Temperature-induced changes in the NIR spectra of hydrous albitic and rhyolitic glasses between 300 and 100 K

Near-infrared (NIR) absorption bands related to total water (4000 and 7050 cm⁻¹), OH groups (4500 cm⁻¹) and molecular H₂O (5200 cm⁻¹) were studied in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the glasses were determined using Karl Fischer titration. Molar absorption coefficients were calculated for each of the bands using albitic glasses containing between 0.54 and 9.16 wt.% H₂O and rhyolitic glasses containing between 0.97 and 9.20 wt.% H₂O. Different combinations of baseline type and intensity measure (peak height/area) for the combination bands at 4500 and 5200 cm⁻¹ were used to investigate the effect of evaluation procedure on calculated hydrous species concentrations. Total water contents calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic and 6.5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H₂O, respectively). In glasses with water contents >1 wt.%, calculated hydrous species concentrations vary by up to 17% relative for OH and 11% relative for H₂O (maximum absolute differences of 0.33 and 0.43 wt.% H₂O, respectively). This variation in calculated species concentrations is typically greater in rhyolitic glasses than albitic. In situ, micro-FTIR analysis at 300 and 100 K was used to investigate the effect of varying temperature on the NIR spectra of the glasses. The linear and integral molar absorption coefficients for each of the bands were recalculated from the 100 K spectra, and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm⁻¹ bands decrease by 16–20% and integral molar absorption coefficients by up to 30%. Depending on glass composition and baseline type, the integral molar absorption coefficients for the absorption bands related to OH groups and molecular H₂O change by up to −5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum change of ∼4%. Using the new molar absorption coefficients for the combination bands to calculate species concentrations at 100 K, the maximum change in species concentration is 0.08 wt.% H₂O, compared with 0.39 wt.% which would be calculated if constant values were assumed for the combination band molar absorption coefficients. Almost all the changes in the spectra can therefore be interpreted in terms of changing molar absorption coefficient, rather than interconversion between hydrous species. Received: 17 December 1998 / Revised, accepted 8 July 1999     

Traces of structural H2O molecules in baryte

The infrared (IR) spectra of gem-quality baryte crystals from different occurrences are characterized by relatively weak but strongly pleochroic absorption bands at 3,280, 3,220, 3,155, and 3,115 cm⁻¹. These bands are assigned to anti-symmetric and symmetric OH stretching vibrations of two types of H₂O molecules localized on vacant Ba sites. The H–H axis of the H₂O I molecule is slightly tilted from the a-axis direction, its twofold axis being nearly parallel to the b-axis, thus defining the plane of the H₂O molecule practically parallel to (001). The H₂O II molecule has its H–H axis parallel to the b-axis direction, with its plane lying approximately parallel to (101). The values of the total water contents of the baryte crystals, calculated on the basis of IR spectroscopic data, are ranging from about 1.7–3.8 wt.ppm. The possible presence of H₃O⁺ ions is also discussed.     

OH defects in forsterite

Polarized FTIR spectra of near endmember forsterite single crystals from Pamir, Tadzikistan show the existence of sharp strongly pleochroic absorption bands in the region of the OH stretching fundamental. Bands centered at 3674/3624, 3647/3598 and 3640/ 3592 cm^-1 are attributed to OH dipoles oriented parallel to [100]. An OH band doublet at 3570/3535 cm^-1 shows both, a strong absorption parallel to [100] and a strong component parallel to [001]. On the basis of the pleochroic scheme and under the assumption of vacancies on Si- and M-sites it is proposed that O1 is partially replaced by OH defects pointing to the vacant Si-site. O3 is donator oxygen of OH dipoles lying near the O3-O1 tetrahedral edge or roughly pointing to a vacant M2-site. Also O2 can act as donator oxygen of an OH group oriented along the O2-O3 edge of a vacant M1 octahedron. The splitting of the bands is explained by the presence of Fe²⁺ in cation sites surrounding the OH defects.     

