Variation of infrared absorption spectra in the system Bi2Al4−x Fe x O9 (x = 0–4), structurally related to mullite

The crystal structure of Bi₂Al_4−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 Bi₂Al_4−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.     

Phase relations in Fe–Ni–C system at high pressures and temperatures

We performed comparative study of phase relations in Fe_1−x Ni _x (0.10 ≤ x ≤ 0.22 atomic fraction) and Fe_0.90Ni_0.10−x C _x (0.1 ≤ x ≤ 0.5 atomic fraction) systems at pressures to 45 GPa and temperatures to 2,600 K using laser-heated diamond anvil cell and large-volume press (LVP) techniques. We show that laser heating of Fe,Ni alloys in DAC even to relatively low temperatures can lead to the contamination of the sample with the carbon coming from diamond anvils, which results in the decomposition of the alloy into iron- and nickel-rich phases. Based on the results of LVP experiments with Fe–Ni–C system (at pressures up to 20 GPa and temperatures to 2,300 K) we demonstrate decrease of carbon solubility in Fe,Ni alloy with pressure.     

Variation of the oxygen content and point defects in olivines, (Fe x Mg 1−x )2SiO4, 0.2 ≤ x ≤ 1.0

 The variation of the oxygen content in olivines, (Fe _x Mg_{1− x} )₂SiO₄, with 0.2 ≤ x ≤ 1.0, was investigated by thermogravimetric measurements. Mass changes occurring upon oxygen activity changes were measured as a function of oxygen activity and cationic composition at 1130 and 1200 °C. During the measurements the samples were in direct contact with gases containing CO, CO₂ and N₂ and, at a few spots at the bottom of the sample stack, also with SiO₂. By fitting experimental data of mass changes to equations derived using point defect thermodynamics, it was shown for olivines with 0.2 ≤ x ≤ 1.0 at 1130 °C and 0.2 ≤ x ≤ 0.7 at 1200 °C within the oxygen activity ranges investigated that the observed variations in the oxygen contents are compatible with cation vacancies and Fe³⁺ ions on M sites and Fe³⁺ ions on silicon sites as majority defects if it is assumed that only three types of point defects occur as majority defects. Different cases were considered, closed systems, taking into account that ξ=[Si]/([Si]+[Fe]+[Mg]) is not necessarily equal to 1/3, and olivines in equilibrium with SiO₂ or pyroxenes. The oxygen content variations observed in this study are significantly smaller than those reported previously in the literature. It is proposed that these differences are related to the dissolution of Fe into noble metal containers used as sample holders in earlier studies and/or to the presence of secondary phases. Received: 1 November 1995 / Accepted: 15 September 2002 Acknowledgements This work was supported by the Cornell Center for Materials Research (CCMR), a Materials Research Science and Engineering Center of the National Science Foundation (DMR-0079992). The authors thank Mr. Daniel M. DiPasquo and Mr. Jason A. Schick for helping in experimental work.     

Microanalysis of Fe3+/ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge

The Fe M _2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe₃(Al_1–xFe_x)₂[SiO₄]₃ and andradite-skiagite (Fe_1–xCa_x)₃Fe₂[SiO₄]₃), pyroxenes (acmite-hedenbergite (Ca_1–xNa_x)(Fe²⁺ _1−xFe³⁺ _x)Si₂O₆), and spinels (magnetite-hercynite Fe(Al_1–xFe_x)₂O₄) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M _2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe³⁺/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe³⁺/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M _2,3 edge is observed. Since the partial cross sections of the Fe M _2,3 edges are some orders of magnitude higher than those of the Fe L _2,3 edges, the Fe M _2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M _2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe³⁺/ΣFe ratio over distances on the order of 100 nm. Received: 14 April 1998 / Revised, accepted: 8 March 1999     

