THE SERPENTINITE MULTISYSTEM REVISITED: CHRYSOTILE IS METASTABLE

Abstract

The two rock-forming polymorphs of serpentine Mg3Si2O5(OH)4, lizardite and chrysotile, occur in nature in virtually identical ranges of temperature and pressure, from surficial or near-surficial environments to temperatures perhaps as high as 400°C. Laboratory evidence indicates that lizardite is the more stable at low temperatures, but the difference in their Gibbs free energies is not more than about 2 kJ in the 300-400°C range. Above about 300°C, antigorite + brucite is more stable than both; in other words, chrysotile is nowhere the most stable.The crystal structures of lizardite and chrysotile give rise to contrasting crystallization behaviors and hence modes of occurrence. The hydration of peridotite at low temperature results in the growth of lizardite from olivine, and (commonly topotactically) from chain and sheet silicates, although the MgO-SiO2-H2O (MSH) phase diagram predicts antigorite + talc in bastite. The activity of H2O during serpentinization may be buffered to low values by the solids, making the reaction of olivine to lizardite + brucite a stable one. Conservation of oxygen and inheritance of the Fe/Mg exchange potential of olivine lead predictably to the precipitation of a highly magnesian lizardite and magnetite, and to the evolution of H2. Volume expansion is made possible by lizardite's force of crystallization, and it is tentatively suggested that this might account for the a-serpentine orientation (length normal to (001)) of lizardite pseudofibers in mesh rims and hourglass pseudomorphs after olivine. Whereas mineral replacements commonly conserve volume, in massif serpentinites the diffusive loss of Mg and Si needed for volume conservation during serpentinization requires chemical potential gradients that are unlikely to exist.