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

A 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.

Substitution of ‘small’ divalent cations (e.g. Mg) for Si and Al in the nepheline tetrahedral framework: 2. The occurrence of Mg-rich nepheline and kalsilite

“…These observations are based on recently assembled databases of published and new wavelength-dispersive spectroscopy (WDS) electron microprobe analyses of these minerals, taken exclusively from alkaline ultrabasic volcanic rocks (see below). Additionally, we verify the significance of the higher amounts of Mg (and Fe) for the calculation of nepheline/kalsilite endmember components using the revised equations presented in our companion paper (Henderson and Oliveira, 2022).…”

“…2). As shown by Henderson and Oliveira (2022), the former numbers reflect the possible atomic proportions of Fe 3+ and the latter values those for Fe 2+ . The 4, composition E) as reference (black cross).…”

“…However, when considering the coupled substitution 2Al 3+ = Mg 2+ + Si 4+ , the excess Si ′ is affected significantly if the MgO concentrations are not given (compare the values of the Q (Si-Al) parameter in Table 1, particularly for columns 4, 6, 9 and 15). The Si total is related directly to this coupled substitution as demonstrated by Henderson and Oliveira (2022;table 1). Thus, robust analyses of T 2+ cations (mainly Mg, though also Mn and reliably-estimated Fe 2+ ) are essential to determine more realistic values of excess Si ′ , particularly, for K-rich nepheline and kalsilite crystallised from Mg-rich alkaline ultrabasic magmas (i.e.…”

“…As noted by Henderson and Oliveira (2022), the revised Spreadsheet 1 for calculation of nepheline-group minerals atomic formulae and stoichiometry parameters including the T 2+ cations (e.g. Mg 2+ ) gives more sensible compositional parameters for analyses which report MgO contents, particularly for the ΔAl cc / ΔT charge ratio.…”

“…The analysed Fe-oxide components reported are those given in the original publications, however note that the choice of using Fe 2 O 3 or FeO values for these EMP data is a personal choice of the analyst; in all cases the atomic formulae are given here in Table 1 with all iron as Fe 3+ . The atomic formulae, stoichiometry parameters, and end-member compositions are calculated with the revised spreadsheet 1 (Henderson and Oliveira, 2022) and the data for the sample set are attached here as the version denoted Spreadsheet 1A; in this spreadsheet we also show how the Fe valence could be assessed on the basis of Fe calculated as both Fe 3+ or Fe 2+ 'end-members' (see Henderson and Oliveira, 2022). Note that Spreadsheet 1A is attached to this paper as a supplementary file together with an additional file explaining how to use the EXCEL file.…”