A model of the OH positions in olivine,derived from infrared-spectroscopic investigations

Polarized infrared (IR) spectroscopy of olivine crystals from Zabargad, Red Sea shows the existence of four pleochroic absorption bands at 3,590, 3,570, 3,520 and 3,230 cm^?1, and of one non pleochroic band at 3,400 cm^?1. The bands are assigned to OH stretching frequencies. Transmission electron microscopy (TEM) shows no oriented intergrowths in this olivine; it is concluded that OH is structural. On the basis of the pleochroic scheme of the absorption spectra it is proposed that [□O(OH)₃] and [□O₂(OH)₂] tetrahedra occur as structural elements, assuming that the vacancies are on Si sites. If M2 site vacancies were assumed [SiO₃(OH)] and [SiO₂(OH)₂] tetrahedra occur as structural elements.     

Hydroxyl in mantle olivine xenocrysts from the Udachnaya kimberlite pipe

The incorporation of hydrogen in mantle olivine xenocrysts from the Udachnaya kimberlite pipe was investigated by Fourier-transform infrared spectroscopy and secondary ion mass spectrometry (SIMS). IR spectra were collected in the OH stretching region on oriented single crystals using a conventional IR source at ambient conditions and in situ at temperatures down to −180°C as well as with IR synchrotron radiation. The IR spectra of the samples are complex containing more than 20 strongly polarized OH bands in the range 3,730–3,330 cm⁻¹. Bands at high energies (3,730–3,670 cm⁻¹) were assigned to inclusions of serpentine, talc and the 10 Å phase. All other bands are believed to be intrinsic to olivine. The corresponding point defects are (a) associated with vacant Si sites (3,607 cm⁻¹ E || a, 3,597 E || a, 3,571 cm⁻¹ E || c, 3,567 E || c, and 3,556 E || b), and (b) with vacant M1 sites (most of the bands polarized parallel to a). From the pleochroic behavior and position of the OH bands associated with the vacant M1 sites, we propose two types of hydrogen—one bonded to O1 and another to O2, so that both OH vectors are strongly aligned parallel to a. The O2–H groups may be responsible for the OH bands at higher wavenumbers than those for the O1–H groups. The multiplicity of the corresponding OH bands in the spectra can be explained by different chemical environments and by slightly different distortions of the M1 sites in these high-pressure olivines. Four samples were investigated by SIMS. The calculated integral molar absorption coefficient using the IR and SIMS results of 37,500±5,000 L mol H₂O cm⁻² is within the uncertainties slightly higher than the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003) (28,450±1,830 L mol H₂O cm⁻²). The reason for the difference is the different distributions of the absorption intensity of the spectra of both studies (mean wavenumber 3,548 vs. 3,570 cm⁻¹). Olivine samples with a mean wavenumber of about 3,548 cm⁻¹ should be quantified with the absorption coefficient as determined in this study; those containing more bands at higher wavenumber (mean wavenumber 3,570 cm⁻¹) should be quantified using the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003).

OH absorption coefficients of rutile and cassiterite deduced from nuclear reaction analysis and FTIR spectroscopy

Summary The OH content of four rutile and two cassiterite single-crystals was studied by nuclear reaction analysis (NRA) and by polarised FTIR microspectroscopy. The OH absorption bands of both minerals are centered around 3300 cm⁻¹ with different absorption features. The analytical H₂O content determined by NRA ranges from 70 to 820 wt.ppm. The integrated molar absorption coefficients deduced from the total integrated OH absorbances are equal to 38000 lċmol⁻¹ _H2Oċcm⁻² for rutile and 65000 lċmol⁻¹ _H2Oċcm⁻² for cassiterite. For both minerals the absorption coefficients are significantly smaller than those expected from the linear calibration curves given by Paterson (1982) and by Libowitzky and Rossman (1997).

