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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.).

 

ocean_1.jpg (389536 bytes) 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 samples
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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