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1.
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   

2.
In the present work, a combination of various techniques is utilized for the study of nano-mineralogy and -geochemistry of high-grade karst-type bauxite (Al-rich and Fe-depleted samples; Al2O3 ca. 80 wt.%) from the Parnassos-Ghiona mines located in Greece. Initial characterization using PXRD and electron microscopy in microscale and mesoscale (SEM-EDS including STEM mode), proved the presence of “Fe-Cr-Ti-containing diaspore”, anatase and minor rutile. The study by means of 57Fe Mössbauer spectroscopy, in correlation with magnetic susceptibility measurements and, complemented, with Synchrotron-based spectroscopies at the microscale (SR micro-XRF and micro-XANES/-EXAFS), indicated that Fe3+, in contrast to [6]Cr3+, is not exclusively a component of the diaspore structure. While Cr3 + substitutes Al3 + in octahedral sites of diaspore ([6]Cr3+ ↔ [6]Al3+), the electron microscopy in nanoscale (TEM-EDS & EELS) revealed that Fe exists in the form of peculiar Fe3+-bearing nanominerals (most likely maghemite-type phases) between 25 and 45 nm in size, in addition to the Fe3+ ions substituting Al3+ in the diaspore structure. Moreover, it was proven that TiO2 polymorph mineral nanoparticles, particularly rounded anatase mesocrystals and nanocrystals and individual needle-shaped rutiles, are dispersed into the diaspore matrix. Thus, diaspore in the studied bauxite concerns -in fact- a distinct Fe3+-Cr3+-AlOOH low-T authigenic phase, demonstrated for the first time in literature. On the other hand, the observed TiO2 mineral nanoparticles (formed, together with diaspore, during diagenesis) and Fe nanominerals (formed during epigenesis) were hitherto unknown not only for the allochthonous karst-type bauxite deposits of Greece, but also for the overall bauxite deposits, worldwide.  相似文献   

3.
Synthetic melilites on the join Ca2MgSi2O7 (åkermanite: Ak)-Ca2Fe3+AlSiO7 (ferrialuminium gehlenite: FAGeh) were studied using X-ray powder diffraction and 57Fe Mössbauer spectroscopic methods to determine the distribution of Fe3+ between two different tetrahedral sites (T1 and T2), and the relationship between ionic substitution and incommensurate (IC) structure. Melilites were synthesized from starting materials with compositions of Ak100, Ak80FAGeh20, Ak70FAGeh30 and Ak50FAGeh50 by sintering at 1,170–1,350 °C and 1 atm. The average chemical compositions and end-member components, Ak, FAGeh and Geh (Ca2Al2SiO7), of the synthetic melilites were Ca2.015Mg1.023Si1.981O7 (Ak100), Ca2.017Mg0.788Fe 0.187 3+ Al0.221Si1.791O7 (Ak78FAGeh19Geh3), Ca1.995Mg0.695Fe 0.258 3+ Al0.318Si1.723O7 (Ak69FAGeh25Geh6) and Ca1.982Mg0.495Fe 0.449 3+ Al0.519Si1.535O7 (Ak49FAGeh44Geh7), respectively. Rietveld refinements using X-ray powder diffraction data measured using CuK α -radiation at room temperature converged successfully with goodness-of-fits of 1.15–1.26. The refined Fe occupancies at the T1 and T2 sites and the Mg and Si contents determined by electron microprobe analysis gave the site populations of [0.788Mg + 0.082Fe3+ + 0.130Al]T1[0.104Fe3+ + 0.104Al + 1.792Si]T2 for Ak78FAGeh19Geh3, [0.695Mg + 0.127Fe3+ + 0.178Al]T1[0.132Fe3+ + 0.144Al + 1.724Si]T2 for Ak69FAGeh25Geh6 and [0.495Mg + 0.202Fe3+ + 0.303Al]T1[0.248Fe3+ + 0.216Al + 1.536Si]T2 for Ak49FAGeh44Geh7 (apfu: atoms per formula unit), respectively. The results indicate that Fe3+ is distributed at both the T1 and the T2 sites. The mean T1–O distance decreases with the substitution of Fe3+ + Al3+ for Mg2+ at the T1 site, whereas the mean T2–O distance increases with substitution of Fe3+ + Al3+ for Si4+ at the T2 site, causing decrease in the a dimension and increase in the c dimension. However, in spite of the successful Rietveld refinements for the X-ray powder diffraction data measured using CuK α-radiation at room temperature, each Bragg reflection measured using CuK α1-radiation at room temperature showed weak shoulders, which were not observed in those measured at 200 °C. The Mössbauer spectra of the melilites measured at room temperature consist of two doublets assigned to Fe3+ at the T1 site and two or three doublets to Fe3+ at the T2 site, implying the existence of multiple T1 and T2 sites with different site distortions. These facts can be interpreted in terms of the IC structure in all synthetic melilites at room temperature, respectively. The results of Mössbauer analysis indicate that the IC structure in melilite is caused by not only known multiple T1 site, but also multiple T2 site at room temperature.  相似文献   

