SCAPOLITE STOICHIOMETRY

Over the range Ca/(Ca + Na) from zero to 0.75, scapolites vary linearly between Na4Al3Si9O24Cl and NaCa3Al5Si7O24CO3, by means of a coupled replacement of Na3Si2Cl by Ca3Al2CO3. Ca includes minor Sr, Na includes K, and CO3 may include substantial SO4. From Ca/(Ca + Na) = 0.75 to 1.00, variation is between NaCa3Al5Si7O24CO3 and Ca4Al6Si6O24CO3, by means of a coupled replacement of NaSi by CaAl, as in plagioclase. Anion substitution is complete at the 0.75 point. Recalculation of scapolite analyses on the basis of Al + Si atoms = 12.0, as suggested by computed unit cell contents, offers a reasonable basis for normalization of atomic proportions. On this basis, Ca + Sr + Na + K + Fe atoms total very close to 4.0. New limits on the minor element content of scapolite are proposed. In view of the inflection at 0.75, subdivision of the scapolites at Ca/(Ca + Na) ratios of 0.25, 0.5, and 0.75 seems more logical than the 0.2, 0.5, and 0.8 limits used hitherto for marialite, dipyre, mizzonite, and meionite. © 1969 Springer-Verlag.