SUBSTITUTIONAL MECHANISMS, COMPOSITIONAL TRENDS AND THE END-MEMBER FORMULAE OF SCAPOLITE

Abstract

Compositional trends of natural samples of scapolite extend toward the ideal (anhydrous) end-members marialite (Ma) Na4Al3Si9O24Cl and meionite (Me) Ca4Al6Si6O24CO3. Intermediate members do not follow the plagioclase substitution, although Si contents closely correlate with Me% [calculated as 100{Σ(divalent cations)}/4]. The solid solution may be explained by substitutional interactions among the M, T and A sites, as constrained by net charge-balance and maximum site-occupancy requirements of the structural formula M4T12O24A. Two changes in compositional trends located at [(Na3.4Ca0.6)(Al3.6Si8.4)O24]+1 and [(Na1.4Ca2.6)(Al4.7Si7.3)O24]+1.9 divide the series into three portions. Nomenclature consistent with these divisions is marialite (Me 0–15), calcian marialite (15 < Me ≤ 50), sodian meionite (50 < Me < 65) and meionite (Me 65–100). A large number of scapolite occurrences have Si values near 7.3 apfu, corresponding to the 14/m-P42/n symmetry transition and to the stabilization of scapolite in a Cl-poor environment. Trends of Cl with Si or (Na + K) apfu indicate complex substitutional mechanisms, and should not be used to suggest that “mizzonite” or “dipyre” are end-members. Calculation of excess positive charge from measurement of M and T cations indicates that scapolite typically has the maximum content of Cl expected by charge-balance. Anion substitution is complicated by substition of H as hydroxyl or bicarbonate-bisulfate anions. The calculation CO2 = 1−ClFS may apply only to anhydrous scapolites which are rare, otherwise it over-estimates CO2 because it ignores the role of H. CO2 content calculated by charge-balance equations gives better agreement with constraints of the known substitutional mechanism. More H is present than is required by stoichiometry, and molecular H2O may be present in channels along the c-axis.