On Polymorphism of BaAl2Si2O8
The polymorphs of BaAl2Si2O8 known so far are celsian, paracelsian and hexacelsian. Celsian is a monoclinic feldspar mineral with a 14 Å c-axis and a body-centered lattice (Newnham and Megaw, 1960). Paracelsian, which also occurs as a mineral, has a framework structure similar to the feldspar structure (Smith, 1953; Bakakin and Belov, 1961; Craig et al., 1973). Hexacelsian, which is only known as a synthetic product is a layer alumosilicate built up by double sheets of Al2Si2O8 (Ito, 1950). Takéuchi (1958) described a hexagonal and an orthorhombic polytype. According to Lin and Foster (1968), celsian is stable at atmospheric pressure below 1590° C. From 1590° C up to the melting point (~ 1760° C) hexacelsian, is the stable polymorph. It persists metastably when cooled below 1590° C. Paracelsian appears to be a metastable phase.
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