A temperature-dependent single-crystal Raman spectroscopic study of fayalite: evidence for phonon-magnetic excitation coupling

The polarized single-crystal Raman spectrum of synthetic fayalite, Fe₂SiO₄, was recorded between 5 and 773 K in order to investigate its lattice dynamic behavior. A broad absorption envelope is observed at wavenumbers between 800 and 960 cm^–1 and it contains two intense bands at 816 and 840 cm^–1 at 293 K in the (cc) spectrum. The integral area of the envelope decreases upon cooling from 293 K and reaches a minimum around 55 K. It then increases again with a further decrease in temperature down to 5 K. It is proposed that the envelope in the (cc) spectra consists of seven different modes, some of which are symmetry-forbidden, that arise from combination scattering of nonsymmetric internal SiO₄-stretching modes of B_ig symmetry (i = 1, 2, 3) and low-energy excitations. The individual modes can be observed under different polarizations and agree in number and wavenumber with those obtained by fitting the broad envelope with Lorentzians. An analysis of the Raman spectrum as a function of temperature, using the known magnetic properties of fayalite, allows the assignment of the low-energy excitations to short-range magnetic interactions. Modulation of the Fe²⁺(1)–Fe²⁺(2) exchange energy leads to phonon-magnetic excitation coupling and the main role in the Fe²⁺(1)–Fe²⁺(2) magnetic interaction occurs via superexchange through the oxygens. The magnetic excitations are not magnons in the usual sense, that is as quasiparticles having a long wavelength in an ordered system. The degree of observed broadening of the SiO₄-stretching modes is consonant with a Fe²⁺(1)–Fe²⁺(2) exchange energy of 4.7 cm^–1 presented by Schmidt et al. (1992). At temperatures above 300 K the line width of the mode at 840 cm^–1 decreases slightly, whereas those of low energy lattice modes increase. This suggests that a decrease in mode broadening due to weakened magnetic interactions compensates any thermally related broadening. Complete Fe²⁺ spin disorder may not be reached until at least 530 K. Results from this study show that estimates of third-law entropies for silicates using simple crystal-chemical considerations that do not account for magnetic properties cannot give accurate values for many transition-metal-containing phases.     

The effect of silica activity on the incorporation mechanisms of water in synthetic forsterite: a polarised infrared spectroscopic study

Pure forsterite crystals were grown from hydrous melts using controlled cooling experiments at 2.0 GPa and varying the bulk Mg/Si ratio from 2.0 to 1.5. Oriented single crystals were then studied by polarised infrared spectroscopy. The spectra of the samples with the lowest silica activity (aSiO₂) contain the main OH bands in the range 3,620–3,450 cm^–1 only. In contrast, the spectra of the samples synthesised with the highest aSiO₂ contain additional pleochroic bands at 3,160, 3,220 and 3,600 cm^–1. The variations are interpreted in terms of protonated silicon vacancies being dominant at low aSiO₂ and Mg vacancies dominant at high aSiO₂. Xenolithic mantle olivines generally do not have the spectrum expected for orthopyroxene buffered conditions, suggesting that they re-equilibrated with their host melts during ascent, but mantle olivine from the Zabargad peridotite massif probably is in equilibrium with the coexisting orthopyroxene.Editorial responsibility: T.L. Grove     

Micro-FTIR spectroscopy of liptinite macerals in coal

Reflectance FTIR microspectroscopy has been used to investigate the chemical structure of the liptinite macerals, alginite, bituminite, sporinite, cutinite and resinite in bituminous coals of Carboniferous to Tertiary age. In comparison with the spectra of vitrinite in the same coals, the micro-FTIR spectra of liptinite macerals are characterized by stronger aliphatic CH_x absorptions at 3000–2800 and 1460–1450 cm⁻¹, less intense aromatic C=C ring stretching vibration and aromatic CH out of plane deformation at 1610–1560 and 900–700 cm⁻¹ respectively and various intense acid C=O group absorptions at 1740–1700 cm⁻¹. The peaks at 1000–900 cm⁻¹ due to aliphatic CH₂ wagging vibrations in olefins and at 730–720 cm⁻¹ due to CH₂ rocking vibration in long chain aliphatic substances ([CH₂]_n, n≥4), are characteristic of liptinite macerals. Collectively the micro-FTIR spectral characteristics indicate that liptinite is composed of greater numbers of long chain aliphatics, fewer aromatics and a broader range of oxygen-containing groups than other macerals. Marked differences exist in micro-FTIR spectra within the liptinite maceral group. Alginite has the strongest aliphatic and least aromatic absorptions followed by bituminite, resinite, cutinite and sporinite. The aliphatic components in alginite are the longest chained and least branched whereas those in sporinite are the shortest chained and most branched. Bituminite, resinite and cutinite are intermediate. Notable differences in micro-FTIR spectra of individual liptinite macerals, such as intensities and peak locations of aromatic C=C in alginite, C=O groups in bituminite and resinite and substituted aromatic CH and C–O–C groups in cutinite and sporinite, also exist, which are attributed to differences in depositional environments or biotaxonomy.     

