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1.
Silicon-29 magic-angle-spinning NMR spectroscopy has been used to investigate the silicon-aluminum distribution in natural samples of analcite and leucite (before and after heat treatment) as well as a leucite synthesized from a gel. Three different simulation programs have been developed to fit the experimental spectra. For two we assume a different aluminum occupancy fraction g i for each of the three crystallographically distinct tetrahedral sites T i in leucite and some degree of aluminum avoidance, but an otherwise random arrangement of tetrahedral cations. A third program interchanges Al and Si cations on a lattice of 3×3×3 unit cells to generate an optimized fit. All models predict that the T 2 sites in natural leucite are deficient in aluminum: g 1≈0.39, g 2≈0.16, and g 3≈0.42 for the fractional Al occupancy at each site, with apparently strict aluminum avoidance. Heat treatment of the sample at 1673 K for a week has little effect on the g i values but may create some Al-O-Al linkages. In the gel-synthesized leucite, Al occupancies are slightly more uniform than in natural leucite: g 1≈0.36, g 2≈0.20, and g 3≈0.42. For analcite, two distinctly different Si, Al distributions are obtained: (A) g 1=g 3≈0.09, g 2≈0.78 and (B) g 1=g 3≈0.46, g 2≈0.04. Additional NMR measurements on an ion-exchanged sample or an accurate determination of unit-cell dimensions could resolve this ambiguity.  相似文献   

2.
Infrared and X-ray diffraction studies were made on synthetic serpentines (Si4 to Si2Al2 tetrahedral compositions). Changes in cell dimensions and variations in infrared spectra indicate that ordering can occur in the octahedrally coordinated site and possibly the tetrahedral site. Octahedra order into an Mg3 and/or Mg2Al configuration depending upon bulk mineral composition. In the latter cases an Al ion preferentially fills only one of the large M2 sites found in dioctahedral minerals and is thus characteristic of neither di-nor trioctahedral minerals. Ordering in octahedral and possibly tetrahedral sites does not appear to affect the basic higher frequency vibrations (OH stretch, Si-O stretch) by creating new modes but definite band splitting is seen for vibrations of lower frequency. The alumina-free composition was crystallized into monophase products which can be assimilated to natural chrysotile or antigorite forms. Addition of alumina produces lizardite-type serpentines which are very closely related to the chrysotile structure as indicated by cell-dimension and Si-O stretch bands in infrared spectra.  相似文献   

3.
Phases with the composition K2MgSi5O12, belonging to the leucite structure group were synthesised under dry and hydrothermal conditions and studied using 29Si NMR. The 29Si spectrum for the dry-crystallised material (which is cubic) consists of a single broad line, suggesting a high degree of disorder. The hydrothermally crystallised material (which is probably monoclinic, with a distorted leucite lattice) has a 29Si spectrum which consists of ten lines of equal intensity, two of which have small chemical shift anisotropies and are therefore assigned to Q4(4Si) sites. These data have been interpreted in terms of a structure with 12 distinct tetrahedral sites over which 2 Mg atoms and 10 Si atoms are fully ordered. A 2-dimensional COSY spectrum shows correlations between some Q4(3Si) silicon atoms and two other Q4(3Si) silicon atoms. This fully constrains the topology of the unit cell. Two schemes of Si/Mg ordering over the unit cell can give good fits to the COSY spectrum. Using the tetrahedral (T) site notation defined for natural tetragonal leucite, the first of these arrangements involves Mg and Q4(4Si) silicon atoms each occupying one T1-type site and one T3-type site, and Q4(3Si) silicons occupying the remaining sites, i.e. four T2-type sites, two T1-type sites and two T3-type sites. In the second arrangement, the T2-type sites are occupied by Mg atoms and Q4(4Si) silicon atoms and all the T1-and T3-type sites are occupied by Q4(3Si) atoms.  相似文献   

