Abstract
Phase equilibrium considerations derived from studies of progressively metamorphosed serpentinites combined with a theoretical analysis of the system MgO–SiO2-H2O by Evans et al. (1976) indicate that antigorite is the stable phase in olivine–serpentine reactions. Corresponding serpentine–olivine equilibria in which chrysotile is the serpentine phase are metastable relative to the antigorite analogs. I suggest that this conclusion can also be applied to reactions involving lizardite and olivine.
Metastable serpentine–olivine equilibria are inferred to be the result of combinations of crystallographic, energetic, and kinetic factors that vary for different reactions. The contrasting crystal structures of the serpentine minerals probably play an important role in determining reaction paths where kinetics are unfavorable to the attainment of stable equilibrium; e.g., low temperature hydration or static dehydration. The role of crystal structure is manifested in the relative ease of nucleation of the various phases. Antigorite nucleation is hampered by its complex structure whereas lizardite–olivine reactions are favored by a topotactic relationship. Chrysotile is favored by crystallization from gels and oxides in synthetic systems and in dilation veins in serpentinites.
