X-ray Spectroscopy of Nitrogen in Jarosite, Ammoniojarosite, and other NH4-Bearing Sulfate Minerals

Jarosite was detected on Mars in 2004 by NASA’s Opportunity rover in a rock outcrop from the Meridiani Planum region [1]. The chemical signature of jarosite was confirmed using alpha particle X-ray spectrometer (APXS), Miniature Thermal Emission Spectrometer (Mini-TES), and Mössbauer spectrometer [2, 3]. Jarosite has since been identified with remote spectroscopic observations from landforms within the Mawrth Vallis region [4] and again detected in-situ by the Curiosity rover in drill-core samples from Gale crater using the CheMin X-ray Diffractometer [5]. Nitrogen-bearing compounds have been detected in aeolian sediments and subsurface sedimentary units by the Curiosity rover from the Gale Crater [6]. Quantification of N was performed using quadrupole mass spectrometry of gases emitted upon sample pyrolysis. Results indicate approximately 0.01-0.1% by weight as nitrate (NO3-). Discovery of fixed N on Mars is significant because the majority of nitrogen fixation reactions on Earth involve biologically-mediated processes. Multiple forms of reduced N could exist (NO3-, NH3, NH4+) but are unlikely to persist for long duration when exposed on the surface of Mars, unless they are sequestered within a stable mineral structure. The purpose of our study is to perform X-ray spectroscopy analysis of N incorporation into jarosite and other hydrous sulfate minerals to investigate potential phases for long-term preservation of N on Mars. Jarosite is a K-bearing iron sulfate mineral with formula KFe3+3(SO4)2(OH)6. It can accommodate N in its structure through substitution of NH4+ for K+ within crystallographic A sites. To investigate X-ray fluorescence methods for detecting N as ammonium in jarosite, we performed systematic analysis using wavelength dispersive X-ray spectrometry (WDS) and energy dispersive X-ray spectroscopy (EDS) on eight samples of jarosite, four samples of ammoniojarosite, and samples of five other NH4-bearing sulfate minerals (boussingaultite, godovikovite, letovicite, mascagnite, and tschermigite). WDS analysis was performed using a Field Emission Electron Probe Microanalyzer (FE-EPMA) equipped with three large d-spacing synthetic diffraction crystals capable of analyzing low energy X-rays: PC0 (2d=47 Å) measures from 0.321-1.196 keV, PC1 (2d=60 Å) measures from 0.257-0.931 keV, and LPC2 (2d=98 Å) measures from 0.161-0.569 keV. EDS analysis of N was tested using three different EDS detectors mounted on one EPMA and two Scanning Electron Microscope (SEM) instruments. Vacuum sensitivity observed for some of the hydrous NH4-sulfates restricted analysis to chamber pressures of ∼100 Pa using Variable Pressure (VP-SEM) mode. Detection of N-Kα X-rays with energy 0.392 keV is difficult for multiple reasons. Light elements have poor fluorescence yield and their low energy, long wavelength X-rays are readily absorbed by other elements in the sample matrix. Additionally, spectroscopy of low energy X-rays is complicated by X-ray interferences from other elements in the sample. WDS analysis of jarosite requires resolving N X-rays from high-order reflections of multiple Fe L-lines: Fe-Lα1 (n=2) at 0.354 keV, Fe-Lβ1 (n=2) at 0.359 keV, and Fe-Lβ3 (n=2) at 0.397 keV. While these high-order reflections are not an artifact in EDS, the lower spectral resolution of EDS makes it challenging to resolve N-Kα from other elements in jarosite, including interferences with O-Kα at 0.525 keV, C-Kα at 0.277 keV, and K-Ll at 0.260 keV. Figure 1 compares EDS spectra acquired from ammoniojarosite (NMNH 139680; California), jarosite (type locality; Barranco Jaroso, Spain), and boussingaultite ((NH4)2Mg(SO4)2(H2O)6; locality unknown). EDS spectra indicate elevated concentrations of N from ammoniojarosite and boussingaultite, which contain 2.9 and 7.8 % N by weight, respectively. These results show that it is possible for EDS to resolve differences in N content between the three phases, but the close proximity of N-Kα to the large O-Kα peak introduces error when using EDS to quantify low abundance N contents in sulfates. Figure 2 shows results of WDS wavescans across the N-Kα position acquired from ammoniojarosite (NMNH 139680; California) and jarosite (type locality; Barranco Jaroso, Spain) showing (a) the full energy range of the PC1 diffraction crystal and (b) a detailed scan around the N region. These results demonstrate the capability of WDS to resolve N-Kα X-rays emitted from ammoniojarosite and show that N contents of the analyzed jarosite sample are negligible. While it is not possible to fully separate N-Kα from interference with Fe-Lβ3, the low proportion of these X-ray lines results in low intensity interference at the N-Kα position. WDS can effectively measure N contents from jarosite-ammoniojarosite samples but requires careful consideration of X-ray interferences and background models for accurate quantification. X-ray spectra from ammoniojarosite, jarosite, and boussingaultite acquired with Energy Dispersive X-ray Spectroscopy (EDS). The spectra were acquired at 10 kV accelerating voltage with 4096 measurement channels and are plotted over the range 0.1-2.5 keV. X-ray spectra from ammoniojarosite and jarosite acquired with Wavelength Dispersive X-ray Spectrometry (WDS). (a) A total of 2000 measurements were acquired across the full energy range of the PC1 diffraction crystal (0.257-0.931 keV) to identify all peaks emitted from ammoniojarosite and jarosite samples. (b) A high-resolution scan of 2000 measurements acquired from a narrow energy range (0.341 – 0.425 keV) shows the shape and position of the N-Kα peak in ammoniojarosite and the location of the Fe-Lβ3 (n=2) interference.