Abstract
We present revised tremolite powder thermal decomposition kinetics
using previous and newly acquired data from longer time (years instead
of months) and lower temperature experiments (< 1073 K). We also
present kinetic results for decomposition of millimeter- to centimeter-sized
tremolite grains. Natural tremolite samples were heated at ambient
pressure in flowing CO2 or N-2 gas from 1023-1238 K. The tremolite
decomposition products are a physical mixture of two pyroxene solid
solutions (with the bulk composition Dp(59)En(41)), a silica polymorph,
and water vapor. Decomposition rates were calculated by using the
mass loss of the heated samples. Tremolite crystals and crystalline
powder decompositions follow different but related Avrami-Erofe'ev
(nucleation and growth) kinetic models. The rate equations for thermal
decomposition of tremolite crystalline powder and the larger crystal
grains are log(10)k(powder) (h(-1)) = 18.69(+/-0.19) 23.845(+/-833)/T
and log(10)k(crystal) (h(-1)) = 19.82(+/-0.07) - 25,670(+/-916)/T.
The associated apparent activation energies are 456(+/-16) kJ mol(-1)
and 491(+/-18) kJ mol(-1), respectively. We propose a decomposition
mechanism and suggest that decomposition and dehydroxylation occur
simultaneously. The rate-limiting step is proposed to be structural
rearrangement of the amphibole structure to the two pyroxenes and
silica. This step and the overall decomposition rate are predicted
to be independent of pressure from I to 100 bars. These kinetic analyses
strengthen our previous conclusion (Johnson and Fegley, 2000, Icarus
146, 301-306) that if hydrous minerals, such as tremolite. formed
on Venus during a wetter past, then these minerals could still exist
at current conditions on Venus' surface today. (C) 2003 Elsevier
Inc. All rights reserved.
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