OH groups in nominally anhydrous framework structures: An infrared spectroscopic investigation of danburite and labradorite

The pleochroic behaviour of two nominally anhydrous structurally similar minerals, danburite and An59 labradorite, was investigated in the region of the OH stretching frequencies. Danburite shows a sharp absorption band at 3540 cm^?1, labradorite shows a broad band with an absorption maximum at 3230 cm^?1. On the basis of the pleochroic scheme of theinfrared (IR) absorption spectra it is proposed that the OH dipoles in danburite are located within the symmetry plane showing a distinct orientation parallel to [010]; the OH groups in labradorite are oriented approximately perpendicular to (001). The proposed models are in accordance with bond valence calculations showing that in both framework structures the most deficient oxygens, O5 in danburite and O _C m in labradorite, are partially replaced by OH.     

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.     

Mechanisms of OH defect incorporation in naturally occurring,hydrothermally formed diopside and jadeite

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H₂O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm⁻¹. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm⁻¹ and contains about 197 wt ppm H₂O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm⁻¹ implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.     

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.     

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     

The intensity of amphibole OH bands in the infrared absorption spectrum

Polarized IR spectra of single-crystals of amphiboles show that the molar absorptivity of the fundamental vibrational OH band is strongly wavenumber dependent. The intensity increases by a factor of 1.6 from 3674 cm^-1 (tremolite endmember; MgMgMg) to 3618 cm^-1 (grunerite endmember; FeFeFe). Spectra obtained from Ca and Fe-Mg amphiboles consist of sharp, well-resolved bands on a well-defined background. The high intensity of the OH bands in Ca and Fe-Mg amphiboles makes it sometimes necessary to thin the samples to under 2 μm thickness, whereas alkali amphiboles can be measured at 25–50 μm thickness.     

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.     

OH defects in quartz in the system quartz–albite–water and granite–water between 5 and 25?kbar

The incorporation of OH defects in quartz from the systems quartz–water, quartz–albite–water and granite–water at pressures between 5 and 25?kbar and temperatures between 800 and 1,000?°C was investigated by IR spectroscopy. The two most important OH absorption features can be assigned to hydrogarnet defects (absorption band at 3,585?cm^?1) and coupled substitutions involving Al³⁺ (Al–H defects, absorption bands at 3,310, 3,378 and 3,430?cm^?1). Al incorporation in quartz is controlled by mineral/melt partitioning (D _Al ^Qz/Melt ?=?0.01) and exhibits a negative pressure dependence. This trend is not clearly reflected by the concentration of Al–H defects, which shows positive deviations from the theoretical 1:1 correlation of Al/H for some samples. In contrast to the Al–H defects, formation of hydrogarnet defects appears to be positively correlated to pressure and water activity, and may be used a petrological indicator. The overall water concentration exhibits only minor changes with pressure and temperature, but a clear correlation of water activity (controlled by various amounts of dissolved salts) and hydrogarnet substitution could be established.     

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.     

Trace hydrogen zoning in diopside

Summary ¶The trace hydrogen content of a colourless to light-green zoned diopside single-crystal from Zillertal was investigated by IR microspectroscopy. The light-green part of the crystal reveals pleochroic OH absorption bands centred at 3645, 3463, and 3358cm^–1 which are attributed to structural OH defects. The OH absorptions of the colourless crystal part are characterised by weak bands at 3645 and 3662cm^–1 and by a strong band at 3676cm^–1. The bands at 3662 and 3676cm^–1 are attributed to the presence of amphibole lamellae. The analytical water content due to the structural OH defect concentration of the light-green crystal part amounts to 0.0016wt.%, that of the colourless part is lowered by a factor of about 50. According to optical absorption spectra, the light-green colour of the crystal is essentially caused by an Fe²⁺–Fe³⁺ charge transfer. The relatively high concentration of OH defects in the light-green crystal part associated with higher Al contents relative to the colourless part suggests that OH is incorporated as hydrodiopside component, CaMg(SiAlO₅OH). It is concluded that increasing water activity during the crystallisation process causes the formation of amphibole lamellae under consumption of nearly all of the water available in the fluid phase. It is further concluded that the observed hydrogen content of diopside represents a primary incorporation and not the result of late hydrothermal alteration processes.Received July 11, 2002; accepted October 30, 2002 Published online February 24, 2003     

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     

钼矿石物相的快速分析方法

钼矿石的主要矿物有辉钼矿,其次是钼钨钙矿、铁钼华矿、钼华矿和钼酸铅矿。样品经碱熔融分解,硫氰酸盐比色法测定总钼;利用电感耦合等离子体质谱法测定氢氧化铵溶解钼华中的钼;酒石酸溶解钼钨钙矿中的钼;碳酸钠溶解钼酸铅矿中的钼;盐酸溶解铁钼华矿中的钼。方法对不同钼矿石样品进行了5种钼矿石物相分析,结果与实际地质成矿组分符合。     

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 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.     

