CARBONATE GLOBULES

Structured carbonates:

• Typical size, ~ 50 microns in diameter.
• Larger globules (> 10 microns) have
  • Ca-rich cores (which also contain the highest Mn abundance)
  • alternating outer layers of Fe- and Mg-rich bands (Figure 3 of McKay et al.).
  • Near the edge of the globule, see band rich in Fe and S, and a second one rich in Mg with no Fe, and a third one rich in Fe and S.
    • Fe-rich rims contain magnetite (iron oxide) of size 10 to 100 nm and smaller amounts of pyrrhotite (iron sulfide) (Figure 4a and Figure 4b of McKay et al.). The magnetite can be cuboidal, tear-dropped, or irregularly shaped
    • Teardrop-shaped crystals of Magnetite (an iron oxide) and pyrrhotite (an iron sulfide) are found in regions of carbonate where acid seems to have pitted the surface (Figure 5 from McKay et al.). Magnetite and iron sulfide embedded in places where the carbonate is dissolved is hard to do, but is known to happen in biogenic systems.

      The odd-shapes are not common in inorganic processes, but are common for a biogenic origin. These are reminiscent of magneto-fossils from the bacteria which are sensitive to north and south.*

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  *However, the B-field of the Earth is ~ 0.3 Gauss. Mars has no measurable B-field ---> huh?