The orientation and vibrational states of H2O in synthetic alkali-free beryl

 The polarized single-crystal Raman spectra of synthetic H₂O-containing alkali-free beryl were recorded at room and low temperatures, and the polarized single-crystal IR spectra at room temperature. The H₂O molecule in the channel cavities is characterized by a Raman-active symmetric stretching vibration (ν₁) at 3607 cm⁻¹ and an IR-active asymmetric stretch (ν₃) at 3700 cm⁻¹ at room temperature. At low temperatures this ν₃ mode is observed in the Raman. Weak ν₁ and ν₃ modes of a second type of H₂O are also observed in the Raman spectra but only at 5 K. The H⋯·H vector of the most abundant type of H₂O is parallel to the channel axis of beryl along [0 0 0 1]. The components of the polarizability tensor of the ν₁ mode of H₂O are similar to, but not exactly the same as, those of a free H₂O molecule. The Raman measurements indicate that the H₂O molecule is rotationally disordered around [0 0 0 1]. External translation and librational modes of H₂O could be observed as overtones with the internal H₂O-stretching modes. In the case of the librational motions, normal modes could also be observed directly in the Raman spectra at ∼200 cm⁻¹. The energies of the translational modes can be determined from an analysis of the overtones and are about 9 cm⁻¹ in energy (i.e., T_z). The energies of the librational modes are about 210 cm⁻¹ for R_x and 190 cm⁻¹ for R_y. Received: 8 April 1999 / Accepted: 5 April 2000     

Thermal decomposition of Ferrian chamosite: an infrared emission spectroscopic study

The thermal behaviour of ripidolite, an iron-rich chlorite, has been studied in situ by infrared emission spectroscopy up to 800 °C. The more di,trioctahedral nature due to significant amounts of Fe³⁺ is reflected, in addition to the two bands around 3420 and 3560 cm⁻¹, by an extra band around 3345 cm⁻¹. This extra band is absent in pure dioctahedral chlorites without Fe³⁺. These bands have been assigned to (AlAl)O-OH, (SiAl)O-OH and (SiSi)O-OH stretching modes with increasing frequencies. The bands disappear upon dehydroxylation around 650 °C. A similar behaviour is observed for the corresponding libration modes around 716, 759 and 802 cm⁻¹. The stretching and bending modes of the inner-OH of the octahedral sheet in the 2:1 clay-like layer are observed around 3645, 943 and 904 cm⁻¹. Although the bands decrease in intensity, they remain present up to 800 °C as dehydroxylation of the octahedral sheet is not yet complete at this temperature. The presence of two bending modes is explained as being due to a differentiation between Mg-OH and Fe-OH modes. At 650 °C a new sharp band is observed around 502 cm⁻¹ assigned to a (Fe,Mg)-O-Al bending mode caused by the formation of a spinel-like interlayer phase after dehydroxylation. Received: 4 June 1999 / Accepted: 6 August 1999     

The single-crystal,polarized-light,FTIR spectrum of stoppaniite,the Fe analogue of beryl

We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)₄(Be₆Si₁₂O₃₆)(H₂O)₂(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm⁻¹; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm⁻¹, plus minor features at 4,000 and 3,228 cm⁻¹ are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm⁻¹ band is assigned to the ν₃ stretching mode and the 1,620 cm⁻¹ (associated with an overtone 2*ν₂ at 3,230 cm⁻¹) band to the ν₂ bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm⁻¹ is not associated with a corresponding ν₂ bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm⁻¹ feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.     

