Fayalite Formation in Some Primitive Chondrites

Details Fayalite Formation in Some Primitive Chondrites Fayalite Formation in Some Primitive Chondrites

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.367h&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 367

Computer Science

1

Equilibrium, Fayalite, Kaba, Mokoia Cv3 Carbonaceous Chondrite

Scientific paper

Fayalite (up to Fa(sub)99.9) occurs in the Kaba and Mokoia CV3 carbonaceous chondrites. The grains can reach up to 100 micrometers in diameter and are commonly associated with magnetite and sulfides. Since the fayalite is essentially pure, it must have formed in an environment where Mg is absent. We believe it formed by a two-step gas-solid reaction: (i) the decomposition of enstatite to release SiO (g); (ii) reaction of SiO (g) with magnetite or sulfides to form fayalite. Production of SiO (g) occurs via a reaction such as: 2 MgSiO3 (s) + H2 (g) = Mg2SiO4 (s) + SiO (g) + H2O (g) (1) for which the equilibrium constant is K = the equation which appears here in the hard copy. Using the JANAF tables, we can calculate Delta G for different temperatures, followed by a calculation of PSiO for various H2O/H2 ratios: equation appears here. For fayalite formation, we considered the following reactions: 2 Fe3O4 (s) + 3 SiO (g) + H2O (g) = 3 Fe2SiO4 (s) + H2 (g), and (2) 2 FeS (s) + SiO (g) +3 H2O = Fe2SiO4 (s) + H2(g) + 2 H2S (g) (3) Since sulfides will first react with H2O to form magnetite, reaction (3) will proceed via reaction (2), so our calculation is only for reaction (2). Following the same procedure as for reaction (1), we obtain: the equation, which appears here in the hard copy. After plotting the P(sub)SiO vs. temperature for different H2O/H2 ratios we found: (i) If the H2O/H2 ratio is 5.1 X 10^-4 (solar), fayalite starts to form at a temperature of 300 K, at which the vapor pressure of SiO is only 10^-86 atm. We presume under these conditions the reaction is kinetically prohibited. (ii) If H2O/H2 = 100, fayalite forms above 3000 K, which is also unlikely, because at such a high temperature all silicates will decompose. (iii) If H2O/H2 = 1, the formation temperature of fayalite is ~ 640 K, and the vapor pressure of SiO is 10^-36 atm. (iv) If H2O/H2 = 10, fayalite begins to form at 1100 K, and the vapor pressure of SiO is 10^-19 atm. After taking the reaction rates into consideration, we prefer the combination of H2O/H2 = 10 and starting temperature of 1100 K, because for these conditions P(sub)SiO is 17 orders of magnitude greater than that at ~640 K. If we put additional constraints on the temperature based on the coexistence of magnetite, then a maximum temperature of 1200 K for the reaction can also be determined [1] (Fig 4), because above this temperature magnetite is not stable when the H2O/H2 ratio equals 10. In an environment with H2O/H2 = 10 atm and T = 1100 or 1200 K, the vapor pressure of SiO released from the evaporation of enstatite is 10^-17 and 10^-14.8, respectively. The decomposition of forsterite under identical conditions will be negligible [2 log P(sub)Mgo + log P(sub)SiO = 10^-57.4 (at 1100 K) and 10^-50.3 (at 1200 K)]. References: [1] Larimer J. W. (1967) GCA, 1215-1238; JANAF Thermodunamical Tables (1971).

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