Scapolite固溶体系における陽イオン置換

概要

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Scapolite is a series of tetragonal alumino-framework silicates with two formula units per cell, and is a solid solution between marialite, Na4 Al3 Si9 O24 Cl, and meionite, Ca4 Al6 Si6 O24 CO3. There are two substitution systems according to its composition: from marialite end-member to 75% meionite, Ca2 Al2·CO3 are substituted for Na2 Si2·NaCI, and from 75% meionite to meionite end-member, NaSi are replaced with CaAI as in plageoclase feldspars. The crystal structure of scapolite is characterized by the three dimensional framework of (Al, Si) -O tetrahedra and by both large cation and anion groups in the cavity of the framework. The space group of theoretical end-members is I4/m, but intermediate compositions have weak b-reflections, which violate body centered symmetry, and thus their space group is reduced to P42/n. Al and Si atoms distributed in the three distinct tetrahedral sites are in the most highly ordered fashion at 37.5% meionite. Position of Na and Ca atoms in the same site is also most shifted at 37.5% meionite in the solid solution. So at around this composition, the crystal structure is most distorted from those of end-members and, consequently, the intensities of the additional b-reflections become maximum. The a cell dimension and the unit cell volume linearly increase while c decreases with increased meionite component.
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