Nucleation data in a theoretical context

Feldspar nucleation rate data obtained by laboratory decompression of hydrous silicate melt may be interpreted in view of the classical theory of nucleation (CNT) and a non-classical variation, the diffuse interface theory (DIT). The nucleation rate data can be modeled by the CNT formalism only if the interfacial free energy (s) is allowed to vary as a function of composition. The values thus obtained vary by a factor of four (0.024- 0.098 J m^-2) and decrease systematically over a six-fold increase in dissolved H₂O content (0.8- 4.8 wt. %).

The DIT states that the interfacial region between the bulk solid (at the core of subcritical clusters) and bulk melt has thermodynamic properties intermediate between these phases, and that the interfacial free energy s may be defined as the difference between the interfacial enthalpy and interfacial entropy. If the DIT model is correct, the nucleation rate data for feldspar suggest (1) that dissolved H₂O content controls the spatial distribution of enthalpy and configurational entropy around incipient crystals, and (2) the spatial gradients of these potentials diverge during devolatilization.

Bagdassarov et al., [2000] determined values of the vapor-liquid surface free energy in hydrated haplogranite liquid by a sessile drop technique that are identical within error to values obtained by back-solving the CNT equation with bubble nucleation experimental rate data. Close agreement of independent techniques for obtaining surface energy supports the utility of nucleation experiments for continued examination of liquid-crystal surface free energy phenomena. Mangan and Sisson [2000] report a negative linear correlation of s_vapor-liquid with H₂O concentration C_H2O, similar to what we have found between s_liquid-solid and C_H2O.

Hammer, J.E., Crystal nucleation theory applied to hydrous magma, American Mineralogist, in review.

Hammer, JE (2003) Crystal nucleation theory applied to hydrous magma. Fall AGU.