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Seeking phyllosilicates on Mars -- a simulation study
As a major mineral product of weathering and water-related alteration of
igneous rocks, phyllosilicates are anticipated to occur widely on Mars. There are few
reports in literature about Raman spectral features of phyllosilicates, however, mainly
for two rea-sons: (1) Complex structures and chemistry; (2) fine grained and organic
residues caused fluorescent interference.
In this set of simulation study, terrestrial ocean sediments were found to
provide high quality Raman spectra of clays with low interfering fluorescence because most
organic residues had been converted to graphitic carbon. Phyllosilicates on Mars may also
be free of organic materials, and perhaps organic residues (if there were any!) would have
been altered into graphitic carbon there, owing to the harsh environment (oxidizing
atmosphere, temperature cycling, dryness, etc.).
||a. Biotite, a T-O-T - A tri-octahedral phyllosilicate
|b. Glauconite, general formula K1-x(Al, Fe)2[Al1-x
Si3+xO10](OH)2, belongs to the muscovite subgroup, with Fe3+ replacing much of the Al3+ in
the octahedral sites of muscovite. This substitution causes a downshift of the major
Si-Ob-Si peak (to 700 cm-1), and the appearance of double peaks at 605 and 553 cm-1. A
peak at ~443 cm-1 suggests that part of Al3+ remains unsubstituted. The triplet in the OH
stretching vibrational region suggests at least three possible types of cation occupancies
around the OH site, e.g., Fe-Fe-Fe, Fe-Fe-Al, and Fe-Al-Al. Glauconite is a common
constituent of clastic sediment deposited under marine conditions
|c. An Fe-bearing T-O-T - A tri-octahedral type, neither chlorite nor
saponite. Fe-bearing hydrobiotite and vermiculite are possible candidates. These two
minerals can form by hydrothermal alteration of biotite, which was detected in these
|d. A T-O-T - A di-octahedral phyllosilicate with less Fe than glauconite.
The peak pattern in the OH stretching vibration region suggests more complicated cation
occupancies around the OH site than found in muscovite, and different from that in
glauconite. It is tentatively assigned to illite, a hydration product of muscovite.
Reference: Wang A., Valentine R. B., Seeking and Identifying
Phyllosilicates on Mars – A simulation study, Abstract #1370, 33th LPSc, 2002.
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