Geochemical modeling for predicting potential solid phases controlling the dissolved molybdenum in coal overburden,Powder River Basin,WY, U.S.A.

Surface coal mining processes may increase the dissolved Mo concentrations in overburden (mine spoils) and in nearby soils. Spoil and soil samples were subjected to solubility studies for determining potential solid phases controlling the dissolved Mo. Additionally, native soil samples were included in the study for comparison. Ion activity products (IAPs) were calculated from the total elemental concentrations of aqueous extracts and compared with solubility products (K_sps) of Mo solid phases. Effects of dissolved organic carbon (DOC) on metal ion complexation were also evaluated. The pH of aqueous extracts ranged from 7.14 to 8.31. DOC in aqueous extracts varied from 17.3 to 58.7 mg/l. Molybdenum concentrations in aqueous extracts ranged between 11.5 and 35.6 μg/l for spoils, 13.5 and 20.0 μg/l for soils near mine site and 14.5 and 29.6 μg/l for native soils. Initially, IAPs suggested varying degrees of supersaturation with respect to wulfenite (PbMoO₄). However, when DOC-Pb²⁺ complexes were considered, the IAPs indicated a close approach to saturation with respect to PbMoO₄. Furthermore, the IAPs suggested a high degree of undersaturation with respect to powellite (CaMoO₄) and ferrimolybdite (FeMoO₄). Results in this study suggest the dissolved Mo concentrations in mine spoils and soils that are near coal mine may be controlled by PbMoO₄ solid phase.     

Optical spectroscopy study of natural Fe, Ti-bearing calcic amphiboles

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O^2?→ Fe³⁺, Fe²⁺, O^2?→ Ti⁴⁺ and Fe²⁺ + Ti⁴⁺→ Fe³⁺ + Ti³⁺). A broad intense Y-polarized band ～22?000?cm^?1 (ν_1/2?≈?3700?cm^?1) in spectra of two low iron amphiboles with a relatively low Fe³⁺/Fe_total ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe²⁺(M3) + Ti⁴⁺(M2)→Fe³⁺(M3)+Ti³⁺(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti⁴⁺ and Fe²⁺ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.     

Structure and cation disorder of hydrous ringwoodite, γ-Mg1.89Si0.98H0.30O4

The structure of a single crystal hydrous ringwoodite, Mg_1.89Si_0.98H_0.30O₄ synthesized at conditions of 1300?°C and 20?GPa has been analyzed. Crystallographic data for hydrous ringwoodite obtained are; Cubic with Space group: Fd3m (no. 227). a= 8.0693(5)?Å, V=526.41(9)?ų, Z=8, D_calc= 3.48?g?cm^?3. The results of site occupancy refinement using higher angle reflections showed the existence of a small degree of Mg²⁺-Si⁴⁺ disorder in the structure such as (Mg_1.84Si_0.05□_0.11)(Si_0.93Mg_0.05□_0.02)H_0.30O₄. The IR and Raman spectra were measured and OH vibration spectra were observed. A broad absorption band was observed in the IR spectrum and the maxima were observed at 3160?cm^?1 in the IR and at 3165?cm^?1 and 3685?cm^?1 in relatively sharp Raman spectra, which suggest that locations between O-O pairs around the octahedral 16c and 16d sites are possible sites for hydrogen.     

Empirical study of the infrared lattice vibrations (1100–350 cm−1) of phlogopite

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

An infrared and <Superscript>1</Superscript>H MAS NMR investigation of strong hydrogen bonding in ussingite,Na<Subscript>2</Subscript>AlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>(OH)

The mineral ussingite, Na₂AlSi₃O₈(OH), an interrupted tectosilicate, has strong hydrogen bonding between OH and the other nonbridging oxygen atom in the structure. Infrared spectra contain a strongly polarized, very broad OH-stretching band with an ill-defined maximum between 1500 and 1800 cm^–1, and a possible OH librational bending mode at 1295 cm^–1. The IR spectra confirm the orientation of the OH vector within the triclinic unit cell as determined from X-ray refinement (Rossi et al. 1974). There are three distinct bands in the ¹H NMR spectrum of ussingite: a predominant band at 13.5 ppm (TMS) representing 90% of the structural hydrogen, a second band at 15.9 ppm corresponding to 8% of the protons, and a third band at 11.0 ppm accounting for the remaining 2% of structural hydrogen. From the correlation between hydrogen bond length and ¹H NMR chemical shift (Sternberg and Brunner 1994), the predominant hydrogen bond length (H...O) was calculated to be 1.49 Å, in comparison to the hydrogen bond length determined from X-ray refinement (1.54 Å). The population of protons at 15.9 ppm is consistent with 5–8% Al–Si disorder. Although the ussingite crystal structure and composition are similar to those of low albite, the bonding environment of OH in low albite and other feldspars, as characterized through IR and ¹H NMR, is fundamentally different from the strong hydrogen bonding found in ussingite.     

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).