4.
Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO2, 8 wt.% Cr2O3 and 4 wt.% ZrO2, representing a solid solution of andradite (Ca3Fe3+ 2Si3O12 ≈46 mol%), uvarovite (Ca3Cr2Si3O12 ≈23 mol%), grossular (Ca3Al2Si3O12 ≈10 mol%), schorlomite (Ca3Ti2[Si,(Fe3+,Al3+)2]O12 ≈15 mol%), and kimzeyite (Ca3Zr2[Si,Al2]3O12 ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al3+, Fe3+ and Ti4+. Cr3+ and Zr4+ are found at the octahedral site along with Fe3+, Al3+ and Ti4+. The measured H2O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca2.99Mg0.03)Σ=3.02(Fe3+ 0.67Cr0.54Al0.33Ti0.29Zr0.15)Σ=1.98(Si2.42Ti0.24Fe0.18Al0.14)Σ=2.98O12OH0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.  相似文献   

5.
Absorption bands are determined in polarized optical spectra of vivianite Fe3(PO4)2·8H2O, recorded at room and low temperatures. These bands are caused by spin-allowed d-d transitions in structurally nonequivalent Fe A 2+ (～11000 cm-1 (γ-polarization) (and) ～12000 cm-1 (β-polarization)) (and) Fe B 2+ (～8400 cm-1 (γ, α-polarization) and ～11200 cm-1 (α-polarization)) ions. A charge transfer band (CTB) Fe B 2+ +Fe B 3+ →Fe B 2+ +Fe B 2+ (～15000 cm-1) also determined, has polarizing features giving evidence of a change in the Fe B 2+ -Fe B 3+ bond direction, when compared with Fe B 2+ -Fe B 2+ . Bands of exchange-coupled Fe3+-Fe3+ pairs (～19400, ～20400, ～21300 and ～21700 cm-1) which appear on oxidation of Fe2+ in paired Fe B octahedra are also characterized.  相似文献   

6.
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   

7.
Majorite of bulk composition Mg0.86Fe0.15SiO3 was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO2 buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm?1. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm?1 due to the 5Eg → 5T2g transition of Fe2+ in a distorted dodecahedral site. (2) A band at 9340 cm?1 due to the transition 5T2g → 5Eg of octahedral Fe2+. (3) A band at 22784 cm?1 resulting from Fe3+, probably in an octahedral site (6A1g → 4A1g, 4Eg). (4) A very intense system of Fe2+ → Fe3+ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm?1. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe3+ in two different octahedral sites. The crystal field splitting Δ of Fe2+ in dodecahedral coordination is 5717 cm?1. If a splitting of the ground state in the order of 1000 cm?1 is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm?1, comparable to the CFSE of Fe2+ in pyrope-rich garnet. However, the splitting of 5T2g is significantly higher than in pyrope. This would be consistent with Fe2+ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe2+, Δ= 9340 cm?1 and CFSE=3736 cm?1, assuming negligible splitting of the ground state.  相似文献   