4.
A number of leucite group materials with the formula X2YSi5O12 where X=K, Rb, Cs and Y=Mg, Zn, Cd have been synthesized by dry and hydrothermal crystallization of glass starting materials. 29Si MASNMR has been used to obtain structural information, such as the number of distinct tetrahedral sites, degree of cation ordering, and estimates of the mean T-O-T bond angles of the tetrahedra. X-ray powder diffraction gave information on cell volumes and degree of distortion from cubic symmetry for all the samples and space group and structural information for some samples. Integration of the different length-scale data obtained using these two complementary techniques allows greater reliance to be placed on the structures deduced for these leucite samples, which are only available as fine-grained powders. Hydrothermally synthesized K2MgSi5O12, K2ZnSi5O12 and Rb2ZnSi5O12 have structures with 12 distinct tetrahedral sites (T-sites) and are monoclinic P21/c while the dry-synthesized equivalents are disordered with single T-sites and are cubic, Ia3d. Most of the other members of the group have structures with 6 tetrahedral sites with Cs2CdSi5O12 being orthorhombic, Pbca. Cs2ZnSi5O12 has an intermediate “5+2” structure. Decreasing the size of the X+ cation for a given Y2+ cation gives more collapsed and distorted frameworks. 133Cs NMR was used to show that samples with 6 T-sites have 2 alkali sites. It is deduced that samples with 12 T-sites will undergo a displacive phase transition to a 6 T-site structure (possibly via a 5+2 intermediate in some cases) with no change in the framework topology or degree of T-site ordering.  相似文献   

5.
Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 ? Al:Si=0.80:0.20, T2?Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.  相似文献   

6.
Orthopyroxene has two tetrahedral sites, designated A and B, and two octahedral sites, M1 and M2. Crystallographic studies of synthetic and natural orthopyroxenes (opx) suggest that the tetrahedral Al is ordered nearly completely in the B site, but the octahedral Al disorders between M1 and M2 sites with a preference for M1. If the aluminum avoidance principle is obeyed, then the tetrahedral Si-Al ordering limits the Al substitution in opx to 25 mol%, thus leading to an end-member stoichiometry of Mg3Al2Si3O12 instead of MgAl2SiO6.The enthalpy of formation of these two components has been deduced from the available phase equilibrium data. The thermodynamic properties of the opx solid solution approximates ideal solution behavior more closely when treated in terms of the components Mg4Si4O12(QEn)-Mg3Al2Si3O12(Py) than when expressed in terms of the components Mg2Si2O6-MgAl2SiO6. A model has been developed for the octahedral disordering of Al as function of temperature and composition. These data enable calculation of the configurational entropy and molar entropy of Al-opx; distinction has been made between the cases of completely random mixing of Al and Si in the tetrahedral B site, and of random mixing without violation of the aluminum avoidance principle. The second model yields entropy of the Mg3Al2Si3O12 end member which agrees almost exactly with the value derived from phase equilibrium data. The partial molal entropies of the Orthopyroxene components QEn and Py can be derived from these data; their implications with respect to the P-T slopes of Al2O3 isopleths for the equilibrium of Orthopyroxene with forsterite and spinel/garnet have been discussed.  相似文献   

7.
Ti-spessartite, Ti-andradite and its indium homolog, Ca3In2(Si, Ti)3O12, have been synthesized and investigated by infrared (ir) spectroscopy. The use of isotopic species (46Ti-50Ti) gives the unequivocal proof that some of the additional bands observed in the 800–600 cm?1 region are due to TiO4 tetrahedra. For Si-Ti replacements up to 10 mol%, the localization of Ti on the available tetrahedral sites depends on the nature of the cations. For the In garnet all (or nearly all) Ti is located on tetrahedral sites; in Ti-andradite Ti is distributed over tetrahedral and octahedral sites, tetrahedral sites being thus occupied, in part by Ti (ir band near 700 cm?1) and in part by Fe (ir band near 650 cm?1); and Ti-spessartite the presence of Ti on tetrahedral sites is doubtful, these sites being essentially occupied by Al.  相似文献   

