Monika Koch-Müller

Hydroxyl in omphacites and omphacitic clinopyroxenes of upper mantle to lower crustal origin beneath the Siberian platform

Abstract A series of clinopyroxenes from the lower crust and upper mantle beneath the Siberian platform was investigated by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The IR spectra of all our samples exhibit three groups of absorption bands at (1) 3445−3465, (2) 3500-3540, and (3) 3600-3624 cm-1. Using synchrotron IR radiation, which utilizes a spot size of only 5 × 5 μm, we realized that the intensities of the absorption bands, mostly those of group 3, showed extreme variation within one crystal. TEM as well as polarized and high-pressure IR spectroscopy indicated that the OH groups that cause the bands of group 3 were not intrinsic but due to nm-sized inclusions of sheet silicates. The intensities and peak positions of the bands of group 2 correlate with the amount of tetrahedral Al3+ indicating that the charge-deficient substitution of Al for Si is responsible for the bands of group 2. The intensities and peak positions of the bands of group 1 correlate with the concentration of vacancies at M2 indicating that the cation vacancies at M2 control the incorporation of hydroxyl responsible for the bands of group 1. The bands of groups 1 and 2 are caused by the same type of OH dipole, however, occurring in different structural environments. The concentration of the structurally bound water of the omphacitic clinopyroxene is in the range from 31 to 514 ppm H2O (by weight). Surprisingly, the lowest concentration was found in clinopyroxene, that comes from the highest pressure region, i.e., the diamond-bearing eclogite xenoliths of the Mir kimberlite pipe. The highest values were obtained in omphacites of the lower-pressure grospydites of the Zagadochnaya kimberlite pipe and in omphacitic clinopyroxene of the lower pressure granulites of the Udachnaya kimberlite pipe, respectively. The low water content of clinopyroxene from the high-pressure region seems to be controlled by low water activity during crystallization. However, hydrogen loss during the uplift cannot be ruled out.

Application of Raman spectroscopy to quantify trace water concentrations in glasses and garnets

Abstract We present a new technique for the quantification of water in glasses down to the parts per million level, using confocal microRaman spectroscopy with the recently developed “Comparator Technique.” To test this method, we used a suite of glasses and gemstone-quality garnets with varying chemical compositions. Water contents were independently determined with proton-proton (pp) scattering and infrared (IR) spectroscopy. Moreover, water concentrations obtained for the garnets were compared to data from a study by Maldener et al. (2003) using nuclear reaction analysis (NRA). For each sample, we recorded Raman spectra in the frequency range from 3100 to 3750 cm-1 and standardized them using an independently well-characterized glass. In this paper, we demonstrate the usefulness of this technique for quantifying water concentrations in natural and synthetic glass samples and garnets, and verify its adaptability for concentrations from 40 wt ppm up to 40 wt% H2O. However, in the case of absorbing material (e.g., Fe-bearing samples), the suggested method needs to be modified to overcome problems due to heating and melting of those phases. Furthermore, we propose an integrated molar absorption coefficient for water in quartz glass, εitot = 72 000 ± 12 000 Lmol-1H₂Ocm-2, for quantitative IR spectroscopy that is higher than a previously reported value of Paterson (1982) or that predicted by the general calibration trend determined by Libowitzky and Rossman (1997).

OH- in synthetic and natural coesite

Abstract The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 °C using Al and B doped SiO2 starting materials. The spectra show four sharp bands (ν1, ν2a, ν2b, and ν3) in the energy range of 3450-3580 cm-1, consistent with the hydrogarnet substitution [Si4+(T2) + 4O2- = vaT2 + 4OH-], two weak sharp bands at 3537 and 3500 cm-1 (v6a and ν6b) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm-1 (ν4 and ν5) attributed to substitution of Si4+ by Al3+ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 °C, 1335 H/106 Si is incorporated into the coesite structure. At 8.5 GPa and 1200 °C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm-1 (ν7-ν10) and the ν1-ν3 bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH- in coesite. The polarization and high-pressure behavior of the ν7-ν10 OH bands is quite different from that of the ν1-ν3 bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 ζ ± 300 H/106 Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.

