Influence of time and temperature on reactions and transformations of muscovite mica

Abstract

Members of the mica group of clay minerals are important constituents of clays used as raw materials for the production of bricks, tiles, and clay pipes. Using muscovite, K₂ Al₄ (Si₆ Al₂ )O₂₀ (OH)₄ ), as the pure end member representative of the mica group, their thermal behaviour has been determined and is expressed as a time–temperature–transformation (TTT) diagram. Water loss occurs over a prolonged temperature range with no discrete event resulting in significant weight loss. The thermal decomposition of muscovite is dominated by the presence initially of the three sheet types which make up the layered mica structure. Initially γ-Al₂O₃ and then corundum form from the octahedrally co-ordinated aluminous sheet which is sandwiched by two tetrahedral sheets. With potassium from the interlayer site, the tetrahedral sheets break down to form a melt, from which leucite crystallises. Topotactic influences of this type account for the failure of muscovite to produce potassium feldspar and mullite as predicted by the equilibrium phase diagram for the system K₂O–Al₂O₃–SiO₂.