Abstract Hydrothermal alteration of basalt on the northern Gorda Ridge produced an alteration crust up to 1mm thick that can be divided into two layers based on mineralogy. An inner layer next to the basalt surface is an Al-rich interlayered smectite/chlorite similar to high temperature Al-rich clays recovered from the East Pacific Rise, 21°N with minor quartz, zeolite (phillipsite ?) and relict plagioclase. An exterior layer of boehmite, AlO(OH) and minor anatase results from severe leaching of the underlying clays. The alteration crust is progressively enriched in Al, Ti, V, and Cr from interior to exterior by residual weathering. The presence of boehmite and absence of a silica-rich phase in the exterior of the crust indicates the hydrothermal fluids were silica undersaturated or silica was maintained in solution by extremely acidic fluids. This constitutes the first reporting of boehmite as a major alteration mineral in a mid-ocean ridge hydrothermal environment.