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Thermal expansion of highly silicic nepheline-kalsilite crystalline solutions

Published online by Cambridge University Press:  05 July 2018

G. L. Hovis*
Affiliation:
Department of Geology &, Environmental Geosciences, Lafayette College, Easton, PA 18042, USA
E. Person
Affiliation:
Department of Geology &, Environmental Geosciences, Lafayette College, Easton, PA 18042, USA
A. Spooner
Affiliation:
Department of Geology &, Environmental Geosciences, Lafayette College, Easton, PA 18042, USA
J. Roux
Affiliation:
Physique des Minéraux et des Magmas, IPGP, 4, place Jussieu, 75252 Paris cedex 05, France
*

Abstract

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 Na-nepheline, 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.

Type
Editorial
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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