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 a few micrometers in size in backscattered 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 1s) 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 an 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 to convert the intensity ratio of the water bands over the structural bands (OH/Si) to water content. The conversion factors from OH/Si to parts per million by weight of water (H2O) are 93 108 ± 24 005 for olivine, 250 868 ± 53 827 for iron-bearing wadsleyite, and 57 862 ± 12 487 for iron-free wadsleyite, showing the strong effect of iron on the spectral intensities.

中文翻译：

---

用拉曼光谱法对橄榄石和辉石中的水进行定量分析以及误差和不确定性的研究

如果没有评估观察到的与非立方矿物中与光相互作用的各向异性所引起的值的分散性，则使用振动光谱法对名义上的无水矿物进行研究，尽管其灵敏度很高，但往往会产生很大的不确定性（吸光度或强度）。在这项研究中，我们集中于拉曼光谱学，它可以测量反向散射几何结构中尺寸低至几微米的晶体，并且在任何水含量下，按水的重量计低至200 ppm。使用合成的橄榄石和辉石的含水单晶，我们证明在理想的测量和采样条件下，橄榄石和辉石的数据分散度达到平均值的±30％（在1s时）和辉石的平均值为±32％，这主要是由于它们的天然性各向异性。由于这种各向异性与矿物的物理性质有关，如果不进行处理，则不应将其完全视为错误。通过使用3D模型（针对橄榄石和辉石的OH / Si光谱强度比作为方向的函数）模拟大量测量，我们观察到，尽管当增加样品中的测量点数时色散会增加，但分析误差会降低，并且各向异性对该误差的贡献减小。如果有足够的点数（五到十，取决于测量方法），则对测量强度误差的最大贡献与仪器的偏差有关，在理想情况下达到12％到15％，这表明激光和功率漂移更正必须谨慎进行。我们最终将这种知识应用于已知水含量的橄榄石和沃兹利石标准品的误差源（将数据分散转化为分析误差），为每种矿物建立校准线，以转换水带在结构带上的强度比（OH / Si） ）到含水量。从OH / Si到百万分之几的水（H2O）的转换因子是：橄榄石为93 108±24 005，含铁辉石为250 868±53 827，无铁辉石为57 862±12 487，表现出铁对光谱强度的强烈影响。