8.
Two synthetic series of spinels, MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 have been studied by Raman spectroscopy to investigate the effects of Fe2+ and Fe3+ on their structure. In the first case, where Fe2+ substitutes Mg within the tetrahedral site, there is a continuous and monotonic shift of the Raman modes A1g and Eg toward lower wavenumbers with the increase of the chromite component into the spinel, while the F2g modes remain nearly in the same position. In the second series, for low Mg-ferrite content, Fe3+ substitutes for Cr in the octahedral site; when the Mg-ferrite content nears 40 %, a drastic change in the Raman spectra occurs as Fe3+ starts entering the tetrahedral site as well, consequently pushing Mg to occupy the octahedral one. The Raman spectral region between 620 and 700 cm?1 is associated to the octahedral site, where three peaks are present and it is possible to observe the Cr–Fe3+ substitution and the effects of order–disorder in the tetrahedral site. The spectral range at 500–620 cm?1 region shows that there is a shift of modes toward lower values with the increase of the Mg-ferrite content. The peaks in the region at 200–500 cm?1, when observed, show little or negligible Raman shift.  相似文献   

9.
The electron paramagnetic resonance (EPR) spectra of Fe3+ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B 2 0 = 0.112 cm?1, B 2 2 = 0.0688 cm?1 for one and B 2 0 = 0.116 cm?1, B 2 2 = 0.0766 cm?1 for the second. These two spectra arise from Fe3+ substituted for Al3+ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.  相似文献   

10.
Examination of schorlomite from ijolite at Magnet Cove (USA) and silicocarbonatite at Afrikanda (Russia), using electron-microprobe and hydrogen analyses, X-ray diffraction and Mössbauer spectroscopy, shows the complexity of substitution mechanisms operating in Ti-rich garnets. These substitutions involve incorporation of Na in the eightfold-coordinated X site, Fe2+ and Mg in the octahedrally coordinated Y site, and Fe3+, Al and Fe2+ in the tetrahedrally coordinated Z site. Substitutions Ti4+Fe3+Fe3+–1Si–1 and Ti4+Al3+Fe3+–1Si–1 are of major significance to the crystal chemistry of schorlomite, whereas Fe2+ enters the Z site in relatively minor quantities (<3% Fe). There is no evidence (either structural or indirect, such as discrepancies between the measured and calculated Fe2+ contents) for the presence of [6]Ti3+ or [4]Ti4+ in schorlomite. The simplified general formula of schorlomite can be written as Ca3Ti4+2[Si3-x(Fe3+,Al,Fe2+)xO12], keeping in mind that the notion of end-member composition is inapplicable to this mineral. In the published analyses of schorlomite with low to moderate Zr contents, x ranges from 0.6 to 1.0, i.e. Ti4+ in the Y site is <2 and accompanied by appreciable amounts of lower-charged cations (in particular, Fe3+, Fe2+ and Mg). For classification purposes, the mole percentage of schorlomite can be determined as the amount of [6]Ti4+, balanced by substitutions in the Z site, relative to the total occupancy in the Y site: ([6]Ti4+–[6]Fe2+–[6]Mg2+– [8]Na+)/2. In addition to the predominant schorlomite component, the crystals examined in this work contain significant (>15 mol.%) proportions of andradite (Ca3Fe3+2Si3O12), morimotoite (Ca3Fe2+TiSi3O12), and Ca3MgTiSi3O12. The importance of accurate quantitative determination and assignment of Fe, Ti and other cations to the crystallographic sites for petrogenetic studies is discussed.
A. R. ChakhmouradianEmail: Phone: +1-204-4747278Fax: +1-204-4747623
  相似文献   

11.
Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al2O3 (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al2O3), aluminous orthopyroxene (up to 8.93 wt% Al2O3), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al2O3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe2+-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).  相似文献   

12.
A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R1, R2-sharp line luminescence and spectra of excitation of λ3- and λ4-components of R1-line of Cr3+-emission had the following results: (1) The Fe2+–Ti4+ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm−1 if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm−1 in crystals with  ≥ . The energy difference is explained by an expansion of the Of–Ok, and Ob–Om edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr3+ substitutes for Al compared to the chromium-free case. (2) The Cr3+ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr3+ 4A2g→4T2g(F)(I), →4T1g(F)(II), and →4T1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm−1. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm−1, Dq(b)=1600 cm−1, B(a)=790 cm−1, B(b)=620 cm−1 (errors ca. ±10 cm−1), again in agreement with values extracted from the λ3, λ4 excitation spectra. The CF-values of set a are close to those typical of Cr3+ substituting for Al in octahedra of other silicate minerals without constitutional OH− as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr3+ in the kyanite structural matrix. The CF-data of Cr3+ type b, expecially B, resemble those of Cr3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997  相似文献   

