Determination of Hydrogen Concentrations by Fourier- Fourier-Transform Infrared Absorption Spectroscopy

The detection and quantification of hydrous species in minerals by means of infrared spec-troscopy has evolved to a standard technique in mineralogy (e. g. Rossman, 1988; Rossman, 2006). As explained in section 1.3.1, hydrogen is incorporated into the crystal structure of wadsleyite in the form of hydroxyl groups (Smyth, 1987; McMillan et al., 1991; Young et al., 1993; Jacobsen et al., 2005). While the strength of infrared absorption is related to the concentration of the absorbing species through the Beer-Lambert law (Rossman, 1988;

Libowitzky and Rossman, 1997), frequency and polarization of the absorption bands reflect the atomic environment of the absorbing species such as bond length (Nakamoto et al., 1955; Libowitzky, 1999) and orientation (Libowitzky and Rossman, 1996; Jacobsen et al., 2005; Libowitzky, 2006) of hydroxyl groups. Polarized infrared absorption spectra there-fore allow discriminating between different types of structurally bonded hydroxyl groups and to recognize hydroxyl groups that are present in phases other than the mineral of inter-est, for example on grain boundaries or in inclusions. The ability to identify and quantify hydrogen concentrations independently for different types of hydroxyl groups brings about important advantages over other analytical techniques that are blind to the original state of counted hydrogen atoms in the sample such as Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection Analysis (ERDA) (Rossman, 2006). Both SIMS (Inoue et al., 1995; Demouchy et al., 2005; Deon et al., 2010; Bolfan-Casanova et al., 2012) and ERDA (Bolfan-Casanova et al., 2018) have been used to quantify hydrogen contents of wadsleyite beside numerous infrared spectroscopy studies (e. g. McMillan et al., 1991; Young et al., 1993; Kohlstedt et al., 1996; Jacobsen et al., 2005; Deon et al., 2010).

Figure 2.13: Photomicrographs of oriented wadsleyite single crystals (a,b) and FTIR absorption spectra (c,d) recorded at the spots indicated in a) and b). Spectra in c) were recorded on the crystal section shown ina); spectra ind) were recorded on the crystal section shown inb).

2.2 Characterization of Samples for High-Pressure Experiments

Table 2.3:Hydrogen concentrations in wadsleyite crystals from FTIR spectroscopy Crystal Number Polarized Unpolarized Ratio Numbers initalicswere calculated based on the mean ratio [H₂O]_P/[H₂O]_Ufor the respective crystal orientation.

aBased on the calibration by Libowitzky and Rossman (1997).

bH₂O molecules per formula unit.

Descriptions on the theory and instrumentation of Fourier-Transform Infrared Spec-troscopy (FTIR) can be found for instance in McMillan and Hofmeister (1988) and Kuz-many (2009). Here, I concentrate on those aspects essential for this study. Figure 2.12 shows a schematic drawing of the FTIR setup that consisted of an infrared microscope with reflecting optics coupled to a Bruker¹² IFS 120 HR FTIR spectrometer. A detailed descrip-tion of the experimental procedure has been published in Buchen et al. (2017) (Chapter 4, sections 4.2.4 and A.1.1). FTIR absorption spectra were recorded on plane-parallel wad-sleyite single-crystal thin sections that were oriented parallel to the (120) or (243) crys-tallographic planes and double-sided polished. Polarized and unpolarized spectra were recorded in transmission covering a spectral range from 2500 to 4000 cm⁻¹ with a reso-lution of 4 cm⁻¹. Immersing the single-crystal thin sections in polychlorotrifluoroethylene oil substantially reduced interference fringes but did not suppress them completely. Intents to reduce inference fringes by placing the thin sections above a small circular aperture in a metal foil instead of using a CaF₂ support plate were to no avail indicating that interference originated within the sample by internal reflections on the plane-parallel polished sample surfaces.

The quantitative analysis of recorded FTIR spectra is explained in detail in section A.1.1 including background correction, deconvolution, and integration of absorption bands aris-ing from structurally bonded hydroxyl groups in wadsleyite. Integrated band absorbances of polarized FTIR absorption spectra were combined and converted to hydrogen concentra-tions according to the principles laid out by Libowitzky and Rossman (1996) and Libowitzky

12Bruker Corporation, Billerica, Massachusetts, USA, www.bruker.com

Figure 2.14:Photomicrograph of a twinned wadsleyite crystal (a) and unpolarized FTIR absorption spectra (b) recorded at the spots shown ina). Ina), part A (pale green color) was oriented parallel to the (120) crystallographic plane using X-ray diffraction while the tentative orientation of part B (dark green color) was inferred from extinction positions and pleochroism.

and Rossman (1997). In Buchen et al. (2017) (Chapter 4, section 4.2.4), we applied differ-ent calibrations to quantify hydrogen in minerals (Paterson, 1982; Libowitzky and Rossman, 1997) and wadsleyite in particular (Deon et al., 2010) by FTIR absorption spectroscopy.

Comparing the results for individual absorption bands (Table A.4 on page 186), we con-cluded that a wave number-dependent absorption coefficient is needed to take into account the band-specific absorption strength, in particular for iron-bearing wadsleyite.

In addition to the determination of hydrogen concentrations, I used FTIR spectroscopy to examine the homogeneity of the synthesized wadsleyite crystals in terms of amount and speciation of structurally bonded hydroxyl groups (see also Kawazoe et al., 2015). Fig-ure 2.13 shows two wadsleyite single-crystal sections and the FTIR absorption spectra that were recorded on different locations on these crystals. Although the two crystals differ substantially in size, both are internally homogeneous in terms of hydrogen concentrations and speciations as can be directly inferred from the similarity of polarized FTIR absorption spectra collected on different spots on each crystal.

Figure 2.14 shows unpolarized FTIR absorption spectra recorded on a more hydrous wadsleyite crystal (run #2 in Table 2.1). Polarized FTIR absorption spectra recorded on this and on another crystal from the same synthesis run showed indications for total ab-sorption of infrared radiation at wave numbers of maximum abab-sorption. The combination of hydrogen concentration and thickness of these crystal sections together with the addi-tional reduction of transmitted light intensity caused by the polarizer might have pushed the transmitted light intensity below a minimum level. When the transmitted light inten-sity drops below this minimum level, the resulting absorption spectra might be distorted or even cut off at a maximum threshold absorbance. Water contents of these hydrous crystals were therefore estimated from unpolarized spectra and an empirical ratio based on water