THERMAL EXPANSION OF NEPHELINE - KALSILITE CRYSTALLINE SOLUTIONS

We have studied the thermal expansion from room temperature to 1100 °C of eleven members of a nepheline - kalsilite solid solution series that had been synthesized previously for solution calorimetric research ("synthetic" series of Hovis & Roux, American Journal of Science, 1993). The four most sodium-rich nephelines expand at an equivalent rate with respect to temperature, regardless of K:Na ratio. Three nepheline specimens having higher potassium contents, in which the smaller of the two alkali crystallographic sites is in part occupied by potassium, expand at a slightly greater rate than the more sodic members of the series, as does the tetrakalsilite member of the series. [However, rapid in-situ exsolution in several of the latter prevented the collection of valid data above 500 °C.] At the potassic end of the series pure kalsilite has the lowest expansion rate of any series member, as the introduction of sodium systematically increases the rate of expansion. For pure kalsilite the c unit-cell dimension steadily decreases with temperature, whereas for sodium-bearing kalsilite, c first increases then eventually decreases with temperature. Thus, although different kalsilite members have systematically distinct values of c at room temperature, all members converge to a common value of c at high temperature. Although they are fellow framework silicates, then, nepheline - kalsilite solid solutions behave differently than alkali feldspars, in which thermal expansion systematically decreases with increasing potassium content (Hovis, Brennan, Keohane, & Crelling, The Canadian Mineralogist, 1999). This difference in behavior is related to the structures of nepheline - kalsilite minerals, which are stuffed derivatives of tridymite in which large channel cations play a different role than is the case for alkali feldspars.

These measurements were made using an Inel 4K-PSD position-sensitive detector at the Department of Earth Sciences of Cambridge University, UK. The projects served as a series of undergraduate research experiences for the various coauthors of this abstract. We give our sincere thanks to Ekhard Salje, Michael Carpenter, Simon Redfern, Tony Abraham, Ann Graeme-Barber and Margaret Johnson for use of and help with the Cambridge X-ray facilities.