A Raman spectroscopic study of H/D isotopically substituted hydrous aluminosilicate glasses

We have obtained high quality Raman spectra for two H/D isotopically substituted hydrous aluminosilicate glasses with compositions along the NaAlSi₃O₈-SiO₂ join. Consistent with the results of previous studies, the isotope shift for the band near 900 cm^–1, whose intensity grows with increasing water content, is extremely small: v _h /v _d = 1.004 ± 0.004. The lack of a definite H/D isotope shift for this band does not, however, preclude its association with a vibration of a hydrous species in the glass, because of likely strong coupling between different vibrational modes of hydrated framework species. The 900 cm^–1 band could well be due to a T — OH (T = Si, Al) stretching or bending vibration in the hydrous glass, as required by the presence of a combination band near 4500 cm^–1 in near-infrared spectra.     

Characterization of vivianite from Catavi,Llallagua Bolivia

Summary Vivianite from Catavi Mine, Llallagua, Bolivia, has a near ideal composition with traces of Mg, Zn and Mn. Total rare-earth elements are < 1,gmg/g. Mössbauer spectroscopy shows Fe^III/(Fe^II + Fe^III) is approximately 0.04.a = 10.030Å,b = 13.434Å,c = 4.714Å, = 102.73dg. The middle-infrared powder spectrum shows H₂O-related bands at 3490, 3290, 3130 cm^–1 (stretch), 1618 cm^–1 (bend), 825 cm^–1 (rock), and at 665 cm^–1 a possible M-OH₂ twist. P0₄ bands occur at 1045-940 cm^–1 (stretch) and 570-450 cm^–1 (bend). Corresponding laser Raman microprobe bands occur at 1051 (ms), 986 (m), 948 (vs), 867 (mw), 828 (w), 568, 532, 453 (m), 442 (mw). Weak Raman bands at about 342, 303, 270 (w), 235 (ms), 227 (sh, ms), 196 (ms), 187 (sh, m), 162 (mw), and 126 (m) may arise from lattice vibrations. Differential thermal responses include a major endotherm from 115–235°C with a shoulder at 170°C and a maximum at 210°C resulting from loss of structural water combined with oxidation of Fe²⁺, and two small exotherms with maxima at 605 and 780°C related to structural transformations.

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Charakterisierung des Vivianits von Catavi, Llallagua, Bolivien

Zusammenfassung Vivianit von der Catavi Mine, Llallagua, Bolivien zeigt annähernd ideale Zusammensetzung mit Spuren von Mg, Zn und Mn. Der gesamte Gehalt an seltenen EvolElementen ist < 1 ppm. Die Mössbauer Spektroskopie liefert ein Fe³⁺/(Fe²⁺ + Fe³⁺) Verhältnis von ungefähr 0.04.a = 10.030,b = 13.434,c = 4.714 Å, = 102.73°. Das Infrarot-Pulverspektrum zeigt dem H₂0 zuzuordnende Banden bei 3490, 3290, 3130 cm^–1 (Streckschwingungen), 1618 cm^–1 (Deformationsschwingung), 825 cm^–1 (Schaukelschwingung) und eine mögliche M-OH₂ Torsionsschwingung bei 665 cm^–1. PO₄ Banden liegen bei 1045-940 cm^–1 (Streckschwingung) und 570-450 cm^–1 (Deformations-schwingung). Entsprechende Banden der Laser Raman Mikrosonde liegen bei 1051 (mst), 986 (m), 948 (sst) 867 (mschw), 828 (schw), 568, 532, 453 (m), 442 (mschw). Raman Banden bei etwa 342, 303, 270 (schw), 235 (mst), 227 (Schulter, mst), 196 (mst), 187 (Schulter, m), 162 (mschw) und 126 (m) können auf Gitterschwingungen zurückgeführt werden. Differential-thermoanalytische Untersuchungen zeigen einen endothermen Bereich von 115–235°C mit einer Schulter bei 170 und einem Maximum bei 210°C, was auf den Verlust von strukturellem Wasser, das an eine Oxidation des Fe²⁺ gebunden ist, zurückzuführen ist; zwei auf strukturelle Transformationen zurückzuführende exotherme Maxima liegen bei 605 und 780°C.