Single-crystal electronic absorption spectroscopy of synthetic chromium-, cobalt-, and vanadium-bearing pyropes at different temperatures and pressures

 Single crystals of synthetic vanadium-, chromium- and cobalt-bearing garnets, Pyr:V0.06, Pyr:V0.13, Pyr:Cr0.04, Pyr:Co0.10, and Gt:Co3.00, and a natural vanadium-bearing grossular, Gross:V0.07 (Cr³⁺ < 0.005), were studied by electronic absorption spectroscopy in the wavenumber range 35 000–5000 cm⁻¹ under ambient conditions and at temperatures up to 600 K and pressures up to 8 GPa. The T and P behavior of the absorption band energies and intensities shows the following for the different transition metal-bearing garnets: Cr: The thermal expansion of chromium octahedra are similar to and the Racah parameter the same in synthetic Cr-doped pyrope, α_poly≅ 1.3 × 10⁻⁵ K⁻¹, and in natural pyrope, α_poly≅ 1.5 × 10⁻⁵ K⁻¹, and B=655 cm⁻¹, respectively. Ca^2+[8]-free garnets have a slightly stronger crystal field at the Y^[6] site and, therefore, the energies of the two spin-allowed Cr³⁺ dd bands are ca. 300 cm⁻¹ higher in Mg-pyrope than in natural Ca-bearing pyrope. Co: Increasing temperature causes only a small thermal expansion of the cobalt dodecahedra. Increasing pressure gives rise to appreciable compression, which is similar to that of the Fe²⁺-dodecahedra in almandine, where k=125 ± 25 GPa. T and P dependence of the Co band intensities may be caused by strong spin-orbit coupling. V: Occurs in at least two valence states and structural sites: (1) V³⁺ in octahedral sites gives rise to two spin-allowed bands, at 17 220 cm⁻¹ and 24 600 cm⁻¹, whose temperature dependence is typical for spin-allowed dd transitions in centrosymmetric sites. (2) V⁴⁺, which causes a set of dd absorption bands similar to those observed in the spectrum of V⁴⁺-doped Zr[SiO₄]. The P behavior of the V absorption bands indicates an interaction between V³⁺ and V⁴⁺ species. Received: 27 June 2001 / Accepted: 19 December 2001     

The Kinetics of the Interaction Between Iron(III)-Ethylenediaminetetraacetate and Peroxynitrite

The kinetics of the formation of the purple-colored species between Fe^III-EDTA and peroxynitrite were studied as a function of pH (10.4–12.3) at 22°C in aqueous solutions using a stopped-flow technique. A purple-colored species was immediately formed upon mixing, which had an absorbance maximum at 520 nm. The increase in absorbance with time could be fit empirically by a power function with two parameters a and b. The power equation determined was absorbance = a*t ^b , where a increased linearly with pH and the concentration of peroxynitrite, while b almost remained constant with a value of ~0.25. The molar extinction coefficient ε_520 nm for the colored species was determined as 13 M⁻¹cm⁻¹, which is much lower than ε_520 nm = 520 M⁻¹ cm⁻¹ for the [Fe^III(EDTA)O₂]³⁻, a purple species observed in the Fe^III–EDTA–H₂O₂ system. The results of kinetics and spectral measurements of the present study are briefly discussed and compared with those of the reaction between Fe(III)-EDTA and hydrogen peroxide.     

Distribution of Fe among octahedral sites and its effect on the crystal structure of pumpellyite

