Abstract
Isothermal nucleation experiments using liquid beads suspended in 5 mm-diameter platinum-wire loops were carried out on several compositions near the thermal minima in the system diopside–anorthite–forsterite. Homogeneous nucleation was never observed, at temperatures from 400° C to the liquidi. Crystals of diopside, anorthite and forsterite all nucleate predominantly on the platinum wire. Nucleation on the surface of the bead occurs less often and is apparently promoted by mechanical stress. Similar isothermal experiments on a natural diabase also did not show homogenous nucleation, nor did dynamic crystallization experiments in nickel and alumina crucibles. Homogeneous nucleation probably never occurs in the laboratory or in nature in basaltic compositions. The principal question concerning natural basaltic rocks is whether nucleation in each case was internal, on suspended crystals, or external, on wall rocks or other surfaces.
