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1.
Quantum mechanical calculations based on the density functional theory (DFT) are used to study the crystal structures of dioctahedral 2:1 phyllosilicates. The isomorphous cation substitution is investigated by exploring different substitutions of octahedral Al3+ by Mg2+ or Fe3+, tetrahedral substitution of Si4+ by Al3+, and different interlayer cations (IC) (Na+, K+, Ca2+, and Mg2+). Samples with different kinds of layer charges are studied: only tetrahedrally charged, only octahedrally charged, or mixed octahedral/tetrahedral charged. The effect of the relative arrangements of these substitutions on the lattice parameters and total energy is studied. The experimental observation of segregation tendency of Fe3+ and dispersion tendency of Mg2+ in the octahedral sheet is reproduced and explained with reference to the relative energies of the octahedral cation arrangements. These energies are higher than those due to the IC/tetrahedral and IC/octahedral relative arrangements. The tetrahedral and octahedral substitutions that generate charged layers also tend to be dispersed. The octahedral cation exchange potentials change with the IC-charge/ionic radius value.  相似文献   

2.
We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg3Al2Si3O12 (pyrope) — Mg4Si4O12 (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO3 garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state 27Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.  相似文献   

3.
A well crystallized and homogeneous specimen of lizardite from Monte Fico, Elba, Italy, has been studied by Mössbauer and Fourier transform infrared (FTIR) spectrometries. One of the aims was the determination of the oxidation state and the distribution of iron in the structure of this reference sample. Mössbauer data indicate the presence of octahedral ferrous iron, octahedral ferric iron and tetrahedral ferric iron (59.9, 31.3 and 8.8% of total iron, respectively). The existence of only one octahedral site, previously suggested by X-ray structure refinement, is confirmed. The occurrence of tetrahedrally coordinated iron is indicated also by FTIR spectrometry, in particular by the presence of an absorption band at 790 cm–1. Based also on new electron microprobe data, the improved crystal chemical formula for lizardite from Monte Fico is: (Mg2.74Fe2+ 0.10Fe3+ 0.05Al0.11)Σ=3.00 ?· (Si1.94Al0.05Fe3+ 0.01)Σ=2.00O5.05(OH)3.95.  相似文献   

4.
The 29Si and 27Al nuclear magnetic resonance (NMR) analysis of synthetic trioctahedral phyllosilicates 2:1, with tetrahedral ratios Al T/(Si + Al T) ranging from 0 to 0.5, has shown that the ditrigonal distortion of tetrahedral rings (angle ) is the main factor controlling chemical shift values of tetrahedral components in both signals. The increase of ditrigonal rotation angle shifts these components towards more positive values. For each sample, the composition of tetrahedral and octahedral sheets determine the value of , and from this parameter, the mean tetrahedral Tot angle and the chemical shift values of components are deduced. For a given environment, variations on ditrigonal angle are responsibles for the observed evolution of chemical shift values with bulk composition. The comparative analysis of micas and saponite samples has demonstrated that the location of compensating charge (interlayer and octahedral sheet) does not affect chemical shift values.  相似文献   

5.
 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K2 [6](Al3Mg)[4](Si7Al)O20(OH)4. Values of the atomic exchange interaction parameters J n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.  相似文献   

6.
The crystal structure of a synthetic CaFe3+Al-SiO6 pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe3+ and 0.18 Al3+. Within the tetrahedral T site, Si4+ (0.50), Al3+ (0.41) and Fe3+ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe3+ ions with respect to Al3+ ions in CaFe3+AlSiO6 is about 10 kcal/mole, which is much higher than that found in Y3Al x Fe5–2O12 garnets. Topologically the structure of CaFe3+AlSiO6 is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO6. For CaM3+ AlSiO6 clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.  相似文献   

