Prehnite: Structural similarity of the monoclinic and orthorhombic polymorphs and their Si/Al ordering

Abstract Two modifications of the naturally occurring mineral prehnite, Ca 2 Al(Si 3 AlO 10 )(OH) 2 , are characterized by precise single crystal diffraction methods. The two forms are topologically identical. They differ only in the siting of one Al and one Si atom per unit cell: these switch their places in two of the tetrahedrally coordinated sites. This appears to be the only known case where a silicate occurs with two differently ordered tetrahedral distributions of Al and Si atoms. The two forms are not polytypes according to the current definition of polytypes. The two modifications are so similar to each other that they differ only in the eighth coordination sphere.