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Zusammenfassung OH-Absorptionskoeffizienten von Rutil und Cassiterit ermittelt durch Kernreaktions-Analyse und FTIR Spektroskopie Der OH-Gehalt von vier Rutil- und zwei Cassiterit-Einkristallen wurde mittels Kernreaktions-Analyse (NRA) und polarisierter FTIR Mikrospektroskopie untersucht. Die OH Absorptionsbanden beider Minerale sind um 3.300 cm⁻¹ zentriert, mit unterschiedlichen Absorptionserscheinungen. Der analytische H₂O-Gehalt, der mit NRA bestimmt wurde, schwankt von 70 bis 820 Gew.ppm. Die integrierten molaren Absorptionskoeffizienten, die auf den gesamten integrierten OH-Absorptionen basieren, betragen etwa 38.000 lċmol⁻¹ _H2Oċcm⁻² für Rutil and 65.000 lċmol⁻¹ _H2Oċcm⁻² für Cassiterit. Für beide Minerale sind die Absorptionskoeffizienten signifikant kleiner als die, die auf Grund der linearen Kalibrationskurven von Paterson (1982) und Libowitzky und Rossmann (1997) zu erwarten sind.

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Received January 4, 2000; revised version accepted April 10, 2000     

A model of water allocation in alkali feldspar,derived from infrared-spectroscopic investigations

Polarizedinfrared (IR) spectra of sanidine crystals from Volkesfeld, Eifel show the existence of two broad pleochroic absorption bands at 3,400 and 3,050 cm^?1. Because overtones near 5,150 cm^?1 were observed, the former bands are assigned to OH stretching frequencies of H₂O molecules. On the basis of the pleochroic scheme of the bands it is proposed that H₂O molecules occur as structural constituents entering theM site of the sanidine structure; the plane of the H₂O molecules lies parallel to the symmetry plane.     

Crystal chemistry and OH defect concentrations in spodumene from different granitic pegmatites

Thirty spodumene samples of distinct paragenetic types (primary magmatic, secondary after petalite and hydrothermal) from variety of granitic pegmatites were characterized by electron microprobe, polarized FTIR spectroscopy and Mössbauer spectroscopy. The FTIR spectra of OH (weak sharp pleochroic bands at 3,425, 3,410, 3,395 cm⁻¹ and in the 3,500–3,470 spectral region) are strongly polarized with maximum absorption parallel to n_γ. The majority of OH dipoles are presumably generated by a partial replacement of O2 oxygen atoms with an orientation pointing above the Li vacancy site. The separation of the bands probably resulted from a replacement of the coordinating Al by Fe and Si by Al. Homogeneous spodumene mostly close to its ideal formula LiAlSi₂O₆ shows Fe (0.00–0.10 apfu as Fe³⁺; Fe³⁺ >> Fe²⁺) and Na (0.00–0.04 apfu) as the only minor cations and Fe³⁺Al₋₁ substitution up to 10 mol% of the LiFe³⁺Si₂O₆ component. Hydrogen concentrations (from 0.1 up to <5 ppm H₂O by weight) vary as a function of genetic type with the highest amounts in high-temperature magmatic spodumene. Differences among particular genetic types of spodumene are related to maximum solubility of OH in spodumene structure at given P–T conditions and at actual chemical composition of spodumene. OH defect concentrations in spodumene follow a trend, LT/LP pyroxenes containing lower hydrogen contents compared to HT/HP ones. The hydrogen contents in particular genetic types of spodumene and their decrease with decreasing T and P are consistent with petrologic models of the pegmatite (sub)types formations.     