Location and quantification of hydroxyl in wadsleyite: New insights

Abstract Anhydrous and hydrous wadsleyite were synthesized at 13.3-13.5 GPa and 1150-1200 °C in a multianvil press and investigated by Fourier transform infrared (FTIR) spectroscopy, single-crystal X-ray refinement (SC-XRD), and electron microprobe analyses (EMPA). The FTIR spectra agree with previous data, i.e., the spectra are dominated by a broad band around 3380 cm-1, resolvable in three bands 3326 (ν2), 3382 (ν3), and 3546 (ν4) cm-1 besides some weaker OH-bands around 3600 cm-1. We confirm that wadsleyite incorporates water in the wt% range and that the concentration strongly increases with decreasing temperature when using secondary ion mass spectrometry (SIMS) and Raman spectroscopy. The quantifications combined with FTIR spectra led us to develop the first IR calibration for water in wadsleyite, i.e., calculating an εi,tot of 73 000 ± 7000 (L mol-1H₂O cm-2). A SC-XRD determination of hydrous wadsleyite FD0718, bearing 8000 ± 1000 wt ppm H2O, certifies the presence of Mg vacancies at the M3 sites as previously suggested. Furthermore, we found maxima in the electron density map close to the O atoms O1 and O3 of an M3 octahedron assuming the anhydrous structure. Based on our new data we suggest that the main protonation in wadsleyite occurs along the O1···O4 (3.1 Å) and O3···O4 (3.05 Å) edges of a vacant M3 octahedron. H-incorporation seems to be random leading to protonation of either two O1, two O3, or one O1 and one O3 of the vacant M3 octahedra. With this assignment, the observed ambient and high-pressure IR pattern can now be explained.

IR absorption coefficients for water in nominally anhydrous high-pressure minerals

Abstract Infrared spectroscopic quantification of traces of OH and H2O in minerals and glasses is based on the Beer Lambert law A = ε·c·t, where ε is the molar absorption coefficient. Numerous experimental and theoretical studies show that ε generally increases with decreasing wavenumbers. However, this general trend seems to be valid only for hydrous minerals and glasses and should not be applied to water quantification in nominally anhydrous minerals (NAMs) that incorporate traces of water in their structures. In this study, we analyze ε-values from literature data and propose that within a polymorphic mineral series of the same composition ε negatively correlates with the molar volume and positively correlates with the density of the respective mineral phase. To test this hypothesis, we determined ε-values for synthetic hydrous ringwoodite samples ranging in composition from XMg = 0.0 to 0.6 by combining results of FTIR-spectroscopy with those of secondary ion mass spectrometry. The ε-values plot well below the general calibration curves but follow the same trend, i.e., they increase with decreasing wavenumbers of the OH bands from 59 000 ± 6000 for the Fe end-member to 85 800 ± 10 000 L/mol(H2O)/cm-2 for the Mg-richest sample. From this relation we can predict an absorption coefficient for the iron-free Mg end-member as 100 000 ± 7000 L/mol(H2O)/cm-2. This value together with ε-values for forsterite and wadsleyite taken from literature confirm the proposed correlation with the molar volume and density within this polymorphic series. This allows us to predict absorption coefficients for some minerals, where coefficients for one or better two of their polymorphs, either high- or low-pressure, are available.

