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

Abstract

Infrared spectroscopic quantification of traces of OH and H 2 O 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 X Mg = 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(H 2 O)/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(H 2 O)/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.

A nearly water-saturated mantle transition zone inferred from mineral viscosity

“…Two or three spectra for each crystal were obtained on the polished surfaces, and one or two crystals from each run were analyzed. After background and baseline subtraction, the water contents were determined from the FT-IR spectra using the Beer-Lambert law of the form${\mathit{C}}_{\mathrm{normalH}2\mathrm{normalO}}=true\int \frac{3\mathrm{italicA}false(\mathrm{normal\upsilon }false){\mathit{M}}_{\mathrm{normalH}2\mathrm{normalO}}}{{\mathrm{\epsilon }}_{\mathrm{i}}\mathrm{normal\tau }\mathrm{normal\rho }}\mathrm{italicd}\mathrm{normal\upsilon }$where C H2O is the water content in wt ppm, and A (ν) is the infrared absorption at wave number ν. M H2O is the molar weight of water (18.02 g/mol); ε i is the absorption coefficient [98,600 liter∙mol −1 ∙cm −2 for ringwoodite ( 37 )]; τ is the sample thickness, which is normalized to 1 cm; and ρ is the density (3900 g/liter). The integration was performed for the wave number range 3740 to 2600 cm −1 ( 3 ).…”

Water and Iron effect on the P-T-x coordinates of the 410-km discontinuity in the Earth upper mantle

“…However, it has been shown that these values should not be applied to water quantification in nominally anhydrous minerals (NAMs); NAMs need mineral-specific ε−values (e.g. Rossman 2006, Koch-Müller and Rhede, 2010. Therefore, absorption coefficients (ε) were taken from Deon et al (2009) (2010) 75,300± 7,000 L mol H 2 O cm -2 for ol and ring, respectively.…”

Stress-induced proton disorder in hydrous ringwoodite

“…1) were synthesized in a multianvil apparatus and characterized by electron microprobe analyses (EMPA) and secondary ion mass spectrometry (SIMS). Details of the experimental procedures and results of EMPA are given in Taran et al (2009) and Koch-Müller et al (2009); details of the SIMS measurements are given in Koch-Müller and Rhede (2010).…”