Tremolite decomposition on Venus II. Products, kinetics, and mechanism

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 CO 2 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 59En 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 1 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.