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1.
Density-functional electronic structure calculations are used to compute the equilibrium constants for 26Mg/ 24Mg and 44Ca/ 40Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10 3ln ( K) at 25 °C, of −5.3, −1.1, and +1.2 for 26Mg/ 24Mg exchange between calcite (CaCO 3), magnesite (MgCO 3), and dolomite (Ca 0.5Mg 0.5CO 3), respectively, and Mg 2+(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For 44Ca/ 40Ca exchange between calcite and Ca 2+(aq) at 25 °C, the calculations predict values of +1.5 for Ca 2+(aq) in 6-fold coordination and +4.1 for Ca 2+(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe 3+-hematite and Fe 2+-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe 3+(aq) and Fe 2+(aq) species. 相似文献
2.
The use of ultrasonically modulated electron resonance (UMER) to study S-state ions in substitutional sites of mineral single crystals is discussed. Mn 2+ and Fe 3+ in natural single crystals of tremolite are used as examples. Combined electron paramagnetic resonance (EPR) and UMER measurements establish almost certainly that Mn 2+ enters predominantly into the distorted M4 sites occupied by Ca 2+ in the ideal tremolite structure and only to a minor extent into the M1, M2 and M3 sites normally occupied by Mg 2+. Fe 3+ in tremolite gives rise to the well known high spin resonance with g eff?4.3 but there is considerable uncertainty as to the site of the impurity ion. 相似文献
3.
Consideration of experimental data on the distribution of Mg 2+ between olivine and silicate liquid clearly demonstrates that the distribution coefficient (K Mg) is dependent upon variations in temperature, pressure and melt composition, largely because these variables control the solubility of Mg 2+ in the melt phase. Attempts to minimize composition dependence of K Mg, utilizing various activity-composition models for silicate melts, have been partially successful. Composition-related effects do not appear to be large, however, for melts of restricted range in composition (e.g., tholeiitic or lunar basalts) as long as the contents of alkalis and the alkali/alumina ratio are relatively small (on a molar basis). For such melts, K Mg may be used as a reliable geothermometer. By analogy, these conclusions can be extended to the distribution of other divalent cation such as Fe 2+, Mn 2+, Ni 2+ and Co 2+. 相似文献
4.
Multiple regression analysis on an extended dataset has been performed to refine the relationship between temperature, pressure, composition and the Fe–Mg distribution between garnet and clinopyroxene. In addition to a significant dependence between the distribution coefficient KD and X GrtCa and X GrtMg#, as shown by the experimental data, the effect of X GrtMn has also been incorporated using data from natural Mn‐rich garnet–clinopyroxene pairs. Multiple regression of data ( n=360) covering a large span in pressure, temperature and composition from 27 experimental datasets, combined with 49 natural high‐Mn granulites from Ruby Range, Montana, USA, and Karnataka, India, yields the P–T –compositional relationship ( r2=0.98): where KD=(Fe 2+/Mg) Grt/(Fe 2+/Mg) Cpx, X GrtCa=Ca/(Ca+Mn+Fe 2++Mg) in garnet, X GrtMn= Mn/(Ca+Mn+Fe 2++Mg) in garnet, and X GrtMg#=Mg/(Mg+Fe 2+) in garnet. The Fe 2+–Mg equilibrium between garnet and clinopyroxene does not seem to be affected by variations in the sodic content of the co‐existing clinopyroxene in the range X CpxNa=0–0.51. Comparisons between the new and former calibrations of the garnet–clinopyroxene Fe 2+–Mg geothermometer clearly demonstrate how the various parameters in each case affect the calculated temperatures. Application of the new expression gives reasonable results for natural garnet–clinopyroxene pairs from various rock types and settings, and should be preferred to previous formulations. Using the new calibration to the self‐consistent dataset of Pattison & Newton ( Contributions to Mineralogy and Petrology, 1989, 101, 87–103) suggests a systematic deviation with regard to both temperature and composition between their dataset and the datasets used in the present calibration. 相似文献
5.
Some species of sulphate‐reducing bacteria (SRB) are known to mediate the formation of dolomite and Mg‐calcite. However, their exact role in the mineralization process remains elusive. Here, we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by SRB can occur in the absence of living cells, through passive mineralization of their exopolymeric substances (EPS). SRB capable of mediating dolomite were cultivated in the laboratory, allowing them to secrete EPS. Microbial activity within the cultures was subsequently inhibited with antibiotics. Only after this step, Ca 2+ and Mg 2+ were added to the solution and carbonate minerals could form. Mg‐calcite and disordered Ca‐dolomite precipitated in association with EPS. The mol.% of Mg 2+ in the crystals increased with longer incubation times. This result demonstrates that organic compounds produced by SRB can mediate the formation of Ca‐Mg carbonates in the absence of an active metabolism. 相似文献
6.
