Highly altered, glassy tephras within the active steam vents at Kilauea Volcano, Hawaii, contain subsurface bacteria characterized by small (<500 nm in diameter), epicellular grains attached directly to the cell walls. Compositionally, the grains were dominated by Si, Al, Fe, and K, in a stoichiometry similar to a dioctahedral smectite. The initial dissolution of glass, which may in part have been microbiologically mediated, served as the source for many of the elements sequestered into the biomineralized clays. Overlying the tephras are white crusts (silica and calcite) and green-colored biofilms. The biofilms comprise a filamentous, likely cyanobacterial, community coated with spherical (<100 nm in diameter) grains of amorphous silica directly attached to the sheaths. Individual precipitates can easily be resolved, but quite often they coalesce, forming a dense mineral matrix of amorphous silica. For both the clays and silica, the microbial surfaces are clearly sites for mineral nucleation and growth. These observations imply that microbial biomineralization may be a significant process in the overall alteration of primary basaltic glass in active steam vent systems.