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

Abstract

This is a 'preproof' accepted article for Mineralogical Magazine. This version may be subject to change during the production process.

Crystallisation of Ca-bearing nepheline in basanites from Kajishiyama, Tsuyama Basin, Southwest Japan

“…Finally, following the calculation method of Henderson and Oliveira (2022), the chemical formula of the highest-Ca nepheline in the Kajishiyama basanite could be written as (Ca 0.467 Ba 0.013 Na , where □ Total is the sum of □ Si and □ Ca . Henderson and Oliveira (2022) and Oliveira and Henderson (2022) reported that K-rich nepheline and kalsilite have a trend including relatively high Mg and Fe 2+ and showed a way to estimate the Fe 2+ proportion in total Fe using the variation of the ΔAl cc (change in T charge minus cavity cation charge: T 3+ + 2T 2+ − M + − 2M 2+ ) and ΔT ch (total analysed T charge minus charge for 16T: 4T 4+ + 3T 3+ + 2T 2+ − 16*mean T charge) stoichiometry parameters. In this study, however, we treated all iron as Fe 3+ because the nepheline of Kajishiyama is low in Mg and Fe, and has a high Na/K ratio.…”

“…Henderson (2020) concluded that Mg 2+ , Mn 2+ and Ti 4+ replace Si 4+ in the tetrahedral site, and large cations such as Ca 2+ , Ba 2+ and Sr 2+ replace Na + and K + in the cavity sites and suggested a method to calculate the molecular nepheline formula. Henderson and Oliveira (2022) modified that calculation method in order to deal with the presence of Fe 2+ , Mg 2+ and Mn 2+ in tetrahedral sites, and Oliveira and Henderson (2022) proposed a method to estimate Fe 2+ occurrence in nepheline. However, the content and site occupancies of minor and trace elements such as Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ and Ti 4+ in nepheline have not been studied systematically due to rather limited occurrences of nepheline and the difficulty of preparation of nepheline crystals suitable for detailed structural investigations.…”

“…, the chemical formula of the highest-Ca Ne65.50Ks11.39Q xs 11.22CaNe11.89, where □ Total is the sum of □ Si and □ Ca Henderson and Oliveira (2022). andOliveira and Henderson (2022) reported that K-rich nepheline and kalsilite have a trend including relatively high Mg and Fe 2+ and showed a way to estimate the Fe 2+…”

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

“…This approach is particularly relevant for Mg- and Fe-rich potassic nephelines and kalsilites and we will consider this matter further in Paper 2 (Oliveira and Henderson, 2022, this volume).…”

Iron in Nepheline: Crystal Chemical Features and Petrological Applications

“…In addition, different impurities in the composition of nepheline reflect the composition of the mineral-forming medium and physicochemical conditions of the crystallization, which expands the possibilities of using nepheline as a petrological indicator. In particular, Mg-rich nepheline occurs in the groundmass of strongly SiO 2 -undersaturated, feldspar-free, mafic volcanic rocks (olivine-rich foidites) [35]. Hence, the occurrence of Mg-rich nepheline seems to be related to its derivation from Mg-rich magmas.…”