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

Compositional trends of natural samples of scapolite extend toward the ideal (anhydrous) end-members marialite (Ma) Na2Al3Si9O24Cl and meion ite (Me) Ca4Al6Si6O24CO3. Intermediate members do not follow the plagi oclase substitution, although Si contents closely correlate with Me% [ calculated as 100{Sigma(divalent cations)}/4]. The solid solution may be explained by substitutional interactions among the M, T. and A site s, as constrained by net charge-balance and maximum site-occupancy req uirements of the structural formula M(4)T(12)O(24)A. Two changes in co mpositional trend located at [(Na3.4Ca0.6)(Al3.6Si8.4)O-24](+1) and [( Na1.4Ca2.6)(Al4.7Si7.3)O-24](+1.9) divide the series into three portio ns. Nomenclature consistent with these divisions is marialite (Me 0-15 ). calcian marialite (15 < Me less than or equal to 50), sodian meioni te (50 < Me < 65) and meionite (Me 65-100), A large number of scapolit e occurrences have Si values near 7.3 apfu, corresponding to the I4/m- P4(2)/n symmetry transition and to the stabilization of scapolite in a Cl-poor environment. Trends of Cl with Si or (Na + K) apfu indicate c omplex substitutional mechanisms, and should nor be used to suggest th at ''mizzonite'' or ''dipyre'' are end-members. Calculation of excess positive charge from measurement of M and T cations indicates that sca polite typically has the maximum content of Cl expected by charge-bala nce. Anion substitution is complicated by substitution of H as hydroxy l or bicarbonate-bisulfate anions. The calculation CO2=1-Cl-F-S may ap ply only to anhydrous scapolites which are rare, otherwise it over-est imates CO2 because it ignores the role of H CO2 content calculated by charge-balance equations gives better agreement with constraints of th e known substitutional mechanisms More H is present than is required b y stoichiometry, and molecular H2O may be present in channels along th e c-axis.