Lithian ferrian enstatite with Li 2O = 1.39 wt% and Fe 2O 3 7.54 wt% was synthesised in the (MgO–Li 2O–FeO–SiO 2–H 2O) system at P = 0.3 GPa, T = 1,000°C, fO 2 = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å3. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula M2(Mg0.59Fe 0.21 2+ Li0.20) M1(Mg0.74Fe 0.20 3+ Fe 0.06 2+ ) Si2O6. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe3+ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe3+ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe2+) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe3+, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe2+-Mg ordering process in octahedral sites. 相似文献
7.
Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe 2+ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe 2+/Fe total ratio on An content demonstrates that Fe 2+ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe 3+ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe 3+ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe 3+, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions. 相似文献
8.
A simple theory is developed which relates cation ordering in the olivine structure to compositional trends in the vibrational spectra of olivines. Quantitative results for (Mg, Mn) and (Fe, Mn) olivines indicate that Mn 2+ favors the M2 site relative to both Mg 2+ and Fe 2+. 相似文献
9.
The instability of transition metal dolomites [CaR 2+(CO 3) 2 where R 2+ is Fe, Co, Ni, Cu, or Zn] and the limited substitution of transition metal cations for Mg in the dolomite structure can be accounted for by the effect of octahedral distortion. For example, trigonal elongation of the Fe octahedron, due to the Jahn-Teller effect, observed in siderite and ankerite, results in elongation of the Ca octahedron which is sensitive to distortion because the radius of Ca 2+ is close to the upper limit for octahedral coordination. Co, Ni, Cu, Zn octahedra are also thought to be deformed, relative to Mg octahedra, in carbonates.The free energy of formation (ΔG of) of R 2+CO 3 becomes more positive with increasing octahedral distortion. Estimated ΔG of(dolomite) as well as stabilities and solubility limits of R 2+ in natural and synthetic dolomites suggest a series in order of decreasing stability: Mg > Mn > Zn > Fe > Co > Ni > Cu.ΔG of(est.) for the terminal Fe-dolomite solid solution [72 mol% CaFe(CO 3) 2] in the system CaCO 3-MgCO 3-FeCO 3 may represent an empirical threshold value for dolomite stability which lies between ΔG of for Mn- and Zn-dolomites. While Zn-dolomite is probably not a stable phase, very extensive solid solution toward CaZn(CO 3) 2 is to be expected in the system CaCO 3-MgCO 3-ZnCO 3. The tendency for transition metal dolomites to contain excess CaCO 3 can also be accounted for in terms of octahedral distortion and AG of. 相似文献
10.
Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity ( fO 2) and temperature ( T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg 2+, Mn 2+, Co 2+, Ni 2+, Zn 2+) and Cu 2+/Cu + with increasing fO 2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO 2 implies a reciprocal relationship between a(Fe 2+O) and a(Fe 3+O 1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO 2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO 2 in silicic, but less so in mafic bulk systems. 相似文献
11.
The crystal structure and site preference of Co 2+ in a synthetic Co 1.10Mg 0.90SiO4 olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co 2+ in olivine. The strong preference of Co 2+ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg 2+ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe 2+, Mg 2+, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines. 相似文献
12.
Polarized absorption spectra of natural piemontite (Ca1.802Mn
2+0.178
Mg0.025) (Mn
3+0.829
Fe
3+0.346
Al1.825) [(Si2.992Al0.008) O12OH], viridine (Al1.945Mn
3+0.033
Fe
3+0.063
Mg0.003) [O|Si0.970 O4], and kanonaite (Al1.291Mn
3+0.682
Fe
3+0.019
) [O|Si1.006 O4] were measured at 295 and ca. 100 K. For piemontite, lowering the temperature resulted in a sharpening of broad bands in the 10 000–25 000 cm−1 region supporting their assignment to single ion Mn3+ in M3 non-centrosymmetric sites. Alternatively, in kanonaite, temperature behaviour pointed to a slightly stronger influence of vibronic coupling on strong bands near 16 000 and 22 000 cm−1, which supported an interpretation of Mn3+ in nearly centrosymmetric M1 sites. Measurements at ca. 100 K show pronounced fine structure in the viridine spectra which is attributed to Fe3+. The ɛ values for Mn3+ spin-allowed bands in the three minerals lie in the range 18 to 227 [1·g-atom−1·cm−1]. For the same band and polarisation, ɛ values in Mn3+-bearing andalusite-type minerals viridine and kanonaite are the same, which indicates an absence of strong magnetic coupling effects between Mn3+ ions in the andalusite type structure down to ca. 100 K. In silicates, the high ɛ values for Mn3+ spin-allowed bands, in comparison to those obtained for Fe2+ spin-allowed bands from sites of “similar distortion”, is attributed to a higher degree of covalency in the Mn3+-O bonds compared to the Fe2+-O bonds, as a result of the higher valence state of manganese. 相似文献
13.