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With 4 Figures     

OH defects in cassiterite

Summary Two infrared (IR) spectral types are discerned in cassiterites with OH defect concentrations ranging from several wt. ppm H₂O in synthetic melt-grown samples up to 0.017wt.% in natural samples of hydrothermal origin. Spectrum type I is characterised by one sharp band centred at 3255cm^–1, type II spectra reveal an additional band group centred around 3340cm^–1, which is clearly correlated with the presence of Ti. Both OH band groups are strongly polarised with maximum absorption perpendicular to the c-axis. Type I spectra prevail in nearly colourless samples, type II spectra typically occur in light to dark brown cassiterites, bearing essentially higher amounts of minor elements, especially Ti, Nb, Ta, W, Zr, and Fe. The total amount of water is not significantly correlated with the minor element content. It is concluded that different OH concentrations reflect different water activities prevalent during the formation of cassiterite crystals in different geological environments.     

Local relaxation around [6]Cr 3+ in synthetic pyrope–knorringite garnets, [8]Mg 3 [6](Al 1-X Cr X 3+) 2 [4]Si 3O 12, from electronic absorption spectra

Pyrope-knorringite garnets, Mg₃(Al_1-X Cr³⁺_X)₂Si₃O₁₂ with X=0.25, 0.50, and 1.00, were synthesized between 9 and 16 GPa and 1300 and 1600 °C, using multianvil high-pressure techniques. The garnets with X=0.25 and 0.50 are fine-grained, pink and violet in color. The end-member knorringites with X=1.00 are black when compact and gray when coarse-grained. The fine powder is greenish gray in natural light and pale pink under a tungsten lamp. Powder remission spectra in the wavenumber range 30 000–10 000 cm^–1 on finely powdered crystals were measured by two different methods: (I.) by the use of a small integrating sphere for small samples or (II.) microscope-spectrometric measurement using diffusely reflected radiation from a 45° illuminated microsample. Both methods yielded similar diffuse reflectance spectra. The following crystal-field parameters of ^[6]Cr³⁺ were determined for garnets with X=0.25, 0.50, 1.00: 10 Dq=17 856, 17 596, 17 286 cm^–1; and B=654, 677, 706 cm^–1; nephelauxetic ratio =(B_field/B_free)= 0.71, 0.74, 0.77. The -values indicate decreasing covalency of the Cr–O bond with increasing Cr content. The 10 Dq value for together with the mean Cr–O distance in end-member knorringite, 1.96 Å (Novak and Gibbs 1971), were used to calculate from the spectral data, local mean Cr–O distances (Langer 2001a) as a function of composition. The results indicate relatively strong local site relaxation with a value of =0.77.     

Experimental study on the phase equilibria in the join CaMgSi2O6-CaCrCrSiO6 with special reference to the blue diopside

The behaviour of tetrahedrally coordinated and octahedrally coordinated Cr³⁺ ions in diopside is discussed from the study on the join CaMg-Si₂O₆-CaCrCrSiO₆. The molecule CaCrCrSiO₆ decomposes into uvarovite+eskolaite and its maximum solubility in diopside is 6.7 wt percent at 940 ° C. Crystalline phases are diopside ss (ss is abbreviation of solid solution), uvarovite ss, wollastonite ss, spinel and eskolaite. The diopside ss is blue in colour. Its optical spectra were measured in the wavelenght range of 325–2600 nm, and assigned after tetrahedral configuration Td and octahedral configuration Oh. It is estimated that octahedral Cr³⁺ ions are in high spin state, while tetrahedral Cr³⁺ ions may be probably in low spin state. The _t and B are 10,300–10,370 cm^–1 and 429–432 cm^–1. The CFSE for tetrahedral low spin Cr³⁺ ions is nearly the same as that for octahedral high spin Cr³⁺ ions. The ionic radii of tetrahedral low spin Cr³⁺ ions calculated are 0.47–0.53 Å, shrinked from the radius of octahedral high spin Cr³⁺ ion (0.615 Å) as much as 14–24 percent. Petrologic implications of the result are also discussed.The first half of the D. Sc. dissertation of K. Ikeda presented to Hokkaido University in June, 1976     

