| Summary: | Mgâ€phyllosilicateâ€bearing, dark surface materials on the dwarf planet Ceres have NH_4â€bearing materials, indicated by a distinctive 3.06 μm absorption feature. To constrain the identity of the Ceres NH_4â€carrier phase(s), we ammoniated ground particulates of candidate materials to compare their spectral properties to infrared data acquired by Dawn's Visible and Infrared (VIR) imaging spectrometer. We treated Mgâ€, Feâ€, and Alâ€smectite clay minerals; Mgâ€serpentines; Mgâ€chlorite; and a suite of carbonaceous meteorites with NH_4â€acetate to exchange ammonium. Serpentines and chlorites showed no evidence for ammoniation, as expected due to their lack of exchangeable interlayer sites. Most smectites showed evidence for ammoniation by incorporation of NH_4^+ into their interlayers, resulting in the appearance of absorptions from 3.02 to 3.08 μm. Meteorite samples tested had weak absorptions between 3.0 and 3.1 μm but showed little clear evidence for enhancement upon ammoniation, likely due to the high proportion of serpentine and other minerals relative to expandable smectite phases or to NH_4^+ complexing with organics or other constituents. The wavelength position of the smectite NH4 absorption showed no variation between IR spectra acquired under dryâ€air purge at 25 °C and under vacuum at 25 °C to −180 °C. Collectively, data from the smectite samples show that the precise center wavelength of the characteristic ~3.05 μm v_3 absorption in NH_4 is variable and is likely related to the degree of hydrogen bonding of NH_4â€H_2O complexes. Comparison with Dawn VIR spectra indicates that the hypothesis of Mgâ€saponite as the ammonium carrier phase is the simplest explanation for observed data, and that Ceres dark materials may be like Cold Bokkeveld or Tagish Lake but with proportionally more Mgâ€smectite. © 2018 The Authors. Meteoritics & Planetary Science published by Wiley Periodicals, Inc. on behalf of The Meteoritical Society (MET). This is an open access article under the terms of the ...
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