Variations in the geochemistry and texture of amphibole phenocrysts erupted from Augustine Volcano in 2006 provide new insights into preand syn-eruptive magma storage and mixing. Amphiboles are rare but present in all magma compositions (lowto high-silica andesites) from the 3 month long eruption. Unzoned magnesiohornblende in the highand low-silica andesites exhibit limited compositional variability, relatively high SiO2 (up to 49·7 wt %), and relatively lowAl2O3 (511·1wt %). Intermediate-silica andesites and quenched mafic enclaves contain amphiboles that vary in composition (e.g. SiO2 40·8^48·9 wt %, Al2O3 6·52^15·2 wt %) and classification (magnesiohornblende^magnesiohastingsite^ tschermakite). Compositional variation in amphibole is primarily controlled by temperature-dependent substitutions. Both highand low-silica andesites represent remnant magmas that were stored in the shallow crust at 4^8 km depth, remaining distinct owing to a complex subsurface plumbing system. Intermediate-silica andesites and quenched mafic inclusions represent pre-eruptive hybrids of resident highand low-silica andesite magmas and an intruding basalt. Amphiboles in explosive phase high-silica andesites are largely euhedral and unreacted, consistent with the high magma flux rates from depth during this phase (up to 13 800 m s). Phenocrysts from the other lithologies have reaction rims that range from 1 to41000 m in thickness. Reaction rim microlite sizes correlate with reaction rim thicknesses. Reaction rims550 m thick contain microlites 1^10 m in length whereas reaction rims480 m thick contain microlites 10^100 m in length. Differentiating between heatingand decompression-induced amphibole reaction rim formation is problematic because of a lack of experimental constraints.We attempt a new approach to assessing reaction rim formation, based on a kinetic theory of crystal nucleation and growth, in which the differences in reaction rim textures represent different degrees of amphibole disequilibrium. Large crystals and low number densities suggest relatively lower levels of disequilibrium resulting in growth-dominated crystallization. Smaller crystals and larger number densities are indicative of higher nucleation rates and a high driving force.