8.
Synthetic Fe3+-melilites containing NaCaFe3+-Si2O7-, Ca2Fe3+AlSiO7- or Sr2Fe3+AlSiO7-components have been studied by 57Fe Mössbauer spectroscopy. The spectrum of åkermanite containing an NaCaFe3+Si2O7-component consists of one doublet identified to belong to Fe3+ in T1 sites. The spectra of åkermanite and gehlenite containing Ca2Fe3+ AlSiO7- or Sr2Fe3+ AlSiO7-component consist of two doublets. The inner and outer doublets are identified to belong to Fe3+ in the less distorted T1 and that in the more distorted T2 sites, respectively. The area ratios of the spectra show that the site occupancy of Fe3+ (T1) in gehlenite is less than that in åkermanite in which the distribution of Fe3+ in T1 and T2 sites is apparently random. The different distributions can be explained in terms of competition between minimizing the deficiency in the electrostatic valence and the preference of Al for T1 sites which the isomer shift measurements show to be more ionic.  相似文献   

9.
Sapphirine II     
The crystal structure of aP21/a polymorph of sapphirine (a=11.286(3),b=14.438(2),c=9.957(2) Å, β=125.4(2) °) of composition [Mg3.7Fe 0.1 2+ Al4.1- Fe 0.1 3+ ]IV[Si1.8Al4.2]IVO20 was refined using structure factors determined by both neutron and x-ray diffraction methods to conventionalR factors of 0.067 and 0.031. respectively, forF obs>2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel toz with corner-sharing wing tetrahedra (T5 andT6) on either side in the (100) plane. The meanT-O distance is highly correlated with Al occupancy, XAl, as determined from the neutron site refinement: $$\langle T - O\rangle = 1.656 + 0.105X_{Al} (r^2 = 0.995).$$ Details of the neutron refinement are summarized below.  相似文献   

10.
High-resolution 27Al MAS NMR spectra of natural leucite recorded at H 0=11.7T contain three resolvable resonances at 27Al δ i = 69.2, 64.7, and 61.0±0.5 ppm. These three resonances are assigned to the three inequivalent framework positions of leucite: T3, T2, and T1, respectively. Fitting the observed spectra yields a Si,Al distribution for leucite in which approximately one-half of the Al is in T1 and one-quarter in each of T2 and T3. This Si,Al distribution differs substantially from those obtained by previous workers using 29Si NMR spectroscopy and X-ray diffraction. New 29Si NMR spectra and revision of previously reported 29Si NMR peak assignments, however, make the 27Al and 29Si NMR results consistent. The 27Al δ i correlate linearly with the mean T-O-T′ bond angles of the average structure, which allows the peak assignments to be made. However, this correlation lies distinctly toward higher frequency and larger bond angles than correlations for Si,Al ordered aluminosilicates, suggesting that the mean T(Al)-O-T′(Si) bond angle for each site in leucite is smaller than the mean bond angle of the average structure, which is averaged over T(Al)-O-T′(Si) and T(Si)-OT′(Si,Al) angles.  相似文献   

11.
An exploratory high-pressure study of the join CaTiO3-FeTiO3 has uncovered two intermediate perovskites with the compositions CaFe3Ti4O12 and CaFeTi2O6. These perovskites have ordering of Ca2+ and Fe2+ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe2+ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe3Ti4O12, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn7O12. Fe2+ is in a square-planar A site (similar to Mn3+ in NaMn7O12) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi2O6 crystallizes in a unique tetragonal double perovskite structure (SG P42/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO3 recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe3Ti4O12 is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.  相似文献   

12.
A 29Si and 27Al magic angle spinning nuclear magnetic resonance study is reported for differently synthesized mullites. The 29Si MAS NMR spectra of all samples are essentially identical. They consist of a main resonance at -86.8 ppm, a shoulder around -90 ppm and a second resonance at -94.2 ppm. The main resonance is interpreted as being due to a sillimanite-type geometry around Si and the second one is tentatively assigned to a Si environment typical for mullite. The 27Al MAS NMR spectra of sinter- and fused-mullite measured at different Larmor frequencies revealed clearly the presence of three distinct Al sites in mullite, i.e. of octahedral (M1), tetrahedral (M2) and distorted tetrahedral (Al*) sites.  相似文献   

