We present details of an experimental study of crystallization adjacent to a cooled
boundary from an aqueous solution of potassium nitrate and sodium nitrate. This
transparent system is typical of many ternary melts that do not form solid solutions,
including examples in igneous petrology and metallurgy. We have measured the rates
of advance of the front of crystallization and the eutectic front, behind which the
system is completely solid. From careful measurements of the concentration and
temperature fields, we have been able to infer the location of an internal phase
boundary: the cotectic front separating a region in which only one component of the
ternary system forms crystals from a region in which two components form crystals.
Our experiments were conducted under conditions in which fluid flow is minimal,
so that rates of crystallization are determined principally by the diffusive transport
of heat. We have confirmed that the thicknesses of the various regions all grow in
proportion to the square root of time, as is expected of diffusion-limited growth,
and have determined the constants of proportionality for a range of different initial
concentrations and boundary temperatures. We have found evidence to suggest that
there may be a significant nucleation delay in the secondary and tertiary crystallization.
Our measurements of concentration provide much more information about the ternary
phase diagram than has hitherto been available.
