X-ray and Raman spectroscopy of jarosite and ammoniojarosite from several terrestrial localities

Abstract

Jarosite, a hydrous K-rich sulfate mineral, was found in fine-grain quantities on the Martian surface by the Opportunity rover in 2004. Jarosite found in some terrestrial settings can form a complete solid solution with ammoniojarosite through substitution of the ammonium ion (NH4+) which replaces K+ within its crystal structure. Requiring water and acidic conditions to form, jarosite stands as a potential indicator of ancient N-bearing environments on the Martian surface. The Mars Perseverance rover is currently exploring the Martian surface with analytical instrumentation that may be capable of detecting N-rich jarosite, including the SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) and the PIXL (Planetary Instrument for X-ray Lithochemistry) detectors. To investigate the detection of N in jarosite with Raman and X-ray spectroscopy, we analyzed several terrestrial samples of jarosite and ammoniojarosite using laser Raman spectroscopy and electron-excited Wavelength Dispersive X-ray Spectroscopy (WDS). Comparison of high wavenumber regions of Raman spectra with WDS spectra of N K-α X-rays on the same samples will be used to investigate detection and quantification of N as ammonium in jarosite.