13.
Kyzylkumite has been found in Cr-V-bearing metamorphic rocks of the Sludyanka Complex, Southern Baikal region; it has been identified by X-ray powder diffraction method. This is a late secondary mineral developed after Ti-V-oxides (schreyerite, berdesinskiite) and V-bearing rutile and titanite. Kyzylkumite represents a new structural type with composition Ti4V 2 3+ O10(OH)2 corresponding to octahedral coordination of Ti4+ and V3+. Its unit-cell dimensions are: a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) Å, β = 93.174(1)°. The ideal formula of kyzylkumite Ti4V 2 3+ O10(OH)2 corresponds to composition, wt %: 65.56 TiO2, 30.75 V2O3, 3.69 H2O. Indeed, the contents (wt %) of these constituents range from 62 to 70 TiO2 and from 23 to 33 V2O3. Variations in contents and the Ti/V value are caused by partial substitution V3+ for V4+, isovalent substitutions Ti4+ and V3+ for V4+ and Cr3+, respectively, and coupled substitution V3+ + OH? ? Ti4+ + O2?. Smyslova et al. (1981)—the discovereres of kyzylkumite—assumed its composition to be the same as for schreyerite V 2 3+ Ti3O9 that principally different from kyzylkumite from the Sludyanka Complex. Therefore, re-examination of the kyzylkumite holotype or cotype from its type locality is needed.  相似文献   

14.
A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe2+ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Aliv to balance the effect of increasing (Fe3++Ti4++Alvi+Cr3+)M1.The Fe2+/Fe3+ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe2+ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe2+ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.  相似文献   

15.
Electrical properties of natural alexandrite (BeAl2O4:Cr3+) are investigated by the thermally stimulated depolarization current (TSDC) technique. Samples are submitted to consecutive annealing processes and TSDC is carried out after each annealing, yielding bands with different parameters. These bands are fitted by a continuous distribution of relaxation parameters: activation energy and pre-exponential factor of the Arrhenius equation. It has been observed that annealing influences the dipole relaxation behavior, since it promotes a modification of Fe3+ and Cr3+ impurity distributions on sites of distinct symmetry: Al1 and Al2. In order to have a reference for comparison, TSDC is also carried out on a synthetic alexandrite sample, where the only impurity present is Cr3+ ion.  相似文献   

16.
Complex oxides of the pyrochlore (space groups Fd3m, [8]A2 [6]B2O7) and garnet (Ia3d, [8]A3 [6]B2 [4]T3O12) structures (“A” = Ca2+, Ln3+/4+, An3+/4+; “B” = (Ti, Sn, Hf, and Zr)4+ in pyrochlore, and Al3+, Ga3+, and Fe3+ in garnet alone; “T” = (Al3+, Ga3+, and Fe3+) are promising matrices for actinide-bearing wastes. In order to identify optimal compositions of these phases, their isomorphic capacity with respect to REE, actinides, and other components of wastes was examined. The long-term behavior of the matrix at a repository was predicted based on data obtained on the behavior of pyrochlores and garnets under ion irradiation and 244Cm decay and on the determined leaching rates of REE from the matrices because of their interaction with aqueous solutions, including that after amorphization. In order to propose efficient synthesis techniques, samples prepared with the use of various methods were studied. The possibility of incorporating long-lived decay products of 99Tc into the crystalline matrices was analyzed.  相似文献   

