Amorphous silica scale enriched in aluminum is deposited from a variety of geothermal brines. Scale deposits examined in the present study are formed by tetrahedrally-coordinated aluminum substitution within an amorphous silica framework. There is no evidence that aluminum in these scales derives from distinct aluminum minerals, such as gibbsite, or from aluminum silicate minerals transported in brine from the reservoir. The formation of aluminum-rich amorphous silica scale is dependent on brine pH, temperature and aluminum concentration. Silica and aluminum exhibit concentration minima in brines at near-neutral pH. Aluminum-rich silica scales disserve more slowly than pure amorphous silica in water and brine. Lowering brine pH below 5 or increasing it above 9 retards the kinetics of silica polymerization and the formation of aluminum-rich silica. Laboratory studies demonstrate that sequestering agents such as citric acid, acetic acid, and ethylenediaminetetraacetic acid (EDTA) may inhibit aluminum silicate scale formation.