Reflectance spectroscopy of ammonium-bearing phyllosilicates.

Highlights • Laboratory experiments were made to study the NH 4 -carrier phase detected on Ceres. • An ammoniation technique was tested on ten different phyllosilicate samples. • The ammonium treated samples were analysed by means of VIS-NIR spectroscopy. • Only the smectite samples incorporated the ammonium ion in their structure. • A scenario for the thermal evolution of a smectite-bearing surface is presented. Abstract The identification of NH 4 -bearing phyllosilicates on Ceres poses the question on the NH 4 -carrier phase(s) and in this study we describe the laboratory production and IR spectroscopic measurements of a suite of ten NH 4 -phyllosilicates, starting from the corresponding NH 4 -free minerals. For each mineral, we prepared three types of powder samples: raw (R), ammoniated (A), and leached (L). All samples have been spectrally characterized by means of visible/infrared spectroscopy in the INAF-IAPS laboratories with the FieldSpec Pro in the 0.35–2.5 µm range, and with the FT-IR, using a Vertex 80 spectrometer operating in the range of 2–14 µm. The samples were also measured with the SPectral IMager, an imaging spectrometer operating in the spectral range 0.2–5.1 µm, which is a replica of the VIR spectrometer on-board Dawn spacecraft. Reflectance spectra of the ammoniated clays show bands near 1.56 µm, 2.05 µm, 2.12 µm, 3.06 µm, 3.25 µm, 3.55 µm, 4.2 µm, 5.7 µm and 7.0 µm that are related to the presence of nitrogen complexes. Treatment of phyllosilicates with ammonia shows that different minerals behave in different ways: NH 4 + ions are easily accepted by the smectites, while other non-expandable structures do not accept these ions. The obtained results can be used to better constrain the NH 4 -bearing species present on Ceres and, possibly, other bodies of the solar system. [ABSTRACT FROM AUTHOR]