We have investigated the thermal expansion of nepheline-kalsilite crystalline solutions having 12.5% excess silicon relative to the stoichiometric composition. It is proposed that differences in the thermal expansion among various members of the series, and also between this series and a previously studied low-Si series, can be explained by three factors: (1) shrinkage of the tetrahedral rings caused by the substitution of Na for K, especially in the case of kalsilite, allows for greater expansion from the increased vibrational amplitude of Na ions with increasing temperature. (2) The occupancy of alkali sites by ions rather than vacancies draws tetrahedral rings inward via electrical attraction to these ions, providing the potential for greater expansion with ionic vibration as temperature increases. (3) Structural differences between nepheline and kalsilite, in particular the existence of two alkali sites in nepheline, account for the increased thermal expansion of K-enriched nepheline relative to Nanepheline, as the occupation of the larger alkali position by K+ results in greater thermal expansion than is the case with Na+ occupancy of the same sites.
Lafayette Coll, Dept Geol & Environm Geosci, Easton, PA 18042 USA; IPGP, Phys Mineraux Magmas, F-75252 Paris 05, FranceArticleEnglish