The influence of OH in coesite on the kinetics of the coesite-quartz phase transition

Abstract Metastable coesite is an important pressure indicator for ultrahigh-pressure rocks. However, in many cases coesite does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that incorporation of H in coesite increases with increasing pressure, most coesite relics found in nature are essentially dry (i.e., OH concentrations are below detection limit, <1 wt ppm H2O). Thus, does the incorporation of H promote the back-reaction of coesite to quartz during exhumation? The influence of intrinsic OH on the kinetics of the coesite-quartz phase transition was determined using synthetic “dry” coesite with ≈ 10 wt ppm H2O and “wet” coesite with ≈ 105 wt ppm H2O. TEM analysis of the quenched samples proved the presence and absence of water in the .wet. and .dry. samples, respectively. The kinetics of the coesite-quartz transition was investigated in-situ using the multi-anvil apparatus MAX 80 at the Hamburger Synchrotron Radiation Laboratory (HASYLAB). The transition rates were measured by observing changes in selected diffraction line intensities as a function of time. The transformation and growth rates were derived using Cahns model of nucleation and growth at grain boundaries. Under the same experimental conditions the transformation rate of the “wet” coesite is more than ten times higher than that of the “dry” coesite. This difference may explain why OH-bearing natural coesite is rare. This study reveals the importance of structurally bound OH for the kinetics of phase transitions of nominally anhydrous minerals.

Cathodoluminescence properties and trace element signature of hydrothermal quartz: A fingerprint of growth dynamics

Abstract Relationships between cathodoluminescence spectra and trace element contents of hydrothermal α-quartz including hydrogen species are characterized for crystals from Gigerwald (Switzerland) and Rohdenhaus (West Germany) grown under highly different physico-chemical conditions and related to growth fabrics visualized by classical cathodoluminescence microscopy. Distinct emission bands at 395, 448, 503, 569, and 648 nm determine the spectral characteristics of cathodoluminescence images. Aluminum, Li, and H are the most important trace elements as determined by LA-ICP-MS and IR spectroscopy, reaching up to 6000 μmol/mol Al3+, 3300 μmol/mol Li+, and 5000 μmol/mol H+. Germanium, B, and Na are present at less than a few μmol/mol concentrations. A large amount of H is present in structurally bound water. AlOH-defects are also common, whereas LiOH- and SiOH-defects play only a minor role. Fast grown zones contain Li+ and H+ concentrations too low to compensate the charge deficit if all measured Al substitutes for Si4+ in the quartz structure. This indicates the occurrence of intrinsic defects such as oxygen deficiency centers, which are assumed to affect the luminescence properties. Lithium abundances correspond to [AlO4|Li]-defects, correlated to the unstable intensity at 395 nm, but the correlation is different for both localities. This is inconsistent with a simple causal relationship between Al-Li-centers and the emission at 395 nm. Conversion of [AlO4|Li]-defects to [AlO4]0-defects by natural irradiation is a possible explanation for this discrepancy. The increase of the intensity at 648 nm is not proportional to SiOH concentration as suggested in the literature, indicating that other precursor defects such as peroxy-linkages are more important. The decay of the intensity at 395 nm is much more rapid than the increase at 648 nm, excluding a coupling between these processes. Trace element incorporation in slowly grown hydrothermal quartz crystals is a direct function of fluid chemistry and temperature for a specific growth sector. Because quartz grows during extended periods of hydrothermal activity, changes in trace element inventory as visualized by cathodoluminescence may identify significant changes in growth conditions, which likely remain unrecognized during sample characterization with conventional microscopy.

Optical spectroscopic study of tetrahedrally coordinated Co2+ in natural spinel and staurolite at different temperatures and pressures

Abstract Optical absorption spectra of natural Co-bearing spinel and staurolite were studied at different temperatures and pressures. In both minerals, two broad, intense structured bands in the range 5500-8000 and 15 000-19 000 cm-1, caused by electronic spin-allowed transitions 4A2 → 4T1(4F) and 4A2 → 4T1(4P) of IVCo2+ are the predominant absorption features. In addition, in both cases broad bands, derived from spin-allowed electronic transitions 4E → 4T2 of IVFe2+, appear in the near infrared range partly overlapping the bands caused by IVCo2+. In staurolite the NIR range of the spectra are complicated by intense sharp lines of OH-vibrations at around 3400 cm-1. In spinel, with a regular tetrahedral site, the splitting of the spin-allowed bands I and II of IVCo2+ is assumed to be caused by spin-orbit and vibronic coupling. In staurolite, the splitting is stronger due to the additional low-symmetry crystal field effect of IVCo2+. It is found that the effect of temperature and pressure on the behavior of the 4A2 → 4T1(4P) bands of IVCo2+ in the two minerals are rather similar, in contrast to our findings for the spin-allowed bands of IVFe2+ in spinel and staurolite. This is interpreted as a manifestation of a dynamic Jahn-Teller effect for IVFe2+ and lack of it in case of IVCo2+

