ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract 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 AlIV-O-AlIV 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, 63〈eqan〈73, did not detect any reflections violating the P42/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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371201
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