Natroalunite containing substantial amounts of Fe occurs as a prominent secondary phase during acid-sulfate alteration of pyroclastic basalts in volcanic fumaroles in Nicaragua and elsewhere, and has been observed in laboratory simulations of acid-sulfate alteration as well. Reaction path models constrained by field and experimental observations predict that Fe-rich natroalunite should also form as a major secondary phase during alteration of martian basalt under similar circumstances. Here, we evaluate the potential to use spectroscopic methods to identify minerals from the alunite group with chemical compositions intermediate between natroalunite and natrojarosite on the surface of Mars, and to remotely infer their Fe contents. X-ray diffraction and spectroscopic measurements (Raman, visible/near infrared, mid-infrared, Mössbauer) were obtained for a suite of synthetic solid solutions with a range of Fe contents ranging from natroalunite to natrojarosite. In the visible/near infrared, minerals with intermediate compositions display several spectral features not evident in end-member spectra that could be used to remotely identify these minerals and infer their composition. In addition, Raman spectra, mid-infrared spectra, and X-ray diffraction peaks all show systematic variation with changing Fe content, indicating that these methods could potentially be used to infer mineral compositions as well. The results suggest that alunite group minerals with intermediate Fe compositions may be able to account for some visible/near-infrared and Mössbauer spectral features from Mars that had previously been unidentified or attributed to other phases. Overall, our findings indicate that consideration of solid solutions may lead to new identifications of alunite group minerals on the surface of Mars, and raise the possibility that minerals with compositions intermediate between natroalunite and natrojarosite may be widely distributed on the planet.
Bibliographical noteFunding Information:
This research was supported by funds from NASAMars Fundamental Research Program grants NNX12AI02G (T.M.M.) and NNX10AM89G (A.W.), NASA Exobiology Award NNX08AQ11G (T.M.M. and B.M.H.), and NASAEarly Career Award NNX12AF20G (B.M.H.). Thanks to George Rossman for assistance and use of his FTIR spectrometer and to Frieder Klein for help with the Raman analysis of natural samples. The authors are grateful for reviews by Gregg Swayze and Stuart Mills, whose thoughtful comments helped to improve the manuscript. The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities Program, Division of Earth Science, National Science Foundation.
- Acidsulfate alteration
- Alunite group
- Raman spectra
- Visible/near infrared spectra