Structural changes and valence states in the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–FeCr<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution series

The influence on the structure of Fe²⁺ Mg substitution was studied in synthetic single crystals belonging to the MgCr₂O₄–FeCr₂O₄ series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mössbauer spectroscopy. The Mössbauer data show that iron occurs almost exclusively as ^IVFe²⁺. Only minor Fe³⁺ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr²⁺ and Cr³⁺ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a₀, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) Å corresponding to the Cr³⁺ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe²⁺ content (apfu) is described by the following linear relations: a₀=8.3325(5) + 0.0443(8)Fe²⁺ (Å) and T–O=1.9645(6) + 0.033(1)Fe²⁺ (Å) Consequently, Fe²⁺ and Mg tetrahedral bond lengths are equal to 1.998(1) Å and 1.965(1) Å, respectively.     

Holocene Siliceous Sediments in the Sea of Okhotsk

The mineral and chemical compositions and physical properties of diatomaceous clayey–siliceous sediments in the Sea of Okhotsk are studied. Absolute masses of silica accumulation are determined. Their compositional model based on the silica content is similar to that of Late Jurassic and Olenekian–middle Anisian cherts of the Sikhote Alin region. The thickness of the Holocene siliceous unit and the absolute mass of siliceous deposits depended on bioproductivity in the upper water column and the seafloor topography. Absolute masses of SiO₂ am (0.05–5.7 g cm^–2 ka^–1) and SiO₂ fr (0.5–11.6 g cm^–2 ka^–1) are minimal on seamounts and maximal in depressions near foothills. These values match absolute masses of SiO₂ fr accumulations in Triassic and Late Jurassic basins of the Sikhote Alin region (0.33–3 g cm^–2 ka^–1). Comparison of the composition and absolute masses of silica shows that Triassic and Late Jurassic siliceous sequences of Sikhote Alin could be accumulated in the marginal marine basin near a continent.     

The combined effects of ƒO2 and melt composition on SnO2 solubility and tin diffusivity in haplogranitic melts

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO₂ solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log ƒ_O2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn²⁺ species, whereas at oxidized conditions SnO₂ is much less soluble, and tin is present dominantly as a Sn⁴⁺ species. There is a strong melt composition control on SnO₂ solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log ƒ_O2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO₂ for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO₂ solubility is 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn0₂ solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn⁴⁺/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from 0.4 at log ƒ_O2 of FMQ + 3.49 to 0.01 at FMQ - 0.57. Over the same log foZ range the Sn⁴⁺/ΣSn ratio for the A.S.I. 0.6 composition ranges from 0.98 to 0.4 and for the A.S.I. 1.25 composition, from 0.8 to 0.02.Tin diffusivity is dependent on both f_O2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10^−7.5 cm²/sec for the peraluminous compositions and 10^−8.2 cm²/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10^−8.2 and 10^−9.0 cm²/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn²⁺ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn⁴⁺ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn²⁺ and Sn⁴⁺ is the same, but the bond strengths are different. At fixed f_O2 the faster diffusivity in the peraluminous compositions reflects the lower Sn⁴⁺/Sn²⁺ ratio. The fact the Sn⁴⁺/Sn²⁺ ratio in melts varies greatly with ƒ_O2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.     

Single crystal infrared spectra in the range of OH fundamentals of paragenetic garnet,omphacite and kyanite in an eklogitic mantle xenolith