The two most abundant network-modifying cations in magmatic liquids are Ca 2+ and Mg 2+. To evaluate the influence of melt structure on exchange of Ca 2+ and Mg 2+ with other geochemically important divalent cations ( m-cations) between coexisting minerals and melts, high-temperature (1470-1650 °C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg 2SiO 4-CaMgSi 2O 6-SiO 2 with ?1 wt% m-cations (Mn 2+, Co 2+, and Ni 2+) substituting for Ca 2+ and Mg 2+. The bulk melt NBO/Si-range ( NBO/Si: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca 2+, Ca 2+- NBO) is linearly related to NBO/Si, whereas fraction of Mg 2+- NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of KD(m−Mg) with NBO/Si(Ca) for the exchange equilibrium, mmelt + Mg olivine ? molivine + Mg melt, is linear. KD(m−Mg) decreases as an exponential function of increasing ionic potential, Z/ r2 ( Z: formal electrical charge, r: ionic radius—here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, Δ H, decreases linearly with increasing Z/ r2 [Δ H = 261(9)-81(3)· Z/ r2 (Å −2)]. From existing information on (Ca,Mg)O-SiO 2 melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Qn-species in the melts. The negative ∂ KD(m−Mg)/∂( Z/ r2) and ∂(Δ H)/∂( Z/ r2) is because increasing Z/ r2 is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Qn-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts. 相似文献
14.
Water quality and hydrochemistry of Shariatpur district were evaluated in terms of hydrochemical composition and some important physico-chemical parameters. The groundwater of the study area is good for drinking, domestic as well as for irrigation purposes. Among the major ions, shallow tube well waters give higher concentration of Ca 2+ which ranges from 24 to 260 mg/L. The deep tubewell waters show higher concentration of Na + which varies from 74 to 582 mg/L during dry season. Among the trace elements most of the shallow aquifer samples show higher concentration of Fe 2+, Mn 2+ and As. Concentration of Fe 2+ varies from 0.655 to 18.8 mg/L, and Mn 2+ from trace to 0.868 mg/L during dry period. Hydrochemical analyses reveal significant seasonal variation in water quality of shallow aquifer. Both the shallow aquifer and the surface water of the study area are predominantly of Ca–Mg–HCO 3 type, while the deep aquifer water is mainly of Na–K–Cl–SO 4 type with slight inclination to Ca–Mg–HCO 3 type. The study area is suitable for groundwater development if comprehensive and holistic approaches towards water resource management are taken into consideration. 相似文献
15.
Partitioning of Ca, Mn, Mg, and Fe 2+ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400°C with melts in equilibrium with forsterite-rich olivine (Fo >95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q 4, Q 3, and Q 2 type with nonbridging oxygen, therefore, in the Q 3 and Q 2 units.For melts with high Q 3/Q 2-abundance ratio (corresponding to NBO/T near 1), increasing Na/(Na+Ca) [and Na/(Na+Ca+Mn+Mg+Fe 2+)] results in a systematic decrease of the partition coefficients, K Caol/melt, K Mnol/melt, K Mgol/melt, and K Fe2+ol/melt, because of ordering of the network-modifying Ca, Mn, Mg, and Fe 2+ among nonbridging oxygen in Q 3 and Q 2 structural units. This decrease is more pronounced the smaller the ionic radius of the cation. With decreasing Q 3/Q 2 abundance ratio (less-polymerized melts) this effect becomes less pronounced.Activity-composition relations among network-modifying cations in silicate melts are, therefore, governed by availability of energetically nonequivalent nonbridging oxygen in individual Q n-species in the melt. As a result, any composition change that enhances abundance of highly depolymerized Q n-species will cause partition coefficients to decrease. 相似文献
16.
The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca 0.1Mn 1.9 Mg 1.25Fe 2+ 3.56Fe 3+ 0.38Si 7.81O 22(OH) 2 and Ca 0.24Mn 1.57 Mg 2.27 Fe 2+ 2.76Fe 3+ 0.32Si 7.84O 22(OH) 2. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ? M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of ?16.6 kJ mol ?1 and ?14.9 kJ mol ?1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples. 相似文献
17.