7.
Summary Fine-grained homogeneous powder samples of thirteen trioctahedral micas, mostly intermediate members of the phlogopite – annite solid solution series, and samples close to the phlogopite, fluor-phlogopite and tetra-ferriphlogopite end members have been examined at the potassium K-edge by X-ray absorption fine structure spectroscopy. The interlayer K+ cation is in a coordination that is certainly lower than 12, in contrast to what is expected from the ideal hexagonal symmetry model of the mica structure, and approaches – but it does not reach – coordination 6, as it should be when the effective ligands are the three nearest outer bridging oxygens of two facing upper and lower tetrahedral sheets. The observed range of coordinations implies that only some of the three inner bridging oxygen atoms in each sheet are involved, thus leading to 6±(1 … 6) effective configurations depending on the composition of the individual mica terms. The effective coordination number was found to vary continuously with composition from 11 to 7 and to be related to the tetrahedral rotation angle (α) according to two different linear relationships for the phlogopite – annite series (Fe2+Mg−1 exchange vector, involving the octahedral sheet only) and the phlogopite – tetra-ferriphlogopite series (Fe3+Al−1 vector, involving the tetrahedral sheet), respectively. Substitutions affecting either the A cation in the interlayer or the X anion in the octahedral sheet also affect the observed trends. In particular, the latter substitution effect is best seen in two near end member phlogopites, where the fluorine to hydroxyl substitution (F− (OH)−−1 exchange vector), which greatly changes the α tetrahedral rotation angle is, reflected in the experimental K XANES spectra by modifying not only the energy but also the intensities of most multiple scattering features.  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.
Bulk and slab geometry optimizations and calculations of the electrostatic potential at the surface of both pyrophyllite [Al2Si4O10(OH)2] and talc [Mg3Si4O10(OH)2] were performed at Hartree–Fock and DFT level. In both pyrophyllite and talc cases, a modest (001) surface relaxation was observed, and the surface preserves the structural features of the crystal: in the case of pyrophyllite the tetrahedral and octahedral sheets are strongly distorted with respect to the ideal hexagonal symmetry (and basal oxygen are located at different heights along the direction normal to the basal plane), whereas the structure of talc deviates slightly from the ideal hexagonal symmetry (almost co-planar basal oxygen). The calculated distortions are fully consistent with those experimentally observed. Although the potentials at the surface of pyrophyllite and talc are of the same order of magnitude, large topological differences were observed, which could possibly be ascribed to the differences between the surface structures of the two minerals. Negative values of the potential are located above the basal oxygen and at the center of the tetrahedral ring; above silicon the potential is always positive. The value of the potential minimum above the center of the tetrahedral ring of pyrophyllite is ?0.05 V (at 2 Å from the surface), whereas in the case of talc the minimum is ?0.01 V, at 2.7 Å. In the case of pyrophyllite the minimum of potential above the higher basal oxygen is located at 1.1 Å and it has a value of ?1.25 V, whereas above the lower oxygen the value of the potential at the minimum is ?0.2 V, at 1.25 Å; the talc exhibits a minimum of ?0.75 V at 1.2 Å, above the basal oxygen.  相似文献   

10.
This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition IV(Si2.76Al1.24) VI(Al0.64Mg0.72 $ {\text{Fe}}_{1.45}^{2 + } $ Mn0.03Ti0.15) (K0.96Na0.05) O10.67 (OH)1.31 Cl0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition IV(Si3.14Al0.86)VI(Al0.75Mg0.01 $ {\text{Fe}}_{1.03}^{2 + } $ $ {\text{Fe}}_{1.03}^{3 + } $ Mn0.01Ti0.01Li1.09) (K0.99Na0.01) O10.00 (OH)0.65F1.35]. Samples differ for Fe ordering in octahedral sites, Fe2+/(Fe2+?+?Fe3+) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6?K in TK, where Fe is disordered in all octahedral sites, and 8?K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites.  相似文献   

