Serpentinite at Cap Corse. This rock forms at oceanic ridges where magma is altered by hydrothermal fluids. This alteration, which takes the form of veinlets on the rock, corresponds to the serpentinization of mantle peridotites by hydration (transformation of the minerals olivine and pyroxene into serpentine by reaction with these fluids). Serpentinites are particularly common in ophiolites, as seen here on Cap Corse.