Dehydration process and structural development of cordierite ceramic precursors derived from FTIR spectroscopic investigations

 Cordierite precursors were prepared by a sol-gel process using tetraethoxysilane, aluminum sec.-butoxide, and Mg metal flakes as starting materials. The precursors were treated by 15-h heating steps in intervals of 100 °C from 200 to 900 °C; they show a continuous decrease in the analytical water content with increasing preheating temperatures. The presence of H₂O and (Si,Al)–OH combination modes in the FTIR powder spectra prove the presence of both H₂O molecules and OH groups as structural components, with invariable OH concentrations up to preheating temperatures of 500 °C. The deconvolution of the absorptions in the (H₂O,OH)-stretching vibrational region into four bands centred at 3584, 3415, 3216 and 3047 cm⁻¹ reveals non-bridging and bridging H₂O molecules and OH groups. The precursor powders remain X-ray amorphous up to preheating temperatures of 800 °C. Above this temperature the precursors crystallize to μ-cordierite; at 1000 °C the structure transforms to α-cordierite. Close similarities exist in the pattern of the 1400–400 cm⁻¹ lattice vibrational region for precursors preheated up to 600 °C. Striking differences are evident at preheating temperatures of 800 °C, where the spectrum of the precursor powder corresponds to that of conventional cordierite glass. Bands centred in the “as-prepared” precursor at 1137 and 1020 cm⁻¹ are assigned to Si–O-stretching vibrations. A weak absorption at 872 cm⁻¹ is assigned to stretching modes of AlO₄ tetrahedral units and the same assignment holds for a band at 783 cm⁻¹ which appears in precursors preheated at 600 °C. With increasing temperatures, these bands show a significant shift to higher wavenumbers and the Al–O stretching modes display a strong increase in their intensities. (Si,Al)–O–(Si,Al)-bending modes occur at 710 cm⁻¹ and the band at 572 cm⁻¹ is assigned to stretching vibrations of AlO₆ octahedral units. A strong band around 440 cm⁻¹ is essentially attributed to Mg–O-stretching vibrations. The strongly increasing intensity of the 872 and 783 cm⁻¹ bands demonstrates a clear preference of Al for a fourfold-coordinated structural position in the precursors preheated at high temperatures. The observed band shift is a strong indication for increasing tetrahedral network condensation along with changes in the Si–O and Al–O distances to tetrahedra dimensions similar to those occurring in crystalline cordierite. These structural changes are correlated to the dehydration process starting essentially above 500 °C, clearly demonstrating the inhibiting role of H₂O molecules and especially of OH groups. Received: 1 March 2002 / Accepted: 26 June 2002     

IR active orientation of OH bending mode in topaz

Infrared (IR) and nearinfrared (NIR) absorption spectra of hydrous and F-rich topazes were measured to assign an OH bending mode of topaz. Three absorption peaks at 1165, 3650, and 4803 cm⁻¹ are assigned to OH related absorption peaks. Since a peak at 4803 cm⁻¹ can be assigned to a combination mode of 1165 and 3650 cm⁻¹, the 1165 cm⁻¹ peak is harmonic with the 3650 cm⁻¹ peak. Polarized IR absorption spectra of (100), (010), and (001) planes of the hydrous topaz were measured to examine IR active orientation of the 1165 cm⁻¹ OH related mode. Three pleochroic distributions of the absorption peak at 1165 cm⁻¹ on (100), (010), and (001) planes indicate an active orientation of the 1165 cm⁻¹ OH related mode. The IR active orientation of the 1165 cm⁻¹ OH related mode in topaz is normal to the OH dipole. The orthogonality and harmonic combination mode indicate that the 1165 cm⁻¹ peak is OH bending mode. The active orientation of OH bending mode is polarized in the plane normal to the OH dipole. The polarization suggests that anisotropic thermal vibration of protons on the hydroxyl is maximum along the IR active orientation. Received: August 16, 1996 / Revised, accepted: April 20, 1997     

OH point defects in olivine from Pakistan

The infrared (IR) spectra of gem-quality olivine crystals from Pakistan, formed in serpentinised dunitic rocks, are characterised by strongly pleochroic absorption bands at 3,613, 3,597, 3,580 and 3,566 cm^?1. These bands are assigned to O-H stretching vibrations of OH point defects corresponding to H₂O concentrations of about 35 wt ppm. Unlike other olivine spectra, the dominating bands are strongly polarised parallel to the b-axis. The unusual spectra type, excludes the presence of planar defects. This finding is supported by transmission electron microscopy. The 3,613 cm^?1 band is related to vacant Si sites, the slightly lower energetic bands preferentially to vacant M2 sites. The exclusive presence of these bands is not only a characteristic feature of olivines treated under high P,T conditions equivalent to mantle environment, the presence of these bands in untreated natural olivine also indicates formation conditions equivalent to crustal rocks.     