Two pumpellyites with the general formula W ₈ X ₄ Y ₈ Z ₁₂O_56-n (OH) _n were studied using ⁵⁷Fe Mössbauer spectroscopic and X-ray Rietveld methods to investigate the relationship between the crystal chemical behavior of iron and structural change. The samples are ferrian pumpellyite-(Al) collected from Mitsu and Kouragahana, Shimane Peninsula, Japan. Rietveld refinements gave Fe(X):Fe(Y) ratios (%) of 41.5(4):58.5(4) for the Mitsu pumpellyite and 46(1):54(1) for the Kouragahana pumpellyite, where Fe(X) and Fe(Y) represent Fe content at the X and Y sites, respectively. The Mössbauer spectra consisted of two Fe²⁺ and two Fe³⁺ doublets for the Mitsu pumpellyite, and one Fe²⁺ and two Fe³⁺ doublets for the Kouragahana pumpellyite. In terms of the area ratios of the Mössbauer doublets and the Fe(X):Fe(Y) ratios determined by the Rietveld refinements, Fe²⁺(X):Fe³⁺(X):Fe³⁺(Y) ratios are determined to be 22:14:64 for the Mitsu pumpellyite and 27:8:65 for the Kouragahana pumpellyite. By applying the Fe²⁺:Fe³⁺-ratio determined by the Mössbauer analysis and the site occupancies of Fe at the X and Y sites given by the Rietveld method together with chemical analysis, the resulting formula of the Mitsu and Kouragahana pumpellyites are established as Ca₈(Fe _0.88 ²⁺ Mg_0.68Fe _0.77 ³⁺ Al_1.66)_Σ3.99(Al_5.67Fe _2.34 ³⁺ )_Σ8.01Si₁₂O_42.41(OH)_13.59 and Ca₈(Mg_1.24Fe _0.65 ²⁺ Fe _0.46 ³⁺ Al_1.66)_Σ4.01(Al_6.71Fe _1.29 ³⁺ )_Σ8.00Si₁₂O_42.14(OH)_13.86, respectively. Mean Y–O distances and volumes of the YO₆ octahedra increase with increasing mean ionic radii, i.e., the Fe³⁺→Al substitution at the Y site. However, change of the sizes of XO₆ octahedra against the mean ionic radii at the X site is not distinct, and tends to depend on the volume change of the YO₆ octahedra. Thus, the geometrical change of the YO₆ octahedra with Fe³⁺→Al substitution at the Y site is essential for the structural changes of pumpellyite. The expansion of the YO₆ octahedra by the ionic substitution of Fe³⁺ for Al causes gradual change of the octahedra to more symmetrical and regular forms.     

Influence of hydrogen on Fe–Mg interdiffusion in (Mg,Fe)O and implications for Earth’s lower mantle

Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of MgO in contact with (Mg_0.73Fe_0.27)O were annealed hydrothermally at 300 MPa between 1,000 and 1,250°C and using a Ni–NiO buffer. After electron microprobe analyses, the dependence of the interdiffusivity on Fe concentration was determined using a Boltzmann–Matano analysis. For a water fugacity of ∼300 MPa, the Fe–Mg interdiffusion coefficient in Fe _x Mg_1−x O with 0.01 ≤ x ≤ 0.25 can be described by with and C = −80 ± 10 kJ mol⁻¹. For x = 0.1 and at 1,000°C, Fe–Mg interdiffusion is a factor of ∼4 faster under hydrous than under anhydrous conditions. This enhanced rate of interdiffusion is attributed to an increased concentration of metal vacancies resulting from the incorporation of hydrogen. Such water-induced enhancement of kinetics may have important implications for the rheological properties of the lower mantle.

Magnetic properties of the Fe2SiO4-Fe3O4 spinel solid solutions

 Magnetic measurement of Fe_{3− x} Si _x O₄ spinel solid solutions indicates that their Curie temperatures decrease gradually, but not linearly, from 851 to 12 K with increasing content of nonmagnetic ions Si⁴⁺. Magnetic hysteresis becomes more noticeable in solid solutions having a larger content of Fe₂SiO₄. Saturation magnetizations of Fe_{3− x} Si _x O₄ samples increase up to x=0.357 and they are easily saturated in the field of H=0.1 T. However, magnetization of the sample of x=0.794 does not approach saturation even at high field of H=7.0 T and has a large coercive force. The Si⁴⁺ disordered distribution is confirmed to be ^tetr[Fe³⁺ _{1− x + x t} Si⁴⁺ _{x (1− t )}] ^octa[Fe²⁺ _{1+ x} Fe³⁺ _{1− x − x t} Si⁴⁺ _{x t} ] O₄ by the spin moment, which is consistent with site occupancy obtained from X-ray crystal structure refinement. Their molecular magnetizations would be expressed as M _B={4(1+x)+10xt}μ_B as functions of composition parameter x and Si⁴⁺ ordering parameter t of the solid solution. The sample of x=0.794 is antiferromagnetic below the Néel temperature, mainly due to the octahedral cation interaction M _O–M _O, while both M _T–M _O and M _O–M _O interactions induce a ferrimagnetic property. Concerning magnetic spin configuration, in the case of x>0.42, the lowest dɛ level becomes a singlet, resulting in no orbital angular momentum. Received: 20 April 2000 / Accepted: 11 September 2000     

