Serpentinization is a long duration, expansive geologic process that is feasible wherever aqueous solutions interact with ultramafic-mafic protolith. In sheltered subsurface environments where groundwaters of variable chemistry or oceans beneath icy crusts interact with mantle-type rocks, this exothermic, energy yielding transformation of planetary materials has the potential to create habitable niches. Olivine and pyroxene minerals in parent rocks alter to serpentine-dominated assemblages, while ferrous iron is oxidized and hydrogen in molecular water is reduced to diatomic hydrogen, generating daughter materials generally more hydrated, oxidized, and diverse when compared to parent materials. Planetary history and sequential periods of distinctive weathering control the geochemical system, and, by extension, its bioenergetic habitability. Implications for elemental cycling, gas release, and support of a putative extraterrestrial chemosynthetic biosphere are considered.