『Abstract
The detection of phyllosilicates and sulfates on Mars has revealed
a complex aqueous history which suggests distinct geochemical
environments separated temporally and spatially. Recent observations
by MRO CRISM in Mawrth Vallis have shown that phyllosilicate deposits
exhibit a specific stratigraphy, which remains incompletely understood.
Moreover, MER Spirit has evidenced association between phyllosilicates,
amorphous silica and sulfates. We investigated the hypothesis
that these parageneses resulted from the acidic weathering of
older phyllosilicate deposits. We exposed nontronite (Fe-rich
smectite), montmorillonite (Al-rich smectite) and kaolinite to
H2SO4 solutions at pH
0, 2 and 4, and at a temperature of 60℃. After the acid treatment,
a combination of mineralogical techniques was used to assess the
degree of alteration of the three phyllosilicate minerals. XRF,
XRD and ESEM measurements show that nontronite was the most unstable
when acid leached, followed by montmorillonite and then kaolinite.
Progressive acidic leaching of nontronite leads to alteration
of the phyllosilicate to amorphous silica, along with Fe-sulfate
and anatase, and the formation of an acidic Al,Fe-rich solution.
Alteration of montmorillonite resulted in the formation of Fe-,
Al-, Ca- and Mg-sulfates, and a Al-rich leaching solution. Comparatively,
leaching of kaolinite resulted in the formation of Al-sulfates
and a Al-rich solution as well, with only slight alteration of
the primary mineralogical features. The effects of acid leaching
of the phyllosilicates were also observed in NIR reflectance spectra,
allowing a comparison with CRISM spectra from Mawrth Vallis. Based
on our results, we propose a new model where acid leaching of
mixed phyllosilicate deposits leads to kaolinite overlaying montmorillonite,
which in turn caps Fe,Mg-smectites. Leaching of cations and subsequent
evaporation leads to sulfate deposits, as supported by geochemical
modeling, while amorphous silica remains as a residue. Depending
on the intensity (pH) and length of exposure of acidic leaching,
our model can explain the stratigraphic distribution of phyllosilicates,
and the association of sulfates, silica and smectites.』
1. Introduction
2. Methods and materials
2.1. Experimental setup
2.2. Solid-phase analysis
2.3. Liquid analysis
3. Results
3.1. X-ray fluorescence (XRF)
3.2. X-ray diffraction
3.3. Environmental Scanning Electron Microscopy (ESEM and EDAX)
3.4. FT-IR measurements
3.5. Liquid analysis
4. Discussion
4.1. Chemical stability and alteration of phyllosilicates
4.2. Secondary mineralogy
4.3. Geochemical modeling
4.3.1. Nontronite
4.3.2. Montmorillonite
4.3.3. Kaolinite
4.3.4. Comparing model results to experiment results
4.4. Implications for Martian surface deposits
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References