High-pressure Raman spectroscopic study of Fo90 hydrous wadsleyite

Raman spectra of monoclinic Fo₉₀ hydrous wadsleyite with 2.4 wt% H₂O have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium to 58.4 GPa at room temperature. The most intense, characteristic wadsleyite modes, the Si–O–Si symmetric stretch at 721 cm⁻¹ and the symmetric stretch of the SiO₃ unit at 918 cm⁻¹, shift continuously to 58.4 GPa showing no evidence of a first order change in the crystal structure despite compression well beyond the stability field of wadsleyite in terms of pressure. The pressure dependence of these two modes is nearly identical for Fo₉₀ hydrous and Fo₁₀₀ anhydrous wadsleyite. A striking feature in the high-pressure Raman spectra of Fo₉₀ hydrous wadsleyite is the appearance of new Raman modes above 9 GPa in the mid-frequency range (300–650 cm⁻¹ at 1-bar and shifted to 500–850 cm⁻¹ at 58.4 GPa) accompanied by a significant growth in their intensities under further compression. In the OH stretching frequency range Fo₉₀ hydrous wadsleyite exhibits a larger number of modes than the Mg end-member phase. The higher number of modes may be due to either additional protonation sites or simply that we observe a different subset of all possible OH modes for each sample. The high-pressure behaviour of the OH stretching modes of Fo₉₀ and Fo₁₀₀ hydrous wadsleyite is consistent: OH stretching modes with frequencies <3,530 cm⁻¹ decrease with increasing pressure whereas the higher-frequency OH modes show a close to constant pressure dependence to at least 13.2 GPa. The approximately constant pressure dependence of the OH modes above 3,530 cm⁻¹ is consistent with protons being located at the O1···O edges around M3.     

Tantalum-bearing titanite: synthesis and crystal structure data

Synthetic titanite, CaTiOSiO₄, and the series of (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ and Ca(Ti_1−2x Ta _x Al _x )OSiO₄ solid solutions have been prepared by ceramic methods, and their crystal structure determined by the Rietveld analysis. At ambient conditions, titanite can contain up to 20 mol% NaTaOSiO₄ or 60 mol% Ca(Al_0.5Ta_0.5)OSiO₄. These limits might differ in natural samples due to combination with substitutions involving fluorine and/or hydroxyl replacing oxygen together with vacancies at cationic sites. All cations located at the ^vii X- and ^vi Y-sites in the structures of tantalian titanite are disordered. Expansion of the <Si–O> bond from 1.618 to 1.621 Å in CaTi_0.8Ta_0.1Al_0.1OSiO₄ and CaTi_0.6Ta_0.2Al_0.2OSiO₄ to 1.644 Å in the CaTi_0.4Ta_0.3Al_0.3OSiO₄ titanite suggests the possible presence of some Al³⁺ in the tetrahedral site replacing Si⁴⁺ in the latter. All tantalian titanites crystallize in the space group A2/a. This implies that both single-site and complex double-site substitutional schemes induce P2₁/a → A2/a phase transition(s). The (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ substitution scheme incorporates larger cations at both the ^vii X and ^vi Y sites, whereas the Ca(Ti_1−2x Ta _x Al _x )OSiO₄ scheme involves only ^vi Y-site (Al³⁺,Ta⁵⁺) cations with a slightly smaller “average” radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ series, and with an insignificant decrease in the ^viR _Y average cationic radii in the Ca(Ti_1−2x Ta _x Al _x )OSiO₄ series. Both Ta-doped titanite and CaTiOSiO₄ consist of distorted polyhedra with the XO₇, YO₆ coordination polyhedra and the SiO₄ tetrahedron in tantalian titanite being less distorted compared to those of the pure CaTiOSiO₄.     