11.
 A synthesis technique is described which results in >99% pure NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and its deuterium analogue ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder diffraction pattern. Both NH4 + and ND4 + are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH4 + or ND4 + can be explained to a good approximation using Td symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M 1 ) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3112 were found only in the synthesis run for the ND4-phlogopite. Electron microprobe analyses indicate that NH4-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral and octahedral sites, respectively, due to the Tschermaks substitution VIMg2++IVSi4+↔VIAl3++IVAl3+ and with respect to vacancies on the interlayer site due to the exchange vector XII(NH4)++IVAl3+↔XII□+IVSi4+. Received: 30 August 1999 / Accepted: 2 October 2000  相似文献   

12.
The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na1.93Ca0.05Fe0.02) (Mg2.60Fe0.41) (Al1.83Fe0.15Cr0.01) (Si7.92Al0.08)O22(OH)2 was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C p 0 data to values for ideal glaucophane, Na2Mg3Al2Si8O22(OH)2 the third-law entropy S 298 0 -S 0 0 was calculated to be 541.2±3.0 J·mol-1·K-1. Our value for S 298 0 -S 0 0 is 12.0 J·mol-1·K-1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2±3.0, is within 6.2 J·mol-1·K-1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J·mol-1·K-1.  相似文献   

13.
Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO3 perovskite more accurately, we synthesized single crystals of Mg0.946(17)Fe0.056(12)Si0.997(16)O3 perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe2+ and Fe3+) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O3 perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.  相似文献   

14.
This study is devoted to the physicochemical and mineralogical characterizations of palygorskite from Marrakech High Atlas, Morocco. The raw clay and its Na+-saturated <2 μm fraction were characterized using chemical, structural, and thermal analytical techniques. Measurements of specific surface area and porous volume are reported. The clay fraction was found to be made up of 95 % of palygorskite and 5 % of sepiolite. An original feature of this palygorskite is its deficiency in zeolitic H2O. The half-cell structural formula of its dehydrated form was determined on the basis of 21 oxygens to be (Si7.92Al0.08)(Mg2.15Al1.4Fe0.4Ti0.05 $ \square_{1} $ )(Ca0.03Na0.08K0.04)O21, while the hydrated form could be formulated as (Si7.97Al0.03)(Mg2.17Al1.46Fe0.40Ti0.05)(Ca0.03Na0.07K0,03)O20.18(OH)1.94(OH2)3.88·2.43 H2O. These formulas show that the (Al3++Fe3+)/Mg2+ ratio is around 0.84, revealing a pronounced dioctahedral character. Further, inside its octahedral sheet, it was determined that the inner M1 sites are occupied by vacancies, whereas the M2 sites are shared between 90 % of trivalent cations (78 % for Al3+ and 22 % for Fe3+), 7.5 % of Mg2+, and 2.5 % of Ti4+, all of them linked to 1.94 of structural hydroxyls. The two remaining Mg2+ by half-cell occupy edge M3 sites and are coordinated to 3.88 molecules of OH2. Channels of this palygorskite are deficient in zeolitic H2O since they contain only 2.43 H2O molecules. A correlation was found between these results and the observation of very intense and well-resolved FTIR bands arising from dioctahedral domains (mainly Al2OH, Fe2OH, and AlFeOH) along with very small responses from a trioctahedral domain (Mg3OH). Accordingly, a schematic representation of the composition of the octahedral sheet was proposed. The cation exchange capacity, specific surface area, and total pore volume were also assessed to be ca. 21.2 meq/100 g, 116 m2/g, and 0.458 cm3/g, respectively.  相似文献   