Summary Single crystals of paragenetic garnet (gt), omphacite (cpx) and kyanite (ky) were isolated from an eclogite xenolith from the Zagadochnaya kimberlite, Yakutia (grospydite Z13) and studied by polarized FTIR-microspectrometry in the OH valence vibrational region and by microprobe analyses. The coexisting minerals are homogeneous with respect to major and minor elements and have compositions near gross₄₉pyr₂₆alm₂₀uvar₄ (gt), jad₄₅dio₄₇hed₆kos₂ (cpx) and ky_>97. Single crystal spectra show one _OH-band for gt at 3630 cm^–1 (halfwidth ca. 100 cm^–1) which is very likely caused by vibrations of tetrahedral (OH)₄-^4– clusters replacing SiO₄ ^4– tetrahedra. Cpx shows one strong, but weakly pleochroic band at 3464 cm^–1 (halfwidth ca. 160 cm^–1) and a weak satellite band centered at 3620 cm^–1) with a distinct pleochroism. Ky OH spectra exhibit two sets of weak sharp pleochroic bands, a triplet, characteristic for high pressure ky, at 3439, 3410, 3387 cm^–1) and a doublet at 3279, 3264 cm^–1) (halfwidths ca. 10 cm^–1) From integral and linear absorbances in the unpolarized spectra defect-hydroxyl contents in the three coexisting minerals were estimated and found near O.OX wt% H₂O. The distribution scheme of hydrogen in the paragenetic minerals was evaluated to be c^ky < c^cpx < c^gt.

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Einkristall Infrarotspektren im Bereich der OH Grundschwingungen einer Granat-Omphacit-Kyanit Paragenese in einem eklogitischen Mantelxenolith

Zusammenfassung Aus einer Granat (Gt)-Omphacit (Cpx)-Kyanit (Ky) Paragenese eines Eklogit Xenoliths aus dem Zagadochnaya Kimberlit, Jakutien (Grospydit Z13), wurden Einkristalle isoliert und mit polarisierter FTIR-Mikrospektrometrie im OH Valenzschwingungsbereich und mit der Elektronenstrahl-Mikrosonde untersucht. Die koexistierenden Minerale sind hinsichtlich ihrer Haupt- und Nebenelemente homogen und haben Zusammen setzungen von etwa Gross⁴⁹Pyr²⁶Alm²⁰Uvar₄ (Gt), Jad₄₅Dio₄₇Hed₆Kos₂ (Cpx) and Ky_>97 Einkristallspektren von Gt zeigen eine _OH-Bande bei 3630 cm^–1) (Halbwerts breite ca. 100 cm^–1) die wahrscheinlich von Schwingungen tetraedrischer (OH)₄ ^4– Gruppen, die SiO₄ ^4– Tetraeder ersetzen, herrührt. Cpx zeigt eine starke, aber schwach pleochroitische Bande bei 3464 cm^–1 (Halbwertsbreite ca. 160 cm^–1) and eine schwache, deutlich pleochroitische Satellitenbande bei 3620 cm^–1) Ky OH Spektren zeigen zwei Gruppen von schwachen, scharfen pleochroitischen Banden, ein fur Hochdruck Ky charakteristisches Bandentriplett bei 3439, 3410, 3387 cm^–1) and ein Bandendublett bei 3279, 3264 cm^–1) (Halbwertsbreiten ca. 10 cm^–1) Aus den integralen and linearen Extinktionen der nicht-polarisierten Spektren wurde der Defekt-Hydroxyl Gehalt der drei koexistierenden Minerale abgeleitet and mit O.OX Gew% H₂O festgelegt. Das Verteilungsschema des Wasserstoffs kann in der Mineralparagenese mit c^Ky < c^Cpx c^Gt angegeben werden.

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Dedicated to Prof. Dr. J. Zemann on the occasion of his 70th birthday

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With 3 Figures     

Infrared investigation of CO2-bearing cordierites

CO₂ molecules were introduced experimentally into the structural channels of synthetic well ordered Mgcordierite (Mg₂ ^[VI][(Al₄Si₅)^[IV]O₁₈]) at temperatures of 600 and 800° C, and pressures of 7, 8, 10, 12, and 25 kbar. Powder infrared spectra of the run products show five absorption bands in the region of the asymmetric stretching mode of CO₂. Two of them, strong and sharp, occurring at 2353 cm^–1 {2} and 2348 cm^–1 {3}, are related to two different types of CO₂ molecules. The relative intensity of the band {2} (type I) increases with the cell parameter c_o whereas the relative intensity of the band {3} (type II) increases with the parameter a₀ of the crystal. It is concluded that CO₂ molecules of type I may be oriented with their elongation parallel to the c-axis of the crystal, while CO₂ molecules of type II lie with their O-C-O vector parallel to the a-axis. Analytical data indicate that the intensity ratio Z of these two bands ({2}/{3}) is a linear function of the CO₂ content of cordierite. This ratio depends also on the temperature and, to a less extent, on the pressure under which cordierite entrapped CO₂ molecules. It is proposed to combine this infrared parameter Z together with an estimate of the P-T conditions of the incorporation of CO₂ into the channels, in order to determine the CO₂ content of natural cordierites. The samples do not need to be of high purity and only small amounts (<5 mg) are necessary. This semi-empirical analytical method, which does not require complicated data treatments, is suitable for CO₂-rich cordierites of granulite facies rocks.     

