Abstract
Feldspar chemical variations in representative leucite-bearing and related rocks from well-known localities in Italy, Germany, Uganda and Australia demonstrate that “phenocrystal” core to rim variations may not represent the feldspar crystallization trend in the host lava and only the groundmass feldspar zoning trend is a reliable indicator of crystal-liquid relationships. Textural relationships indicate that coexisting plagioclase and alkali feldspar crystallized sequentially, the latter after the former, rather than cotectically.
Groundmass alkali feldspar show Ca-, Na-depletion and K-enrichment zoning trends. Plagioclase crystallization follows Ca-depletion, Na and K-enrichment trends. Typically, Sr and Ba solid solubility is significant, particularly in groundmass feldspar.
The alkali feldspar variation trend from groundmass assemblages is not consistent with the theoretical phase relationships in the system NaAlSiO4-KAlSiO4 CaAl2Si2O8-SiO2 (“The phonolite pentahedron”) proposed by Carmichael et al. (1974).
Factors believed to be important in controlling feldspar crystallization trends are the Sr-Ba feldspar components, the role of the coexisting pyroxene and the presence of F, Cl and/or their alkali compounds.
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Ferguson, A.K., Cundari, A. Feldspar crystallization trends in leucite-bearing and related assemblages. Contr. Mineral. and Petrol. 81, 212–218 (1982). https://doi.org/10.1007/BF00371298
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DOI: https://doi.org/10.1007/BF00371298