Substitution of ‘small’ divalent cations (e.g. Mg) for Si and Al in the nepheline tetrahedral framework: 1. Calculation of atomic formulae and stoichiometry parameters

Abstract

AbstractA recent review paper on nepheline solid solutions dealt mainly with sodic varieties from nepheline syenites and differentiated alkali basaltic sills. Excel spreadsheets were provided for recalculating cell formulae in terms of the stoichiometric parameters ΔAlcavity cation charge and ΔTframework charge and molecular percentages of the end-members Ne, Ks, Ca-nepheline and excess Si (Qxs). Small amounts of tetrahedral divalent species (usually <0.10 wt.% though with up to 0.33 wt.% MgO) were related to a stuffed-tridymite end-member of ideal formula K8Mg4Si12O32 (denoted KsT2+-nepheline), although the equations used to calculate these end-members only considered Si, Al and Fe3+ as tetrahedral framework species. Recently, K-rich nepheline and kalsilite which contain up 0.54 wt.% MgO have been found in some feldspar-free mafic potassic volcanic rocks from central Brazil. Equations have been developed to deal rigorously with the presence of significant amounts of Mg (and Mn) in recalculating nepheline solid-solution parameters. In two related communications reworked spreadsheets are provided with examples of their use. High quality microprobe analyses of nepheline and kalsilite from magmatic rocks must include analyses for Mg and Mn and, if possible, estimates of Fe2+ contents.