Paper No. 132-0
KOZIOL, Andrea M., Geology, Univ of Dayton, 300 College Park, Dayton, OH 45469-2364,

Siderite (sid; FeCO3)-magnesite (magn; MgCO3) carbonates occur in a variety of parageneses, including the unique Ca-Fe-Mg carbonate globules seen in Martian meteorite ALH84001. The decarbonation of this solid solution depends on temperature, oxygen fugacity, and total pressure, although details are unknown. Carbonates with compositions sid100, sid80magn20, sid60magn40, sid20magn80 and sid10magn90 (Vanderbeek et al. (1999) GSA Abstr. w/Prog. 31, p. A162) were loaded into individual noble metal capsules and subjected to temperatures of 470 degrees C for 5 to 15 minutes. One set of experiments was performed at ambient atmospheric compositions. In a second set the individual capsules were surrounded by powdered graphite to simulate conditions at the graphite-carbon monoxide (CO) buffer.

Each sample was examined optically and by X-ray diffraction analysis. Sid100 extensively decarbonated to form magnetite (Fe3O4). For the other compositions, only a tiny amount of decarbonation to magnetite occurred. Neither wüstite nor hematite was observed (with one possible exception) so the decarbonation reaction is inferred to be:

3 FeCO3 + 1/2 O2=Fe3O4 + CO2.

Carbonates heated under reduced oxygen fugacity conditions (i.e. at graphite-CO) all exhibited contraction of the unit cell, in some cases up to 1%. Magnetite formed during these experiments also exhibited contraction of its unit cell, as evidenced by the change in d-spacing of the mt 311 X-ray reflection. This depended on initial carbonate composition. Magnetite formed from sid10magn90 had a d-spacing of 2.5010 Angstroms, whereas magnetite formed from sid80magn20 had a d-spacing of 2.5277 Angstroms. (Compare to 2.5320 Angstroms from PDF card #19-0629.)

Variation in ferric iron content or Mg content may explain these patterns. Preliminary compositional data indicate that magnetite may have a magnesioferrite (MgFe2O4) component. Initial TEM imaging shows submicron euhedral magnetite grains that are 2 orders of magnitude larger than those seen in ALH84001. Further chemical analyses are in progress to detect changes in composition of carbonate and magnetite.

This research supported by NASA grant # NAG5-9808.

GSA Annual Meeting, November 5-8, 2001
General Information for this Meeting
Session No. 132
Planetary Geology (Posters)
Hynes Convention Center: Hall D
1:30 PM-5:30 PM, Wednesday, November 7, 2001

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