THE THERMAL EXPANSION OF ULTRA-SILICIC NEPHELINE – KALSILITE CRYSTALLINE SOLUTIONS

We have investigated the thermal expansion of nepheline - kalsilite crystalline solutions that contain 12.5 % excess Si relative to Al:Si=1:1 stoichiometry. [Results of a solution calorimetric investigation on the same specimens, with sample-preparation descriptions, are found in Hovis & Roux, American Journal of Science, 1999.] High-temperature X-ray powder diffraction measurements made with an Inel 4K-PSD position-sensitive detector at the Department of Earth Sciences, Cambridge University, UK, from room temperature to 1100 °C reveal that nepheline specimens of this ultra-silicic series expand at a high, but similar, rate regardless of potassium content. These ultra-silicic nepheline samples expand less, however, than previously studied lower-Si (1.7 % excess Si) analogs (Hovis, Crelling, Wattles, Driebelbis, Dennison, Keohane, & Brennan, Mineralogical Magazine, 2003). At the potassic end of this Si-rich series both pure-K and Na-bearing kalsilite samples display coefficients of thermal expansion that are lower than those of the nepheline specimens. Similar systematics were shown by lower-Si samples (ibid.). Interestingly, though, elevated excess Si decreases the coefficient of thermal expansion for nepheline while increasing the same for kalsilite. When plotted against molar excess Si (or mole fraction vacancies in the alkali site), the coefficients of thermal expansion for nepheline and kalsilite converge toward one another as Si increases, apparently approaching the thermal expansion coefficient for hexagonal tridymite (derived from the high-temperature X-ray data of de Dombal & Carpenter, European Journal of Mineralogy, 1993). The contrasting expansion behavior of nepheline and kalsilite relative to excess Si must be related to differences in the details of their structures, in particular alkali-site configuration and behavior, as well as the structural location of the excess Si and vacancies.

This and previous thermal expansion research have formed the bases for several undergraduate research projects at Lafayette College. We greatly appreciate support from the National Science Foundation (grant EAR-0000523) and the generous cooperation of the Department of Earth Sciences at Cambridge University.