13.
A detailed evaluation of the assignments given to the infrared (IR) vibrations in the lattice stretching region is presented here based on observations of the effects of various chemical substitutions in synthetic analogues of phlogopite, KMg3(AlSi3)O10(OH)2. As in previous studies, this study has confirmed that the 995, 960, and 460 cm?1 vibrations are influenced by Si, the 822 and 760 cm?1 vibrations by Al, the 915 and 725 cm?1 vibrations by Al and Si, and the 592 cm?1 vibration by OH. Contrary to previous studies, it is shown here that the 690, 495, and 375 cm?1 vibrations are strongly linked with Mg and not just Si. The 655 cm?1 band in phlogopite is attributed to an in-plane Al-O vibration rather than an Al-O-Si vibration. As a check on the band assignments made here, IR spectra were obtained for synthetic clintonite, CaMg2Al(Al3Si)O10(OH)2, as well as its chemical analogues and compared with the IR spectrum of phlogopite. The band intensities for the Si-O, Al-O, and Si-O-Mg vibrations changed in accord with the composition of clintonite. The most intense band in clintonite at 660 cm?1 appears to be associated only with Al and is assigned here to a tetrahedral Al-O-Al vibration which must be present, if not dominant, in this mineral. The near coincidence of an in-plane Al-O vibration at 655 cm?1 (phlogopite) and an in-plane Al-O-Al vibration at 660 cm?1 (clintonite) makes the identification of tetrahedral Al-Si order-disorder in trioctahedral layered silicates by IR spectroscopy very difficult. The ratio of the 822/995 cm?1 bands may, however, prove to be very useful for discerning the amount of tetrahedrally coordinated Al in these types of minerals.  相似文献   

14.
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   

15.
Four samples of synthetic chromium-bearing spinels of (Mg, Fe2+)(Cr, Fe3+)2O4 composition and four samples of natural spinels of predominantly (Mg, Fe2+)(Al, Cr)2O4 composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr2O4 spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr3+ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed 4 A 2g  → 4 T 2g and 4 A 2g  → 4 T 1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions 4 A 2g  → 2 E g and 4 A 2g  → 2 T 1g of Cr3+, intensified by exchange-coupled interaction between Cr3+ and Fe3+ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm?1. A vague broad band in the range from ca. 15,000 to 12,000 cm?1 in synthetic spinels is tentatively attributed to IVCr2+ + VICr3+ → IVCr3+ + VICr2+ intervalence charge-transfer transition. Iron, mainly as octahedral Fe3+, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O2? → Fe3+, Fe2+ transitions). As tetrahedral Fe2+, it appears as a strong infrared absorption band at around 4,850 cm?1 caused by electronic spin-allowed 5 E → 5 T 2 transitions of IVFe2+. From the composition shift of the U-band in natural and synthetic MgCr2O4 spinels, the coefficient of local structural relaxation around Cr3+ in spinel MgAl2O4–MgCr2O4 system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O6 polyhedra known so far. The octahedral modulus of Cr3+ in MgCr2O4, derived from pressure-induced shift of the U-band of Cr3+, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr3+ in natural and synthetic MgCr2O4 spinels indicates that Cr–O-bonding in octahedral sites of MgCr2O4 has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl2O4 spinel, B does not much depend on pressure.  相似文献   

16.
L III edge extended X-ray absorption fine structure (EXAFS) spectra of Gd, Er and Lu environments in synthetic epidotes of composition CaLa0.9 X 0.1Al2MgSi3O13H (X=Gd or Er or Lu) were recorded using synchrotron radiation. The Fourier transforms of the Gd-, Er- and Lu-EXAFS are clearly different from one another indicating wholly or partially different site occupancy. Model fitting of the Fourier-filtered partial EXAFS and comparison of pair distribution functions with those calculated for natural epidote leads to the conclusion that three different sites are probably involved in the accommodation of these elements in the epidote structure, and that site preference is a function of the rare earth ionic size. Gd is located in A2-type sites, whereas the local atomic environment of Er is consistent with A1 site occupancy and the Lu environment has been modelled on an M3-type octahedral site.  相似文献   