Location and quantitative analysis of OH in coesite

 The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater 5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three relatively intense bands at 3575, 3516, and 3459 cm⁻¹ (ν₁ to ν₃, respectively) and two very weak bands at 3296 and 3210 cm⁻¹ (ν₄ and ν₅, respectively). The band at 3516 cm⁻¹ is strongly asymmetric and can be resolved into two bands, 3528 (ν_2a) and 3508 (ν_2b) cm⁻¹, with nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1 0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative analysis of OH in coesite (ɛ _{i ,tot}=190 000 ± 30 000 l mol⁻¹ _H2O cm⁻²). The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions of the ν₁, ν₂, and ν₃ bands decrease linearly with pressure. The mode Grüneisen parameters for ν₁, ν₂, and ν₃ are −0.074, −0.144 and −0.398, respectively. There is a linear increase of the pressure derivatives with band position which follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν₁, ν₂, and ν₃ bands increase from 35, 21, and 28 cm⁻¹ in the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results, a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the substitution Si^4+(Si2)+4O²⁻= ^[4]□^(Si2) + 4OH⁻, which gives rise to four vibrations, ν₁, ν_2a, ν_2b, and ν₃. Because the OH(D)-bearing samples do contain traces of Al and B, the bands ν₄ and ν₅ may be coupled to Al and/or B substitution. Received: 19 December 2000 / Accepted: 23 April 2001     

On the presence of hydrous defects in differently coloured wulfenites (PbMoO4): an infrared and optical spectroscopic study

Several samples of wulfenite, PbMoO₄, varying in colour from colourless to yellow, orange and red, have been characterised by means of IR and optical absorption spectroscopy and by microprobe analyses. A distinct pleochroic band group with absorption maxima centred at 3,380 and 3,150 cm^?1 can be seen in the IR spectra of wulfenite single-crystals, indicating the presence of hydroxyl groups. The pleochroic and thermal behaviour of the OH stretching bands along with deuteration experiments, as well as results obtained from synthetic flux-grown samples, exclude the presence of submicroscopic hydrous mineral inclusions as their primary origin. The pleochroic scheme and the band positions were used to postulate a model for the OH incorporation mode, based on the assumption of vacancies on Mo and Pb sites in the structure of this ‘nominally anhydrous mineral’. Optical absorption spectra of coloured natural samples show a broad and polarised band around 23,000–24,000 cm^?1, preceding the fundamental UV absorption edge, which has been identified as the reason for the colour of the mineral. The comparison with synthetic PbMoO₄ single-crystals, doped with variable amounts of Cr⁶⁺, yielded conclusive evidence that trace amounts of the CrO₄ ^2? anion group, substituting for MoO₄ ^2?, determine the variable colour. Besides, in one sample, trace amounts of Nd³⁺ have been spectroscopically identified.     

Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile

Polarized absorption spectra, σ and π, in the spectral range 30000–400 cm⁻¹ (3.71–0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO₂ rutile powder at 50 kbar/1500 °C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z≥13). The spectra are characterized by an unpolarized absorption edge at 24300 cm⁻¹, two weak and relatively narrow (Δν_1/2≈3500–4000 cm⁻¹), slightly σ-polarized bands ν₁ at 23500 cm⁻¹ and ν₂ at 18500 cm⁻¹, and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the σ-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, σ and π, at 21000 cm⁻¹, i.e. in the blue, which explains the colour of the crystals. Bands ν₁ and ν₂ are assigned to dd transitions to the Jahn-Teller split upper E_g state of octahedral Ti³⁺. The NIR band system can be fitted as a sum of three components. Two of them are partly π-polarized, nearly Gaussian bands, both with large half widths 6000–7000 cm⁻¹, ν₃ at 12000 cm^–1 and the most intense ν₄ at 6500 cm⁻¹. The third NIR band ν₅ of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm⁻¹ forms a shoulder on the low-energy wing of ν₄. Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti³⁺Ti⁴⁺ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm⁻¹ in σ-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti³⁺ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20–295 K show the presence of an electron centre at temperatures above 100 K and Ti³⁺ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are in good agreement with the conclusions based on the electronic absorption data. Received: 24 March 1997 / Revised, accepted: 14 October 1997     

OH incorporation in nearly pure MgAl2O4 natural and synthetic spinels

Four nearly pure MgAl₂O₄ spinels, of both natural and synthetic occurrence, have been studied by means of X-ray single crystal diffraction and FTIR spectroscopy in order to detect their potential OH content. Absorption bands that can be assigned to OH incorporated in the spinel structure were only observed in spectra of a non-stoichiometric synthetic sample. The absorption intensity of two bands occurring at 3350 and 3548 cm⁻¹ indicate an OH content of 90 ppm H₂O. Based on correlations of OH vibrational frequencies and O-H?O distances, the observed absorption bands correspond to O-H?O distances of 2.77 and 2.99 Å, respectively, which is close to the values obtained by the structure refinements for ^VIO-O_unsh (2.825 Å) and ^IVO-O (3.001 Å). This indicates that one probable local position for hydrogen incorporation is the oxygens coordinating a vacant tetrahedral site. The present spectra demonstrate that the detection limit for OH in Fe-free spinels is in the range 10-20 ppm H₂O. However, at appreciable Fe²⁺ levels, the detection of OH bands becomes hampered due to overlap with strong absorption bands caused by electronic d-d transitions in Fe²⁺ in the tetrahedral position.     

High-temperature X-ray diffraction and Raman spectroscopy of diopside and pseudowollastonite

Diopside (CaMgSi₂O₆) and pseudowollastonite (CaSiO₃) have been studied by X-ray powder diffraction and Raman spectroscopy up to their respective melting points. In agreement with previous unit-cell parameters determinations below 1100 K, thermal expansion of diopside along the a and c axis is much smaller than along the b axis. For pseudowollastonite, the axis expansivity increases slightly in the order b>a>c. For both minerals, the change in unit-cell angles is very small and there are no anomalous variations of the other unit-cell parameters near the melting point. With increasing temperatures, the main changes observed in the Raman spectra are strong increases of the linewidths for those bands which mainly represent Si−O−Si bending (near 600 cm⁻¹) or involve Ca−O or Mg−O stretching, in the range 270–500 cm⁻¹ for diopside, and 240–450 cm⁻¹ for pseudowollastonite. At temperatures near the onset of calorimetric premelting effects, this extensive band widening results in a broad Raman feature that can no longer be deconvoluted into its individual components. No significant changes affect the Si−O streching modes. For both diopside and pseudowollastonite, premelting appears to be associated with enhanced dynamics of the alkaline-earth elements. This conclusion contrasts markedly with that drawn for sodium metasilicate in which weaker bonding of sodium allows the silicate framework to distort and deform in such a way as to prefigure the silicate entities present in the melt. Received 16 July 1997 / Revised, accepted: 6 March 1998     

The occurrence of OH absorptions in phenakite—an IR spectroscopic study

Summary IR spectra of phenakite single crystals from different localities show a distinct variability in the region of the OH stretching fundamental. Minute hydrous mineral phases (tourmaline, bertrandite) are included in Piracicaba phenakite. Structural OH, ranging up to 0.005 equivalent wt.% H₂O, is characterized by two extremely pleochroic bands centered at 3380 and 3120 cm^–1. On the basis of their pleochroic scheme it is proposed that (O₂(OH)₂) and (O₃(OH)) tetrahedra occur as structural elements, assuming that the vacancies are on Be sites.

