Ordering and composition of scapolite: Field observations and structural interpretations

Field observations, experimental and crystallographic data and thermodynamic considerations all suggest that Al-Si order-disorder is a crucial factor in explaining composition and stability of the mineral scapolite. Over the whole compositional range, scapolites have Al^IV-O-Al^IV bonds with the exception of one intermediate member with an Al/Si ratio of 1/2. Scapolites of this composition are the lowest temperature form and appear in areas with argillaceous carbonates and evaporites which have been subjected to progressive metamorphism. In similar areas without evaporites, the onset of the CO₃-scapolite stability field is approx. 150° C higher with an Al/Si ratio in the scapolite of about 5/7. This particular CO₃-scapolite is a compromise between the number of Al-O-Al bonds and the volume of the anion site occupied by CO₃. Based on field- and experimental data, temperature-composition diagrams for scapolite, plagioclase and calcite have been constructed. These diagrams may be explained in the light of contrasting Al-Si order-disorder in plagioclase and scapolite, i.e. at low temperature, plagioclase endmembers and intermediate scapolite members are stable, towards higher temperatures the ∩-shaped temperature-composition field of plagioclase and the V-shaped one of scapolite interfere in a complicated way. Electron microscopy of Al-rich scapolite, 632/n extinction rules. But these scapolites (with or without Cl-anion) show domain boundaries. We interpret them as APB's in the Al/Si ordering pattern on T₂-T₃ sites which reverses when a displacement R=1/2 111] is applied.