THERMAL-EXPANSION OF SCAPOLITE

Thermal expansion coefficients were measured for four natural and one synthetic scapolite spanning the solid solution range from marialite to meionite (22-100% Me). Measurements of lattice parameters and unit-cell volumes were carried out at 20-degrees-C before and after heating and at intervals of 100-degrees-C ovet the temperature range 100-800-degrees-C. The measurements were carried out on finely powdered samples using a heating stage designed for examining phase transitions in minerals (Salje et al., 1993). For all scapolite compositions, the unit-cell volume and the a lattice parameter were observed to increase linearly during heating, whereas the c lattice parameter remained essentially unchanged. These effects are reversible, as unit-cell volumes and lattice parameters revert to their initial values after the samples are cooled from 800-degrees-C. The thermal expansion of the unit-cell constants measured in this study, V and a, does not vary linearly with composition. The new data are consistent with the earlier measurements of Graziani and Lucchesi (1982) only for scapolites of relatively sodic composition; for more calcic scapolites, the new data predict a much greater thermal expansion, with alphaV(meionite) being almost four times that originally obtained in the earlier study.