Estimating the water content of hydrated minerals using reflectance spectroscopy: I. Effects of darkening agents and low-albedo materials

The apparent strength of absorptions due to H2O near 1.9 and 3 μm in reflectance spectra is strongly affected by sample albedo. This study uses experimental and analytical approaches to quantify the effects of albedo on estimating the water content of hydrated minerals using various band parameters. We compare spectral band parameters for a series of low-albedo physical and numerical mixtures to measured water contents. Physical experiments consist of montmorillonite, clinoptilolite, and palagonite mixed with lesser amounts of carbon black and ilmenite, whereas numerical mixtures are composed of these host minerals mixed with a material of constant, low albedo. We find the effective single-particle absorption-thickness parameter provides the best correlation to water content, independent of composition and albedo, when derived from continuum-removed single scattering albedo spectra. Uncertainties in estimated water content are on the order of ±1 wt% using this method. The normalized optical path length parameter provides the best correlation to water content when using reflectance spectra, yielding estimates within ±1.6 wt% H2O. The accuracy of these models is related to the physical nature of the darkening material. Scattering and absorption efficiencies are easier to model for intimate mixtures containing relatively large, dark grains than mixtures dominated by coatings of a fine-grained, strongly absorbing material. This suggests the physical properties that give rise to the albedo of a material are an important factor for accurate estimates of absolute water content.