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Das Auftreten von OH Absorptionen in Phenakit—eine IR spektroskopische Untersuchung

Zusammenfassung Die IR Spektren von Phenakit-Einkristallen verschiedener Vorkommen zeigen im Bereich der OH-Streckschwingungen eine deutliche Variabilität. Piracicaba Phenakit enthält feinste Einschlüsse von OH-hältigen Mineralphasen (Turmalin, Bertrandit). Strukturell gebundene OH-Gruppen (bis 0,005 äquivalente Gew.% H₂O) sind durch zwei extrem pleochroitische Banden bei 3380 und 3120 cm^–1 charakterisiert. Unter der Annahme von Be-Leerstellen werden aufgrund des Pleochroismus dieser Banden (O₂(OH)₂) und (O₃(OH)) Tetraeder als strukturelle Baueinheiten vorgeschlagen.

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With 3 Figures     

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.     

Water solubility in pyrope to 100 kbar

The solubility and incorporation mechanism of water in natural, almost pure pyrope from Dora Maira, Western Alps was investigated. The infrared spectrum of the natural, untreated sample (58 ppm water) shows several exceptionally sharp bands in the OH-stretching region, including a single band at 3601.9 cm⁻¹ and a band system with main components at 3640.5, 3650.8 and 3660.6 cm⁻¹. High-temperature and high-pressure infrared spectra suggest that the two absorption features arise from almost free OH groups in sites with different compressibility and thermal expansivity, with the site causing the 3601.9 cm⁻¹ band being much stiffer. Pyrope samples were annealed in a piston-cylinder or multi-anvil apparatus for several days in the presence of excess water, excess SiO₂ and excess Al₂SiO₅ to determine the equilibrium solubility of water in pyrope to 100 kbar. Total solubility increases with pressure, however, this is exclusively due to the high-frequency band system, while the intensity of the low-frequency band decreases with pressure. At 1000 °C and the oxygen fugacity of the Ni-NiO buffer, the bulk solubility can be described by the equation c _OH =Af _H2O ^0.5exp(−PΔV/RT) with A = 0.679 ppm/bar^0.5 and ΔV = 5.71 cm³/mol. This equation implies the incorporation of water in the crystal as isolated OH groups. With increasing temperature, solubility appears to decrease with ΔH = − 14 kJ/mol. At Fe-FeO buffer conditions, solubility is 30 to 50% lower than with the Ni-NiO buffer, suggesting that the incorporation of OH is not coupled to the reduction of Fe³⁺. Possibly, the 3601.9 cm⁻¹ band is associated with the tetrahedral OH B defect and the high-frequency system with the dodecahedral OH Li defect. Based on the experimentally established solubility model, it is estimated that garnet in a hot subducted slab will transport 170 ppm of water into the mantle beyond the breakdown limit of amphibole. In a cold slab, 470 ppm of water can be incorporated into garnet at the breakdown limit of phengite. These numbers imply that a significant fraction of the total water in the hydrosphere has been recycled into the mantle since the Proterozoic. Received: 6 January 1997 / Accepted: 27 March 1997     

On the mechanisms for H and Al incorporation in stishovite

The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al₂O₃ content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454–456, 1995) model, where H docks close to one of the shared O–O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111–3,117 cm⁻¹. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157–3,170 cm⁻¹. All H is present on one site, with interstitial H both coupled to Al³⁺ substitutional defects on adjacent octahedral (Si⁴⁺) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al³⁺ substitution for Si⁴⁺ charge-balanced by oxygen vacancies at low bulk Al₂O₃ contents, and coupled substitution of Al³⁺ onto octahedral (Si⁴⁺) and interstitial sites at high bulk Al₂O₃ contents. Trends in OH stretching frequencies as a function of Al₂O₃ content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. (Am Mineral 87:1486–1489, 2002).