Determining the water content of nominally anhydrous minerals at the nanometre scale
S. Patabendigedara, D. Nowak, M.J.B. Nancarrow and S.M. Clark, Rev. Sci. Inst. 2 023103 (2021).
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The amount and distribution of water in nominally anhydrous minerals (NAMs) is usually determined using Fourier-transform infrared spectroscopy. This method is limited by the spot size of the beam to the study of samples with dimensions greater than a few microns. Here we demonstrate the potential of photoinduced force microscopy for the measurement of water in NAMs with samples sizes down to the nanometer scale with a study of water concentration across grain boundaries in forsterite. This development will enable the study of water speciation and diffusion in small-grained rock matrixes and allow a determination of the influence of nanoscale heterogeneity on the incorporation of water to NAMs.
Figure 1: Lift out of a lamella with the lamella attached to a gold-coated AFM sample holder. (a) SEM image of the sample welded to the tip of the micromanipulator and ready for lift out. (b) SEM image of the polycrystalline sample lamella secured on AFM sample holder and the demarcated area shows the region of PiFM map, which is an enlarged view of AFM topography image b1. (c) SEM image of a single crystal standard olivine lamella mounted on the AFM sample holder.
Figure 2: Schematic overview of the photoinduced force microscopy (adapted from Nowak, et al. 29). The topographic and photoinduced force images are acquired simultaneously when laser modulation frequency satisfy the condition of , where and are the first and second mechanical eigenmode resonances of the cantilever.
Figure 3: FTIR (black line) and PiFM (red line) spectra of olivine standards.
Figure 5: Series of PiFM spectra from the polycrystalline forsterite sample. Two profiles were attained perpendicular to both two-grain boundaries. The water distribution profiles (bottom figure) ware created using estimated water content from point spectra.
Figure 4: The correlation between PiFM signal and water content obtained from FTIR measurements in the OH stretching regions of [Si] [Ti[ and [triv] defect sites. Linear regression line is
Figure 6: Point spectroscopy map of apparent water distribution in the polycrystalline sample. The strength of the signal is proportional to the amount of water.
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