The application of ionic equilibria to metamorphic differentiation: An example

Mineral segregations formed by metamorphic differentiation are an important source of information on diffusion processes in metamorphism. Segregations consisting of andalusite-biotite-quartz cores surrounded by a quartz-feldspar mantle in sillimanite-biotitefeldspar-quartz gneiss near Västervik, Sweden (Loberg, 1963) formed by core-to-mantle migration of K, and mantle-to-core migration of Fe, Mg and Ca. These migrations can be represented by a set of interconnected ionic equilibria involving reaction of microcline and Fe(OH)⁺ in the core to form andalusite plus biotite, and reaction of K⁺, sillimanite and biotite in the mantle to form microcline. Equilibrium constants for these reactions, calculated for conditions inferred from the mineral assemblage and biotite composition, indicate gradients of K⁺ activity (higher in core) and Fe(OH)⁺ activity (higher in mantle). These gradients result simply from the free energy difference between andalusite and sillimanite, without invoking pre-existing megascopic inhomogeneities in the rock or surface energy effects. Although small, these gradients appear to be capable of driving the segregation process.