Temperature dependence of the polarized electronic absorption spectra of olivines. Part I – fayalite

 Polarized electronic absorption spectra of orthorhombic fayalite, Fe₂SiO₄, [E || a(|| Z),E || b(|| X), E || c(|| Y)], space group Pbnm, have been studied in the temperature range 293 ≤T/K ≤ 1273. The spectra were analysed into component bands originating from spin-allowed dd transitions of iron(II) at the different sites, M1 and M2, in the structure. The assignments of bands, made on the basis of the polarization dependence of the spectra and considerations of transition energies, were confirmed by the analysis of the temperature-dependent spectra. The temperature dependencies of integral intensities, half band widths and energy positions of absorptions bands caused by Fe²⁺ on the different octahedral sites, M1 and M2, were evaluated for the individual transitions. Independent of the site symmetry, absorption bands shift to lower energies and half band widths increase on rising temperature. The temperature dependence of band intensities depends on site symmetry. The integral intensities are found to increase with temperature for the transition metal ion on a centrosymmetric site, or remain constant when the site is missing an inversion centre. This is consistent with the general conclusion of Taran et al. (1994). Received: 11 October 2001 / Accepted: 17 January 2002     

Ab initio quantum mechanical study of γ-AlOOH boehmite: structure and vibrational spectrum

The structure and vibrational spectrum of boehmite have been investigated at the quantum-mechanical level with the CRYSTAL code, using a Gaussian-type basis set and the B3LYP Hamiltonian. Three space groups are considered in this study: Cmcm, Cmc2₁, P2₁/c. Cmcm turns out to correspond to a transition state, whereas Cmc2₁ and P2₁/c are minimum energy structures. The difference among them is the position of H atoms only, the Al-O frame being essentially the same. Harmonic frequencies at the Γ point have been computed. The comparison between calculated and experimental frequencies shows a good agreement for the Al-O part of the spectrum (under 790 cm⁻¹). For the Al-OH bending modes (800–1,300 cm⁻¹) an absolute differences of 50–100 cm⁻¹ is observed; for the OH stretching modes (3,200–3,500 cm⁻¹) it increases to 120–200 cm⁻¹: anharmonicity is large because OH groups are involved in strong hydrogen bonds.     

Raman spectroscopy, vibrational analysis, and heating of buergerite tourmaline

Polarized Raman spectra were collected for single crystal buergerite (NaFe₃Al₆(BO₃)₃Si₆O₁₈(O_0.92(OH)_0.08)₃F) from room temperature to near 1,375°C. Vibrational assignments to features in the room temperature spectra were determined by lattice dynamics calculations, where internal BO₃ motions dominate modes near 1,300 cm⁻¹, internal SiO₄ displacements dominate modes between 900 and 1,200 cm⁻¹, while less localized displacements within the isolated Si₆O₁₈ ring mix with motions within Na, Fe, Al, F, and BO₃ environments for fundamental modes below 780 cm⁻¹. At elevated temperatures, most buergerite Raman features broaden and shift to lower frequencies up to 900°C. Above this temperature, the lattice mode peaks evolve into broad bands, while OH stretch modes near 3,550 cm⁻¹ disappear. According to Raman spectroscopy, X-ray diffraction, differential thermal analysis, and scanning electron microscopy, buergerite undergoes a complex transition that starts near 700°C and extends over a 310°C interval, where initially, Al and Fe probably become disordered within the Y- and Z-sites, and most F and all OH are later liberated. A reversible crystal-to-amorphous transition is seen by Raman for buergerite fragments heated as high as 930°C. Buergerite becomes permanently altered when heated to temperatures greater than 930°C; after cooling to room temperature, these altered fragments are comprised of mullite and Fe-oxide crystals suspended in an amorphous borosilicate matrix.     

