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White mica domain formation: A model for paragonite, margarite, and muscovite formation during prograde metamorphism

Christidis Georgios, Kenneth J. T. Livi , Péter Árkai , David R Veblen

Πλήρης Εγγραφή

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Περίληψη

Scanning transmission electron microscopy images of the 00l white mica planes in crystals fromcentral Switzerland and Crete, Greece, reveal that domains of paragonite, margarite, and muscoviteare ordered within the basal plane. Energy dispersive X-ray analyses show that both cations in theinterlayer and in the 2:1 layer have ordered on the scale of tens to hundreds of nanometers. Domainboundaries can be both sharp and crystallographically controlled or diffuse and irregular. A modeloutlining the domain formation process is presented that is consistent with X-ray powder diffractionand transmission electron microscopy data. The domain model incorporates aspects of a mixedlayeredand a disordered compositionally intermediate phase models. The main feature of the modelis the formation of mica species that segregate within the basal plane and contradict the notion ofhomogeneous layers within mixed-layer phases. Implications for the formation of all diagenetic andvery low-grade metamorphic 2:1 sheet silicates are discussed

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