Crystallographic Data and Refractive Indices of Scapolites

Abstract

Scapolites of intermediate composition show sharp b-reflections (h + k + I +2n), in addition to the a-type (h + k + I - 2n), thus establishing P4e/n as a possible space grotp. I4/m was found at both sodic and calcic end members. New cell parameters, determined with Guinier methods, and refractive indices are reported. Determinative charts based on cell dimensions, d-spacings and refractive indices allow estimation of meionite content within about -+7 percent. Regression of cell parameters on chemical data reveals several possible regression models. Treating the whole group as a single population works best for refractive indices, which can be predicted mostly within -r.002. On the other hand, cell parameter regression on composition is best treated assuming two different populations, one on each side of 65-66 percent meionite; predicted cell dimensions are usually within r-.003 A. The possibility of limited miscibility gaps in intermediate scapolites is discussed. P-group specimens, at least, cannot be considered an ideal solid-solution series.