Development and evolution of water vapor vesicles during fast thermal breakdown of muscovite

At 1,175°C-1 bar, muscovite in natural granite powders is completely transformed after 5 min. Due to the overlapping of several processes such as dehydroxylation, mineralogical transformations, collapse and sintering of transformed lamellae, a few parts per thousand of H2O vapor are trapped, generating bubbles in muscovite pseudomorphs. For short durations (5–40 min), the crystallographic properties of the precursor muscovite control the geometry of the bubbles that may be compared to thick disks with a rounded shape in basal sections and an elongated shape in lateral section, parallel to the former cleavage planes of muscovite. With longer durations the bubbles change from thick disks to spheres, which can be explained by the release of constraints perpendicular to basal planes upon growth of the high temperature Si–Al oxides. With time, the number of bubbles decreases while the bubble size and the porosity of the pseudomorphs increase. Bubble behavior was analyzed in terms of ripening, shape transformations and coalescence.