An Experimental Study of Nepheline-Kalsilite Exsolution

The nepheline-kalsilite exsolution reaction was studied isothermallybetween 400 and 700°C. Under nonaqueous conditions the mechanisminvolves nucleation of kalsilite and growth by diffusion ofthe alkalis. As predicted by simple nucleation theory, the nucleationrate and hence the over-all exsolution rate are strongly dependenton the supersaturation of the nepheline. A decrease in temperatureat constant composition increases the supersaturation and therebythe nucleation rate. This increased nucleation rate is opposedby the decrease in the growth rate due to slower volume diffusion.At a supersaturation of more than 8–10 mole per cent thenumber of nuclei is large and the over-all exsolution rate isdetermined primarily by the growth rate. The activation energyfor growth is 28 kcal/mole. An increase of two kilobars in thehydrostatic pressure has little effect on the kinetics of thereaction. Under nonhydrostatic conditions the exsolution rateincreases significantly because the nucleation rate is faster. Under hydrothermal conditions the ‘exsolution’ rateis approximately two orders of magnitude faster due to a modificationin the mechanism. Partial dissolution of the original solidsolution in distilled water creates a condition of nonequilibriumin which the fluid is sodium-rich. Rapid alkali exchange eliminatesthis condition but produces the equilibrium compositions ofthe solids because kalsilite nucleates and grows in contactwith the fluid. The experimental evidence for this mechanismincludes X-ray diffraction data showing a gradual change inthe composition of the initial supersaturated solid, essentiallyidentical activation energies for growth under aqueous and nonaqueousconditions, and a lower percentage of oxygen isotope exchangethan ‘exsolution’ in the same experiment.