17.
Tremolite–tschermakite solid solutions have been synthesized between 700 and 850 °C and 200 and 2000 MPa. The starting materials were oxide–hydroxide mixtures and an additional 0.1–1.8 molal CaBr2 solution. The run products were characterized using SEM, HRTEM, EMP, XRD and FTIR. The synthesized Al tremolites formed needles and lath-shaped crystals of up to 300 × 20 μm. HRTEM investigations showed that the majority of the amphiboles were well ordered. The EMP analysis revealed that the Al tremolites were solid solutions in the ternary tremolite–tschermakite–cummingtonite. The highest observed Al content was close to the composition of magnesiohornblende (Xts=0.54). Different cummingtonite concentrations (Xcum=0.00–0.18) were observed, which generally increased with Al content. Rietveld refinements of the lattice constants showed a linear decrease of the cell parameters a and b with increasing Al content, whereas c and β increased. Small deviations from the linear behaviour were caused by variable amounts of the cummingtonite component. For pure tschermakite lattice parameters of a=9.7438(11) Å, b=17.936(14) Å, c=5.2995(3) Å, β=105.68(9)° and V=891.7 ± 1.4 Å3 were extrapolated by least-squares regression. Using the a and β lattice parameters for tremolite, tschermakite and cummingtonite, it was possible to derive amphibole compositions using powder XRD. IR spectra of the Al tremolites showed a total of 12 individual bands. The FWHMs of all bands increased with increasing Al content. According to their FWHMs, these bands were grouped into three band systems at 3664–3676 cm?1 (I), 3633–3664 cm?1 (II) and 3526–3633 cm?1 (III). Assuming [6]Al substitution at M2 and/or M3 and [4]Al at T1, three principal different configurational groups could be assigned as local environments for the proton. I: only Si4+ at T1 and one or two Al3+ at M2 and/or M3far, II: one Al3+ at T1 and one to three Al3+ at M2 and/or at M3far, III: either Al3+ on M3near and/or two Al3+ on T1 and additional one to four Al3+ at M2. It is assumed that these three configurational groups correspond to the three groups of observed bands. This was quantitativly supported by Monte-Carlo simulations. A model with random distribution at M2 and M3 including Al avoidance at tetrahedral and octahedral sites yielded the best agreement with the spectroscopical results.  相似文献   

18.
The blue colors of several minerals and gems, including aquamarine (beryl, Be3Al2Si6O18) and cordierite (Al3(Mg, Fe)2Si5AlO18), have been attributed to charge transfer (CT) between adjacent Fe2+ and Fe3+ cations, while Fe2+→Ti4+ CT has been proposed for blue kyanites (Al2SiO5). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe2+ and Fe3+ were detected with MS, indicating that Fe2+→Fe3+ CT, Fe2+→Ti4+ CT, and Fe2+ and Fe3+ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe2+ and Fe3+ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm?1 in E∥c spectra to Fe2+→Fe3+ CT between Fe cations replacing Al3+ ions 4.6Å apart along c. A band at 17,500 cm?1 in E⊥c spectra of cordierite is generally assigned to Fe2+ (oct)→Fe3+ (tet) CT between cations only 2.74 Å apart. However, no Fe3+ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm?1 band and reported to contain Fe3+. A quadrupole doublet with parameters consistent with tetrahedral Fe3+ appears in 77K MS, but the Fe3+/Fe2+ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe3+/Fe2+ ratios for minerals exhibiting Fe2+→Fe3+ CT.  相似文献   

19.
The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980 °C, based on the stability of this assemblage in the simplified chemical system FeO–MgO–Al2O3–SiO2 (FMAS) system. However, the potential for sapphirine to contain significant Fe3+ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz‐bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O (FMASTO), expansion into K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (KFMASHTO) allows the effect of the stability of the additional phases, biotite, K‐feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using P–T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P–T pseudosections, and the appropriate H2O and O contents to use in the modelled compositions are investigated using T–MH2O and T–MO pseudosections. The peak P–T estimates from a sapphirine + quartz‐bearing sample are ～960 to 935 °C at ～10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz ± garnet‐bearing samples. Whereas the sapphirine + quartz‐bearing sample is more Fe‐rich than the orthopyroxene + sillimanite‐bearing sample on an all‐Fe‐as‐FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.  相似文献   

20.
应育浦  宋仁奎 《地质科学》1995,30(4):355-362
21个多硅白云母来自4个低温高压变质带。用电子探针、X射线粉末衍射及穆斯堡尔谱测定其化学成分、b0值及Fe2+占位。建立Fe2+(M1)/Fe2+(M2)对b0值相关图,发现Fe2+在八面体晶位有序化并解释其有序机理。  相似文献   

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