Vibrational spectroscopy of beta-eucryptite (LiAlSiO<Subscript>4</Subscript>): optical phonons and phase transition(s)

The structural behavior of -eucryptite (LiAlSiO₄) has been investigated using infrared (IR) spectroscopy over a temperature range of 20 to 900 K and FT-Raman spectroscopy at room temperature. IR reflectance measurements show that -eucryptite possesses high reflectivity in the far-IR region, as is consistent with its reported superionic conductivity along the c-axis. On heating, the Li-related IR bands near 246 and 300 cm^–1 (with A₂ symmetry) broadened and weakened dramatically, presumably as a result of Li⁺ positional disordering along the structural channels parallel to c. The disordering process appears to induce a framework distortion, as is evidenced by the broadening of some vibrations of Si(Al)–O with increasing temperature. A change in slope in the temperature dependence of the phonon frequency near 300 cm^–1 and the linewidth of the 760 cm^–1 band at 715 K indicates that Li becomes completely disordered above this temperature. In addition, the temperature dependence of the linewidth for the 760 cm^–1 band exhibits an additional change in slope at 780 K, implying the existence of an intermediate state within this temperature range. The detailed structure of this intermediate phase, however, needs further study. Our IR data provide no indication of structural changes between room temperature and 20 K.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Reaction mechanisms,physical conditions,and mass transfer during hydrothermal alteration of mica and feldspar in granitic rocks from south-central Maine,USA

Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa. Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X _{CO 2} = 0–0.13; ln([K⁺]/[H⁺ ]) = 10.0; ln([Ca²⁺]/[H⁺]²)=9.1; ln([Na⁺]/[H⁺]) = 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm³ altered rock) of CO₂, H₂O, H⁺, and K⁺ were added to the granites by fluids during the alteration, as well as lesser amounts (< 0.01–0.03 mol/1,000cm³ altered rock) of Mg²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Na⁺, and Ti⁴⁺. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm³ rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm³ fluid/1,000cm³ altered rock to participate in the hydrothermal event.     

Raman spectroscopy and heat capacity measurement of calcium ferrite type MgAl2O4 and CaAl2O4

Raman spectroscopy of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ and heat capacity measurement of CaAl₂O₄ calcium ferrite were performed. The heat-capacity of CaAl₂O₄ calcium ferrite measured by a differential scanning calorimeter (DSC) was represented as C_P(T)=190.6–1.116 × 10⁷T^–2 + 1.491 × 10⁹T^–3 above 250 K (T in K). The obtained Raman spectra were applied to lattice dynamics calculation of heat capacity using the Kieffer model. The calculated heat capacity for CaAl₂O₄ calcium ferrite showed good agreement with that by the DSC measurement. A Kieffer model calculation for MgAl₂O₄ calcium ferrite similar to that for CaAl₂O₄ calcium ferrite was made to estimate the heat capacity of the former. The heat capacity of MgAl₂O₄ calcium ferrite was represented as C_P(T)=223.4–1352T ^–0.5 – 4.181 × 10⁶T ^–2 + 4.300 × 10⁸T ^–3 above 250 K. The calculation also gave approximated vibrational entropies at 298 K of calcium ferrite type MgAl₂O₄ and CaAl₂O₄ as 97.6 and 114.9 J mol^–1 K^–1, respectively.     

