Laboratory emissivity measurements of the plagioclase solid solution series under varying environmental conditions

Abstract

New laboratory thermal infrared emissivity measurements of the plagioclase solid solution series over the 1700 ∼ 400 cm−1 (6–25 μm) spectral range are presented. Thermal infrared (TIR) spectral changes for fine‐particulate samples (0–25 μm) are characterized for the first time under different laboratory environmental conditions: ambient (terrestrial‐like), half‐vacuum (Mars‐like), vacuum, and vacuum with cooled chamber (lunar‐like). Under all environmental conditions the Christiansen Feature (CF) is observed to vary in a systematic way with Na‐rich end‐member (albite) having a CF position at the highest wave number (shortest wavelength) and the Ca‐rich end‐member (anorthite) having a CF position with the lowest wave number (longest wavelength). As pressure decreases to <10−3 mbar four observations are made: (1) the CF position shifts to higher wave numbers, (2) the spectral contrast of the CF increases relative to the RB, (3) the spectral contrast of the RB in the ∼1200–900 spectral range decreases while the spectral contrast of the RB in the ∼800–400 spectral range either increases or remains the same and (4) the TF disappears. A relationship between the wavelength position of the CF measured under simulated lunar conditions and plagioclase composition (An#) is developed. Although its exact form may evolve with additional data, this linear relationship should be applied to current and future TIR data sets of the Moon. Our new spectral measurements demonstrate how sensitive thermal infrared emissivity spectra of plagioclase feldspars are to the environmental conditions under which they are measured and provide important constraints for interpreting current and future thermal infrared data sets.