Second-order zero-field splitting ( ZFS) parameters from the literature for Fe 3+ in twelve and for Cr 3+ in seven minerals substituting for Al were evaluated by application of the superposition model. For Fe 3+ in monoclinic site symmetries a fair agreement of the observed splitting patterns with those calculated from the crystal structure data was observed in most cases, but the distortions for Fe 3+ appear to be usually larger than those of the unrelaxed Al sites. In cases of not too large local relaxation the unknown sign of the axial ZFS parameter b 0 2 could be predicted, in two cases a different sign than that reported was postulated. In cordierite and scolecite the reported EPR spectra could thus be assigned to the sites with larger average bond distances. For Fe 3+ in beryl the relaxation of the axial site can be deduced within narrow limits. For Cr 3+ significantly larger differences between observed and calculated ZFS patterns are found suggesting additional relaxations due to the non-spherical electron distribution in the ground state of this ion. 相似文献
18.
Partitioning of Ni 2+, Co 2+, Fe 2+, Mn 2+ and Mg 2+ between olivine and silicate melts has been determined near the join (Mg 0.5·-Fe 0.5) 2SiO 4-K 2O 4SiO 2 and for seven different basaltic compositions. The experiments were made at 1 atm total pressure, 1500-1100°C, and under moderate to reducing oxygen fugacities. The concentration factor, defined as KMO = (MO) ol/(MO) liq (molar ratio), increases markedly for all the cations studied as the olivine component of the liquid decreases. Much of the increase in KMO is considered as due to the compositional effect of the coexisting liquid: the temperature effect on KMO is probably opposite to the compositional effect ( KMO decreases as temperature decreases).The partition coefficient KMO-MgO = (MO/MgO) ol/(MO/MgO) liq for the reaction, M ol2+ + Mg liq2+ = M liq2+ + Mg ol2+. is relatively constant over a wide range of SiO 2 content of the liquid, except in the case of Ni 2+. The partition coefficients have similar ranges both in synthetic and natural rock systems: KNiO-MgO = 1.8–3.0, KCoO-MgO = 0.6–0.8, KFeO-MgO = 0.27–0.38, and KMnO-MgO = 0.23–0.32. There is a systematic variation in the partition coefficient KMO-MgO with the composition of liquid; KMO-MgO increases with increasing SiO 2 content of melt. The partition coefficients for Co 2+, Fe 2+ and Mn 2+ are useful to test the equilibration of olivine with magma of a wide compositional range. 相似文献
19.
Structural and compositional data as well as 57Fe Mössbauer parameters were determined on a natural Mn-rich monoclinic ilvaite crystal (ideal composition CaFe 2 2+ Fe 3+Si 2O 8(OH)) which was used for electrical conductivity and thermopower measurements (part 2 of this paper). A zonar structure was found by electron microprobe analysis with a strong decrease in Mn concentration from the rim to the centre of the crystal in a plane perpendicular to the [001] direction. X-ray powder diffraction analysis of the most Mn-rich composition was performed. Mn 2+ cations populate preferentially M2 sites of the ilvaite unit cell (space group P21/ a), to a lower extent they reside on M1 and a reduced part is on Ca sites. The monoclinic angle was determined to β=90.178(4)°. The structural results are compared to literature data for other natural Mn-rich as well as low-impurity ilvaites; this concerns in particular the lattice b parameter and the undecided issue of the varying β angle. In the literature, the order parameter σ, which describes the varying degree of ordering of Fe 2+–Fe 3+ pairs on M11 and M12 sites in chains running parallel to the [001] direction, and structural defects are thought to be related to β. The interrelationship between β and σ with respect to a possible twin domain structure is discussed. Various 57Fe Mössbauer spectra were recorded between 151 K and 327 K. Mössbauer parameters and Fe 2+/Fe 3+ concentration ratios were determined from the fits to the spectra. Fitting of subspectra was accomplished with the idea to find assignments of Fe 2+ and Fe 3+ doublets in agreement with X-ray results. The fraction of Mn 2+ substituting Fe 2+ on M1 sites could be estimated. 相似文献
20.
Microprobe analysis, single crystal X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/ mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) Å and chemical formula (K 0.729) C (Na 0.029) B (Si 10.498 Al 1.502) T1 (Al 2.706 Fe 0.294 2+ ) T2 (Mg 0.735 Mn 0.091 Fe 1.184 2+ ) AO 30. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe 2+ content, unlike the c parameter which does not seem to be affected by chemical composition. The determination of the amount of each element on the mineral surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si 2p, Al 2p, Mg 1s and Fe 2p core levels, suggests that Fe presents Fe 2+ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si 4+ and Si 1+ components, respectively, both in tetrahedral coordination. The binding energy of Al 2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L 2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site. 相似文献
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