15.
The hydroxy groups of the crystal lattice of dioctahedral 2:1 phyllosilicates were investigated by means of quantum-mechanical calculation. The standard Kohn-Sham self-consistent density functional theory (DFT) method was applied using the generalized gradient approximation (GGA) with numerical atomic orbitals and double-zeta polarized functions as basis set. Isomorphous cation substitution of different cations in the octahedral and tetrahedral sheet was included along with several interlayer cations reproducing experimental crystal lattice parameters. The effect of these substitutions and the interlayer charge on the hydroxyl group properties was also studied. These structures represent different cation pairs among Al3+, Fe3+ and Mg2+ in the octahedral sheet of clays joined to OH groups. The geometrical disposition of the OH bond in the crystal lattice and the hydrogen bonds and other electrostatic interactions of this group were analyzed. The frequencies of different vibrational modes of the OH group [(OH), (OH) and (OH)] were calculated and compared with experimental data, finding a good agreement. These frequencies depend significantly on the nature of cations which are joined with, and the electrostatic interactions with, the interlayer cations. Besides, hydrogen-bonding interactions with tetrahedral oxygens are important for the vibrational properties of the OH groups; however, also the electrostatic interactions of these OH groups with the rest of tetrahedral oxygens within the tetrahedral cavity should be taken into account. The cation substitution effect on the vibration modes of the OH groups was analyzed reproducing the experimental behaviour.Dr. V. Botella passed away last February  相似文献   

16.
A mica whose structural formula: (K1.76Na0.31)(Fe2.22Mn1.29Mg0.99Ti0.28Al0.240.98) ·(Si7.33Al0.67)O20.26(F2.16OH1.58) closely approximates that of tetrasilicic potassium mica K2(M 5 2+ )Si8O20(OH,F)4 where M2+ represents Mg2+, Fe2+, Mn2+, ..., has been discovered in the matrix of a peralkaline rhyolite (comendite) of the Mont-Dore massif (France). These micas had been obtained previously by synthesis only. In the groundmass of the rock, the micaceous phase is accompanied by a manganoan arfvedsonite, pyrophanite, magnetite, apatite, sphene, zircon and fluorite. The crystallographic properties of the mica are typically that of a tetrasilicic mica, with d 060 = 1.533Å and space group C2/m. There is a regular decrease of d 060 (parameter b) with the ionic radius of the octahedral cation in synthetic micas containing Fe2+, Co2+, Mg2+, Ni2+. The purely Mn2+ end-member could not be synthesised; its instability is discussed on the basis of structural considerations. The conditions of crystallization of the micaceous phase are estimated to be 760 ° C, 800 bars with a f o 2=10–14.7 bar. This mica has crystallized from a residual liquid, with high activity of silica and low activity of alumina, whose origin is discussed. The name MONT-DORITE is proposed for this natural tetrasilicic mica having Fe/Fe+Mg >1/2 and Fe/Fe+Mn >1/2. This name is from the stratovolcano Mont-Dore.  相似文献   

17.
 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   

18.
Neutron powder diffraction experiments in the temperature range 300–1770 K were performed at BENSC, Berlin, Germany, on synthetic (Mg0.70Fe0.23) Al1.97O4. The cation partitioning over the crystallographic tetrahedral and octahedral sites was determined as a function of temperature through joint Rietveld refinements and advanced minimization techniques. The thermal expansion coefficients of the lattice parameter and inter-atomic bond lengths were also obtained from the full-profile structure refinements. The behaviour of the polyhedral bond-lengths, especially the T−O distances, and of the cell constant upon heating, clearly indicate that the interdiffusion of tetrahedral and octahedral Mg/Al cations starts at about 950 K. This result is straightforwardly supported by the direct analysis of the neutron site scattering factors: Fe always retains tetrahedral coordination at all temperatures, and the cation rearrangement is entirely due to Mg and Al diffusion. Received: 18 November 1997 / Revised, accepted: 23 August 1998  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.
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 (K0.729)C (Na0.029)B (Si10.498 Al1.502)T1 (Al2.706 Fe 0.294 2+ )T2 (Mg0.735 Mn0.091 Fe 1.184 2+ )AO30. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe2+ 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 Si2p, Al2p, Mg1s and Fe2p core levels, suggests that Fe presents Fe2+ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si4+ and Si1+ components, respectively, both in tetrahedral coordination. The binding energy of Al2p, 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 L2,3-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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