High-Temperature VIS-IR Spectroscopy of NH4-Phyllosilicates

Ammonium phyllosilicates have been identified on the dwarf planet Ceres, thanks to infrared telescopic and orbital data from the Dawn mission, by means of the 3.06 μm spectral feature. Nevertheless, it is not known which ammonium-bearing phyllosilicate species are present, nor the thermal processing they underwent throughout Ceres history. Identifying the NH4+-hosting mineral species is important for deciphering Ceres’ surface mineralogy, which provides a link to its interior and putative different evolutionary pathways. 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 outer Solar System origin, they can have undergone heating at depth. In this work, we study the visible-infrared spectra of several NH4-treated/untreated phyllosilicates in the range 0.35–5 μm, acquired in vacuum and at temperatures between 298 and 723 K. Previously NH4-phyllosilicates have been mostly studied at ambient condition, preventing the characterization of the NH4+ band at 3.06 μm, due to overlapping bands of water. With this new set of measurements, we investigate how the NH4-phyllosilicates spectra are modified when the mineral’s water is lost, and which temperature is the limit for the releasing of NH4+. We present the first high-temperatures/high vacuum 3-μm reflectance spectra of ammonium phyllosilicates. Our measurements indicate that Mg-phyllosilicates are the best candidates for the ammonium-bearing species. Moreover, the almost complete disappearing of NH4+ absorption feature at ∼3.06 μm for ammoniated phyllosilicates heated at the highest temperatures indicates that such species on Ceres could not have experienced temperatures higher than 623 K. © 2021. The Authors.