17.
We present structural information obtained on spinel and alumina at high temperature (298-2400 K) using in-situ XANES at the Mg and Al K-edges. For spinel, [4](Alx,Mg1−x)[6](Al2−x,Mgx)O4, with increasing temperature, a substitution of Mg by Al and Al by Mg in their respective sites is observed. This substitution corresponds to an inversion of the Mg and Al sites. There is a significant change in the Al K-edge spectra between crystal and liquid, which can be attributed to a change of the [6]Al normally observed in corundum at room temperature, to a mixture of [6]Al-[4]Al in the liquid state. This conclusion is in good agreement with previous 27Al NMR experiments. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.  相似文献   

18.
Synthetic clinopyroxenes of compositions between CaFe3+AlSiO6 and CaFe 0.85 3+ Ti0.15Al1.15Si0.85O6 have been studied by 57Fe Mössbauer spectroscopy. The spectra consist of two doublets assigned to Fe3+ in M1 and T sites. From the area ratios of the doublets the site occupancies of Fe3+ and Al were determined. Si decreases from 1.00 to 0.85 and Al+Fe3+ increases from 1.00 to 1.15 per formula unit with increasing CaTiAl2O6 component of the clinopyroxene. The atomic ratio of Fe3+(T)/Fe3+(total) is 0.11–0.16; 4.5–7.5 percent of the T sites are occupied by Fe3+. Thus the presence of Si-O-Fe3+, Al-O-Fe3+, and Fe3+-O-Fe3+ bonds is expected in addition to Si-O-Si, Si-O-Al and Al-O-Al bonds. However, the possibility of the former bonds being present would be small, because the amount of Fe3+(T) is far less than that of Si and Al. The isomer shift of Fe3+(T) is one of the largest in the values found previously for Fe3+(T) in silicates. It increases with increasing CaTiAl2O6 component and seems to be correlated to the ionic character of the cation — anion bonds calculated from electronegativity. The quadrupole splittings of Fe3+(M1) and Fe3+(T) decrease with the substitution of Fe3+?Ti4+ in the M1 and of Si?Al in the T sites.  相似文献   

19.
Single-crystal electron paramagnetic resonance (EPR) spectra of a natural citrine quartz without any artificial irradiation, measured at W-band frequencies (∼94 GHz) and temperatures of 77, 110 and 298 K, allow better characterization of three previously-reported Centers (#6, #7 and B) and discovery of three new defects (B′, C′ and G′). The W-band EPR spectra reveal that Centers #6 and #7 do not reside on twofold symmetry axes, contrary to results from a previous X-band EPR study. The W-band spectra also show that the previously reported Center B is a mixture of two defects (B and B′) with similar g matrices but different-sized 27Al hyperfine structures. Center C′ has similar principal g values to the previously reported Center C but is distinct from the latter by a larger 27Al hyperfine structure with splittings from 0.10 to 0.22 mT. Also, Center G′ has a similar g matrix to the previously reported Center G but a different 27Al hyperfine structure with splittings from 0.41 to 0.53 mT. These spin-Hamiltonian parameters, together with observed thermal properties and microwave-power dependence, suggest that Centers #6 and #7 probably represent O23− type defects. Centers B and B′ are probably superoxide radicals (O2−) with the unpaired spin localized on the same pair of oxygen atoms around a missing Si atom but linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Similarly, Centers G and G′ are most likely superoxide radicals with the unpaired spin localized on another pair of oxygen atoms around a missing Si atom and linked to a substitutional Al3+ ion each at different neighboring tetrahedral sites. Center C′ is probably an ozonide radical associated with a missing Si atom and linked to a substitutional Al3+ ion at the neighboring tetrahedral site. This study exemplifies the value of  high-frequency EPR for discrimination of  similar defect centers and determination of  small local structural distortions that are often difficult to resolve in conventional  X- and Q-band EPR studies.  相似文献   

20.
A study has been made of the differences produced in leucites by different methods of cooling charges, in the system NaAlSiO4-KAlSiO4-SiO2-H2O, consisting of leucite and silicate liquid. The results indicate that the ion exchange reaction in the system is so fast that metastable leucite-analcite solid solutions may be produced between leucite crystals and hydrous glass.  相似文献   

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