Nepheline solid solution compositions: stoichiometry revisited, reviewed, clarified and rationalised
C. M. B. Henderson
Abstract
Abstract Molecular formulae used to recalculate nepheline analyses generally have different numbers of oxygens (e.g. NaAlSiO 4 (Ne), KAlSiO 4, (Ks), CaAl 2 Si 2 O 8 (An) and SiO 2 (Q)). A 32 oxygen cell has 16 T cations and 8 cavity sites, but ideal nepheline stoichiometry is not necessarily followed. Ca end-member □ Ca CaAl 2 Si 2 O 4 (CaNe) and excess silica end-member □ Si Si 2 O 4 (Q’) calculation requires inclusion of both vacancy species as cavity cation values. Q’ parameter calculations can involve different assumptions and four parameters are described: Q xs ; Q Si ; Q (Si–Al) ; and Q cavity ; these should have closely similar values for high-quality, stoichiometric analyses. Representative published compositions are recalculated to assess whether authors followed ideal nepheline stoichiometry. Phenocrysts from peralkaline rocks and nephelinites typically exhibit Al deficiencies reflected in negative Δ(Al – cavity cation) parameters (ΔAl cc ), negative ‘normative’ corundum (Al 2 O 3 , Cn ), and anomalously low or negative Q xs parameters; for such rock types Q (Si–Al) provides a better estimate of excess silica contents. A ΔT-site (cation charge) parameter (ΔT charge ), is closely coupled to ΔAl cc and end-member NaAlSiO 4 has a ΔAl cc /ΔT charge ratio of 1.4296; the derivation of this value is controlled by strict stuffed-tridymite, unit-cell constraints. Natural nephelines all contain excess silica with a mean ΔAl cc /ΔT charge of ~1.134 reflecting their Si/Al ratio being > 1. Nepheline analyses with relatively low Al and Si and high Na (also Ca) contents are common; this might reflect the presence of small amounts (up to ~5%) of cancrinite as an alteration phase or perhaps even in solid solution. The compositions of alteration lamellae of Ca-rich cancrinite in altered nepheline phenocrysts in phonolites from the Marangudzi alkaline complex, Zimbabwe, are used to define diagnostic parameters for recognising such non-stoichiometry. These alteration lamellae formed hydrothermally from Ca-rich and K-poor fluids. An EXCEL file is provided to help researchers to standardise calculation of nepheline end-member molecular proportions.