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Water quantification in olivine and wadsleyite by Raman 2spectroscopy and study of errors and uncertainties

Abstract : The study of nominally anhydrous minerals with vibrational spectroscopy, despite its sensitivity, tends to produce large uncertainties (in absorbance or intensity) if the observed dispersion of the values arising from the anisotropy of interaction with light in non-cubic minerals is not assessed. In this study, we focused on Raman spectroscopy, which allows the measurement of crystals down to few micrometers in size in back-scattered geometry, and with any water content, down to 200 ppm by weight of water. Using synthetic hydrous single-crystals of olivine and wadsleyite, we demonstrate that under ideal conditions of measurement and sampling, the data dispersion reaches ±30% of the average (at 1σ) for olivine, and ±32% for wadsleyite, mostly because of their natural anisotropy. As this anisotropy is linked to physical properties of the mineral, it should not be completely considered as error without treatment. By simulating a large number of measurements with a 3D model of the OH/Si spectral intensity ratio for olivine and wadsleyite as a function of orientation, we observe that although dispersion increases when increasing the number of measured points in the sample, analytical error decreases, and the contribution of anisotropy to this error decreases. With a sufficient number of points (five to ten, depending on the measurement method), the greatest contribution to the error on the measured intensities is related to the instrument’s biases, and reaches 12 to 15% in ideal cases, indicating that laser and power drift corrections have to be carefully performed. We finally applied this knowledge on error sources (to translate data dispersion into analytical error) on olivine and wadsleyite standards with known water contents to build calibration lines for each mineral in order to convert the intensity ratio of the water bands over the structural bands (OH/Si) to water content. The conversion factor from OH/Si to ppm by weight of water (H2O) is 93108±24005 for olivine, 250868±45591 for iron-bearing wadsleyite, and 57546±13916 for iron-free wadsleyite, showing the strong effect of iron on the spectral intensities.
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https://hal.uca.fr/hal-02996319
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Submitted on : Monday, November 9, 2020 - 3:42:01 PM
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Loïs Martinek, Nathalie Bolfan-Casanova. Water quantification in olivine and wadsleyite by Raman 2spectroscopy and study of errors and uncertainties. American Mineralogist, Mineralogical Society of America, 2020, ⟨10.2138/am-2021-7264⟩. ⟨hal-02996319⟩

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