorporating reactions as 2 Mg M1 2+ ? M M1 3+ + VM1 + H i + . 相似文献
17.
G. Diego Gatta Romano Rinaldi K. S. Knight G. Molin G. Artioli 《Physics and Chemistry of Minerals》2007,34(3):185-200
The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula M2(Mg0.856Ca0.025Fe2+
0.119) M1(Mg0.957Fe2+
0.011Fe3+
0.016Cr0.011Al0.005)Al0.032Si1.968O6, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction
at several temperatures starting from 1,200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α = V
−1(∂V/∂T) P0, varies linearly with T. The axial thermal expansion coefficients, αj = l
j−1(∂l
j/∂T)P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ɛ//a, ɛ//b, ɛ//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at ΔT = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA
n
OA3n
and TB
n
OB3n
, are kinked differently. At room-T, the TB
n
OB3n
chain is more strongly kinked by about 23° than the TA
n
OA3n
chain. With increasing T, the difference decreases by 3° for the TB
n
OB3n
chain. The intersite cation exchange reaction between M1 and M2 (Mg2+ and Fe2+) shows a slight residual order at 1,200°C followed by reordering with decreasing temperature although seemingly not with
a definite progressive trend. At the lowest temperature reached (150°C), reordering has occurred with the same value of partitioning
coefficient K
D as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts
of Fe3+, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with
consequent important petrologic and geological implications. 相似文献
18.
The effect of CaO, Na2O, and K2O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe3+/Fe2+ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe3+/Fe2+ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO2–TiO2–Al2O3–FeO–Fe2O3–MgO–CaO–Na2O–K2O–P2O5 melts at air conditions. 相似文献
19.
The crystal structure of Bi2Al4−x
Fe
x
O9 compounds (x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi2M4O9 compounds (M = Al, Fe3+) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm−1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm−1 band group, which is assigned to Al(Fe3+)–O stretching vibrations of the corner-sharing MO4 tetrahedra. The Bi2Al4O9 and Bi2Fe4O9 endmembers display single bands with maxima centred at 922 and 812 cm−1, respectively. Intermediate Bi2Al4−x
Fe
x
O9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm−1 range are assigned to low-energy stretching vibrations of the MO4 tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm−1 range are essentially determined by M–O stretching modes of the M cations in octahedral coordination. 相似文献
20.
Phase transformations in a natural sample of hedenbergite ((Ca0.93Fe0.61Mn0.34Mg0.08Na0.01Zn0.02Al0.003)Si2O6) have been studied by X-ray diffraction up to 40 GPa at ∼ 1200°C in a diamond anvil cell interfaced with a laser heating
system. The starting hedenbergite phase decomposed into a garnet plus γ-spinel and stishovite at ∼ 14 GPa; then into garnet plus stishovite and wüstite at ∼ 18 GPa; and finally into perovskite
plus stishovite and wüstite at pressures higher than ∼ 24 GPa. On decompression to 0.1 MPa, all the high pressure phases are
retained except for the cubic perovskite, which reverts back into the ɛ-CaSiO3 phase, in accordance with previous reports. Energy-dispersive SEM analyses show that the garnet is present as a calcium-rich
ABO
3-type phase. As no garnet phase has been previously observed either in pure CaSiO3 or in pure CaMgSi2O6, it appears that the observed calcium-rich garnet phase has been stabilized by the presence of other cations such as the
Na+, Zn2+, Mn2+, Fe2+, Mn3+, Fe3+ and Al3+. 相似文献