Abstract
We report the results of a detailed experimental study of the kinetics
and mechanism of pyrite (FeS2) chemical weathering under Venus surface
conditions. Pyrite is thermodynamically unstable on the surface of
Venus and will spontaneously decompose to pyrrhotite (Fe7S8) because
the observed S-2 partial pressure in the lower atmosphere of Venus
is lower than the S-2 vapor pressure over coexisting pyrite and pyrrhotite.
Pyrite decomposition kinetics were studied in pure CO2 and CO2 gas
mixtures (CO-CO2, Ar-CO2, H-2-CO2, CO-CO2-SO2) along five isotherms
in the temperature range 390-531 degrees C. In all gas mixtures studied,
pyrite thermally decomposes to pyrrhotite (Fe7S8), which on continued
heating loses sulfur to form more Fe-rich pyrrhotites. During this
process the pyrrhotites are also being oxidized to form magnetite
(Fe3O4), which converts to maghemite (gamma-Fe2O3), and then to hematite
(alpha-Fe2O3). This reaction sequence is supported by X-ray diffraction
data, Mossbauer spectra, optical microscopy, and prior literature
studies. The reaction rates for pyrite thermal decomposition to pyrrhotite
were determined by measuring the weight loss. The thickness of the
unreacted pyrite in the samples provided a second independent reaction
fate measurement. Finally, Mossbauer spectra done on 42 of the 115
experimental samples provided a third set of independent reaction
rate data. All three independent methods give the same reaction rate
within experimental uncertainties. Pyrite decomposition follows zero-order
kinetics and is independent of the amount of pyrite present. The
rate of pyrite decomposition is apparently independent of the gas
compositions used and of the CO2 number density over a range of a
factor of 40. The derived activation energy of similar to 150 kJ
mole(-1) is the same in pure CO2, two different CO-CO2 mixtures,
and a ternary CO-SO2-CO2 mixture. Based on data for a CO-CO2-SO2
gas mixture with a CO number density similar to 10 times higher than
at the surface of Venus and a SO2 number density approximately equal
to that at the surface of Venus, the rate of pyrite destruction on
the surface of Venus varies from about 1225 +/- 238 days/cm at the
top of Maxwell Montes (similar to 660 K) to about 233 +/- 133 days/cm
in the plains of Venus (similar to 740 K). These lifetimes are very
short on a geological time scale and show that pyrite cannot exist
on the surface of Venus for any appreciable length of time. (C) 1995
Academic Press, Inc.
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