Quantifying absolute water content of minerals using near‐infrared reflectance spectroscopy

Abstract

Fundamental and overtone vibrational absorptions of H2O and OH− observed in reflectance spectra in the 1.3–5.0 μm wavelength region may be used to estimate the absolute or relative amount of water in particulate materials. Laboratory reflectance spectra of five hydrated materials (sodium and magnesium montmorillonite, sulfate, zeolite, and palagonite) with initial water contents ranging from 6 to 20 wt % were measured under decreased relative humidity and heated conditions to control their absolute water content. Relationships between absolute water content and water‐related absorptions were quantified using common band parameters (e.g., band depth and mean optical path length). Parameters calculated using fundamental O‐H stretching absorptions within the 3 μm region show trends with water content that are consistent among the samples, whereas those calculated using combination O‐H stretch and H‐O‐H bend absorptions in the 1.9 μm region do not. There appears to be no correlation between water absorptions near 1.9 μm and absolute water content that is independent of composition or between these absorptions and their counterparts near ∼2.9–3.1 μm. Combination overtone absorptions near 1.9 μm therefore are not expected to be reliable estimators of water content for mixtures of minerals. A parameter we call normalized optical path length (NOPL) exhibits a strong exponential correlation to absolute water content for the 3 μm region. On the basis of our current laboratory data, the NOPL parameter can be used to estimate H2O content within ±1 wt % or better over a wide range of water contents for the five materials examined. In addition to estimating the water content of laboratory samples, these methods can also be used to map the hydration state of planetary surfaces using high‐resolution visible‐near‐infrared spacecraft data.