Fe–Mg partitioning between ringwoodite and magnesiowüstite and the effect of pressure, temperature and oxygen fugacity

 The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200–1600 ^∘C using both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence on the capsule material used due to the variation in Fe³⁺ concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules, where metallic Fe was also added to the starting materials, the difference in the interaction parameters for the two solid solutions (W _FeMg ^mw−W _FeMg ^ring) is calculated to be 8.5±1 kJ mol⁻¹. Similar experiments performed in Re metal capsules result in a value for W _FeMg ^mw−W _FeMg ^ring that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic analyses, however, show Fe³⁺ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and that Fe³⁺ partitions preferentially into magnesiowüstite, with K _{D Fe3+} ^ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, a least–squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W _FeMg ^ring) of 3.5±1 kJ mol⁻¹. The equivalent regular interaction parameter for magnesiowüstite (W _FeMg ^mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe³⁺ concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol⁻¹ for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T−298)+18.9P with T in K, P in kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of ringwoodite to form magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of these results with previous studies shows that the presence of Fe³⁺ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined thermodynamic properties. Received: 13 September 2000 / Accepted: 25 March 2001     

Pyrolytic and spectroscopic studies of Eocene resin from Vastan lignite Mine,Cambay basin,Western India

The molecular structure of an Eocene fossil resin (Vastan, Cambay basin, Western India) has been investigated with complimentary spectroscopic techniques. The FTIR spectrum shows strong aliphatic CH _x (3000–2800 and 1460–1450 cm⁻¹) and CH₃ (1377 cm⁻¹) absorptions and less intense aromatic C=C (1560–1610 cm⁻¹) absorptions. The major products from analytical pyrolysis are cadalene based bicyclic sesquiterpenoids including some bicadinenes and bicadinanes. The polycadinane products confirm the fossil material as an Angiosperm dammar resin, associated with inputs of tropical rain forests supported by past climates.     

Linked four-membered silicate rings: vibrational analysis of Gillespite

Raman spectra and lattice dynamics calculations are presented for the silicate mineral gillespite, BaFeSi₄O₁₀, which contains sheets of linked four-membered silicate rings. The results are analyzed in relation to earlier work done on the isolated four-membered ring silicate BaCuSi₂O₆ and to published claims that vibrational modes of four-membered rings are responsible for the sharp D ₁“defect line” observed in the Raman spectra of SiO₂ glass. The crystal structure of gillespite (space group P4/ncc or D ) consists of puckered Si₄O₁₂ rings, where each SiO₄ tetrahedron is linked to two neighboring tetrahedra within a ring and to a third tetrahedron within a different ring. The rings are linked to each other in a staggered configuration to form sheets, which are also bonded together by Ba²⁺ and Fe²⁺. The calculation adjusts the bond bending and bond stretching force constants so that calculated fundamental mode frequencies best fit observed fundamental frequencies in the Raman spectra; eigenmodes associated with each calculated fundamental mode are then generated. Some eigenmodes calculated are unique to the gillespite structure, but many of the more localized four-membered ring modes are similar to those calculated for the four-membered rings in BaCuSi₂O₆ and for the three- and six-membered rings in the cyclosilicates studied earlier. The results for gillespite indicate that the Raman-active A _1g mode at 450 cm⁻¹ is a four-membered ring bridging oxygen breathing mode that is mixed with other minor ring deformation displacements; because of this mixing, the calculated frequency of this mode is 45 to 61 cm⁻¹ lower than that calculated for bridging oxygen binding breathing modes of puckered four-membered rings in BaCuSi₂O₆ and in simulated glass structures. Received: 9 April 1997 / Revised, accepted: 19 August 1997