Investigation of phase relations in the (Na,K)AlGeO4 system |
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| Authors: | J Barbier M E Fleet |
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| Institution: | 1. Department of Chemistry, McMaster University, L8S 4M1, Hamilton, Ontario, Canada
2. Department of Geology, University of Western Ontario, N6A 5B7, London, Ontario, Canada
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| Abstract: | The phase relations in the (Na,K)AlGeO4 system have been investigated at atmospheric pressure over the temperature range 700–1100° C by X-ray powder diffraction and electron diffraction/microscopy. Four distinct structure-types occur in this system including the beryllonite, nepheline, kalsilite and KAlGeO4 types in order of increasing KAlGeO4 content. In contrast to the (Na,K)Al-SiO4 system, the nepheline structure is only stable over a narrow composition range around 25 percent K at temperatures above approximately 800° C and transforms reconstructively into the beryllonite structure at lower temperatures. The formation of domain microstructures in some K-rich phases has been directly observed by high-resolution electron microscopy and can be associated with the presence of diffuse scattering in their diffraction patterns. The structural trend observed across the (Na,K)AlGeO4 series as a function of composition can be rationalized to a large extent in terms of the dependence of the framework topology of these tridymite-derivative structures on the size of the alkali atoms. |
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