A SIDERITE-MAGNESITE DECARBONATION STUDY

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.