Phase transitions in leucite: Order parameter behaviour and the Landau potential deduced from Raman spectroscopy and birefringence studies

Hard mode Raman Spectroscopy and optical birefringence studies have been used to evaluate the temperature behaviour of order parameters for the cubictetragonal phase transition in leucite. There is a strong temperature-dependence of Raman line frequencies near 498 cm^–1 and 529 cm^–1, and the optical birefringence decreases towards zero on heating. Above the transition temperature leucite still shows slight birefringence, which correlates with the persistence of the 200 X-ray reflexion at these temperatures, and is not compatible with the ideal cubic symmetry. Quantitative observations of the temperature evolution of Raman line-frequencies and the birefringence are consistent with an order parameter showing T _1g symmetry, and a nearly-tricritical phase transition behaviour. The results of earlier X-ray and DSC studies imply the operation of an additional order parameter, with E _g symmetry; this is observed, but its influence on the transition behaviour is significant only at temperatures close to T _c. The two order parameters are related via linear-quadratic or biquadratic coupling. We develop the form of the Landau Potential for leucite — including both order parameters and their couplings — to discuss the present observations and previous structural data.     

Characterization of water in synthetic rhyolitic glasses and natural melt inclusions by Raman spectroscopy

Raman spectroscopy was used to analyze quantitatively water in silicate glasses and melt inclusions and to monitor H₂O–OH speciation. Calibration is based on synthetic glasses with various water contents (0.02–7.67% H₂O); water determination and OH–H₂O differentiation on the area of the Si–O broad band at 468 cm^–1 and the asymmetric O–H band at 3,550 cm^–1. Each Raman spectrum has been decomposed into four Gaussian + Lorentzian components centered at 3,330, 3,458, 3,560, and 3,626 cm^–1 using the Levenberg–Marquardt algorithm. These components are interpreted to be two different types of H₂O molecule sites. The influence of the temperature on the loss of water is more important for molecular water than for the hydroxyl groups. The H₂O–OH partition confirms the typical evolution of water speciation in rhyolitic glasses as a function of the bulk water content. Method limitations have been studied for the application to natural melt inclusions.Editorial responsibility: T.L Grove     

Cordierite IV: structural heterogeneity and energetics of Mg–Fe solid solutions

The local structural heterogeneity and energetic properties of 22 natural Mg–Fe cordierites, ideal formula (Mg,Fe)₂Al₄Si₅O₁₈·x(H₂O,CO₂), were investigated at length scales given by powder infrared spectroscopy (IR) and also by published electronic absorption spectra. The studied samples have iron mole fractions from X_Fe = 0.06 to 0.82 and cover most of the Mg–Fe cordierite binary. Variations in wavenumbers and line widths of the IR bands were determined as a function of composition. Most modes shift linearly to lower wavenumbers with increasing X_Fe, except those at high wavenumbers located between 900 and 1,200 cm^-1. They are vibrations that have a large internal (Si,Al)O₄ character and are not greatly affected by Mg–Fe exchange on the octahedral site. The lower wavenumber modes can be best characterized as lattice vibrations having mixed character. The systematics of the wavenumber shifts suggest small continuous variations in the "average" cordierite structure with Mg–Fe exchange and are consistent with an ideal volume of mixing, V^mix= 0, behavior (Boberski and Schreyer 1990). IR line broadening was measured using the autocorrelation function for three wavenumber regions in order to determine the range of structural heterogeneity between roughly 2 and 100 Å (0.2–10.0 nm) in the solid solution. In order to do this, an empirical correction was first made to account for the effect that small amounts of channel Na have on the phonon systematics. The results show that between 1,200 and 540 cm^-1 the line widths of the IR bands broaden slightly and linearly with increasing X_Fe. Between 350 and 125 cm^-1 nonlinear behavior was observed and it may be related to dynamic effects. These results suggest minimal excess elastic enthalpies of mixing for Mg–Fe cordierite solid solutions. Channel Na should affect measurably the thermodynamic properties of natural cordierites as evidenced by variations in the IR spectra of Na-containing samples. Occluded H₂O (Class I) and CO₂ should have little interaction with the framework and can be considered nearly "free" molecules. They should not give rise to measurable structural heterogeneity in the framework. The contribution of the crystal field stabilization energy (CFSE) of octahedral Fe²⁺ to the energetics of Mg–Fe cordierites was also investigated using published electronic absorption spectra (Khomenko et al. 2001). Two bands are observed between 8,000 and 10,500 cm^-1 and they represent electronic dd-excitations of octahedral Fe²⁺ derived from the ⁵T_2g ⁵E_g transition. They shift to higher wavenumbers with increasing X_Mg in cordierite. An analysis gives slightly asymmetric excess -CFSE across the Mg–Fe cordierite join with a maximum of about –550 J/mole towards iron-rich compositions.Editorial responsibility: J. Hoefs