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Raman detection of fine-grained Carbonates and Sulfates

A fine-grained global soil, rich in sulfur, was found by the Viking and Pathfinder missions. Sulfur is presumably present as sulfate. Carbonate, like sulfate, is present as secondary phases in the SNC meteorites and might be present in the soil. The mean grain size of the wind-blown soil is estimated to be 1.6 Ám.

Two questions need to be answered:
1. Can we identify carbonate and sulfate minerals?
2.Can we identify the fine-grained components of rocks and soil?

Both carbonates and sulfates are strong Raman scatterers; spectra of some common varieties are shown in Figure 1. The many carbonate and sulfate minerals are readily identifiable from their spectra.

sulf_spec1.jpg (325072 bytes) sulf_spec2.jpg (395014 bytes)
Raman spectra of common carbonates Raman spectra of common sulfates

Definitive Raman spectra of sulfate, carbonate, and silicates were obtained from micro grains (individual or mass of micrograins):

sulf_spec3.jpg (379479 bytes) sulf_spec4.jpg (360051 bytes) sulf_spec5.jpg (217013 bytes)
Raman spectra from individual grains of d ~ 1 micrometer Raman spectra from mass of micro grains grain size distributions of micrograins

Results and Conclusions.

1. We obtain strong spectra from epsomite, calcite, quartz, and olivine crystals smaller than 1 micron using a 10 second integration time.

2. Peak broadening at the smallest grain size is less than 1 wavenumber, except perhaps for olivine.

3. No change in peak position was observed with decreasing grain size, down to <1 Ám.

4. We acquired good spectra from bulk "duricrust" samples; both quartz and epsomite are present in all 10 second spectra.

5. The Mars Microbeam Raman Spectrometer should be able to identify 1 Ám sulfate and carbonate minerals present in Martian soils and rocks.

References:  Kuebler, K.E., Wang A., Abbott K., Haskin L. A., Can we detect carbonate and sulfate minerals on the surface of Mars by Raman spectroscopy? 32th LPSc, 2001.

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