The distribution of NaAlSi 3 O 8 between coexisting microcline and plagioclase and its effect on geothermometric calculations

Abstract

The partitioning of NaAlSi/sub 3/O/sub 8/ between coexisting microcline and plagioclase solid-solutions is modeled using published Margules parameters for microcline-low albite solid-solutions. Variations in the structural state of the alkali feldspar may shift the temperature calculated from coexisting feldspar pairs by as much as 100/sup 0/C. The magnitude of this shift varies with the compositions of the coexisting feldspar, but will generally be less than 100/sup 0/C. In nature, the effect of ordering would be less for primary orthoclase, and would not be significant if the feldspar originally equilibrated in the disordered state.