High-Vacuum / High-Temperature Reflectance Spectroscopy of Phyllosilicates and NH4-Phyllosilicates in the Visible-Near Infrared Range

Ammonium phyllosilicates have been identified, besides Earth, on the dwarf planet Ceres, thanks to ground-based and remote sensing infrared spectroscopic data from the Dawn mission. Data show a characteristic absorption feature at 3.06 micron, assigned to NH4-phyllosilicates. The identification of the particular mineral group(s) hosting the NH4+ ion is important to decipher the surface mineralogy of Ceres and its link with the interior and evolution of the body. Different hosting minerals can hint at completely different evolutionary pathways. Phyllosilicates and ammoniated species can have formed in the presence of water/ammonia-rich fluids in different conditions in the interior of the planet; in case of an exogenous origin, as material accreted on Ceres from the outer Solar System, they can have undergone several warming episodes at depth.
In this work we study the spectral reflectance of several ammoniated phyllosilicates in the visible and infrared range 0.35-5 μm, acquired in vacuum and at different temperatures between 298-723K. The corresponding non-ammoniated phyllosilicates were also measured in similar conditions. Previous measurements of NH4-phyllosilicates have been done mostly at ambient condition, preventing the full characterization of the NH4+ band at 3.06 μm, due to overlapping spectral signatures of water. With this new set of measurements, we try to establish in which way the NH4-phyllosilicates spectra are modified when the mineral water is lost, and which is the temperature limit for releasing of NH4+ in different phyllosilicates. We present the first reflectance spectra of ammonium phyllosilicates in the 3-μm region, acquired in vacuum, and at high temperatures.
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