Stability of micas on the surface of Venus
Planetary and Space Science, 47 (1-2):
245--260 (1999)Abstract
Recent thermodynamic modeling shows that some micas might be stable
on Venus' surface. However, prior studies considered only pure micas
and did not consider mica solid solutions, which are commonly observed
on Earth. Here we use chemical equilibrium calculations to evaluate
the stability of mica solid solutions on Venus' surface as a function
of atmospheric chemistry (H2O and HF abundances, and redox state),
and surface elevation. Our prior calculations show that the end-member
micas eastonite (KMg2Al3Si2O10(OH)(2)) and fluorphlogopite (KMg3AlSi3O10F2)
are stable on Venus' surface, while the end-member micas phlogopite
(KMg3AlSi3O10(OH)(2)), annite (KFe3AlSi3O10(OH)(2)), and siderophyllite
(KFe22+Al3Si2O10(OH)(2)) are unstable. Based on these results and
known petrologic phase relationships, we consider binary solutions
of eastonite with either phlogopite or siderophyllite, and fluorphlogopite
with phlogopite. We calculate that micas along all three binaries
are stable on Venus. Micas containing similar to 20 mole% eastonite
and similar to 80% phlogopite are stable in the lower temperature
highlands, and very eastonite-rich micas are stable over Venus' entire
surface. Fluorphlogopite-rich micas are also stable over Venus' surface,
while fluorphlogopite-poor micas are stable at higher elevations.
Iron-poor micas along the eastonite-siderophyllite join, containing
>80 mole% eastonite, are stable in both the highlands and lowlands.
Finally, we use the thermodynamic calculations, terrestrial geology,
and petrologic phase equilibria to discuss plausible geological settings
where micas may be present on Venus. These suggestions are important
for the design of geochemical experiments on future lander and automated
balloon missions to Venus. (C) 1999 Elsevier Science Ltd. All rights
reserved.