The Serpentinite Multisystem Revisited: Chrysotile Is Metastable

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 Fe2+/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. For small bodies of serpentinite, sheared serpentinite, and systems of large water/rock ratio, volume expansion may be much less. Chrysotile is most conspicuously developed in tectonically active environments, where associated lithotypes show marginal greenschist-facies parageneses and antigorite tends to make its first appearance. Chrysotile growth is favored in isotropic stress microenvironments of fluid-filled voids and pores (where it may ultimately crystallize pervasively), and in veins, generally after active hydration in the immediate surroundings has ceased. This nevertheless allows the simultaneous growth of lizardite and chrysotile in adjacent partially and fully serpentinized peridotite, respectively, as in the cores and rims of kernel structures. Although prominent along shear surfaces, chrysotile growth as slip fiber is promoted by the presence of fluid rather than shear stress. Unlike lizardite, whose growth produces the stress associated with expansion, extreme flattening and shear might be expected to destroy the chrysotile structure. Thus, lizardite and chrysotile behave as though they were a stress-antistress mineral pair. Calorimetric, solubility, and reaction-reversal experiments on chrysotile integrate contributions to its free energy arising from surface properties and, most importantly, from its radius-dependent strain energy. Minimally strained chrysotile (r ≈ 90 Å) may in fact be more stable than lizardite, whereas a maximal-radius chrysotile (r ≈ 200 Å) is not. A model for chrysotile in veins cutting lizardite- or antigorite-bearing rock involves nucleation of low-strain chrysotile followed by kinetically favored crystallization of higher-energy layers driven by mild fluid supersaturation maintained by local potential gradients. It is not clear if this explanation adequately accounts for serrate veins and mass-fiber chrysotile. A revised phase diagram for lizardite and antigorite is offered, and possible stable and metastable reactions among the phases in serpentinites are followed on an isobaric diagram of reaction free energy (driving force) as a function of temperature. Composition-induced equilibrium shifts are believed unlikely to be determinative in most occurrences of Mg-rich lizardite and chrysotile. Circumstances of growth rather than temperature and pressure determine the occurrence of chrysotile vis-à-vis lizardite in serpentinites.