The 3.65 Å phase, MgSi(OH)6: Structural insights from DFT-calculations and T-dependent IR spectroscopy

Abstract First-principles calculations based on density-functional theory (DFT) and low-T IR spectroscopy were performed to gain more insight into the structure of the so-called 3.65 Å phase, a high-pressure phase of the composition MgSi(OH)6. DFT-calculations predict a monoclinic symmetry with ordered sixfold-coordinated Mg and Si and six unique hydrogen sites as the most stable structure. Adapting the structural parameters of the DFT-determined lowest-energy configuration and assuming (MgSi)- ordering, a new Rietveld refinement of the powder XRD pattern of the 3.65 Å phase was performed, which resulted in excellent refinement statistics and successful assignment of X-ray reflections that were unassigned in former structural models with orthorhombic symmetry. A configuration with ordered Mg and Si at the octahedral positions causes a small monoclinic distortion of the network of strongly tilted octahedra and thus leads to space group P21. The structural refinement yields the following unit-cell parameters: a = 5.1131(3), b = 5.1898(3), c = 7.3303(4) Å, β = 90.03(1)°, V = 194.52(2) Å3, space group: P21, Z = 2, ρ = 2.637 g/cm3. The structure of the 3.65 Å phase can be considered as a modified A-site defective perovskite with a unique network of corner-sharing alternating Mg(OH)6 and Si(OH)6 octahedra and is probably related to the structure of stottite group minerals. Low-T IR spectroscopy confirms the presence of 6 different H-positions in the proposed structure. Measured IR-spectra and computed spectra compare favorably, which further supports the computed structure as the correct model for the 3.65 Å phase.

In-situ mid/far micro-FTIR spectroscopy to trace pressure-induced phase transitions in strontium feldspar and wadsleyite

Abstract As representatives of nominally anhydrous minerals (NAMs) in the crust and mantle the pressuredependent behavior of strontium feldspar and wadsleyite, containing different amounts of water, was studied in a diamond-anvil cell via mid and far IR spectroscopy up to 24 GPa. The samples were synthesized in a piston-cylinder press at 2 GPa/700 °C (strontium feldspar) and in a multi-anvil apparatus at 13.8 GPa/1000 °C (dry wadsleyite) and 13.2 GPa/1150 °C (hydrous wadsleyite). The water content of the samples was determined by polarized FTIR and Raman spectroscopy. The strontium feldspar crystals (up to 300 μm) contained about 1100(100) wt ppm water. The hydrous wadsleyite crystals (up to 240 μm) contained 12 500(900) wt ppm water. The synthesis of dry wadsleyite yielded a fine-grained powder. A new THz/FIR-microscope for the synchrotron source BESSY was developed to conduct the diamond-anvil cell measurements in the far IR region. Conventional in-situ mid IR spectroscopy was also performed on all samples. The measurements on strontium feldspar showed a phase transformation at 6.5(5) GPa (space group I2/c to P21/c). The wadsleyite analyses revealed a phase transition at approximately 8.4(7) GPa in the hydrous and approximately 10.0(7) GPa in the dry sample. It probably represents a transition from an orthorhombic to a monoclinic structure. The high amount of water incorporated in the hydrous wadsleyite shifts the transformation toward lower pressures compared to the dry one. By comparison, the relatively low amount of water in strontium feldspar does not change the stability relations compared to the dry one. Therefore, water incorporation in nominally anhydrous minerals may have an effect on the pressure of phase transitions, whereas the extent of that influence strongly depends on the structure of the phase and the amount of water carried within the mineral.