An analysis of the crystal structures of natural and synthetic phases with ideal chemical formula A3REE[Si6O15]nH2O (A ¼ usually alkalis, n 2) shows that they are either 2O2 polytypes or polymorphs of sazhinite, which represents the 1O1 polytype. The polytypic and polymorphic relations are essentially determined by the topology of the [Si6O15]6 anion which is a corrugated sheet and varies according to the type of coalescence of the basic xonotlite-like module. The sheet can consist of: five- and eight-rings (5281 ring symbol); four-, six- and eight rings (416181); four-, five, six- and eight-rings (41516282). The occurrence of weak reflections with h þ k ¼ 2n þ 1 in the diffraction patterns of synthetic phases (space group Cmm2 and b¼31A ° ) shows an isoriented presence of a minor sazhinite-type structure (space group Pmm2 and b ¼ 15.5 A ° ).
Polytypic and polymorphic relations between sazhinite and isochemical synthetic compounds.
CADONI, MARCELLA;FERRARIS, Giovanni
2011-01-01
Abstract
An analysis of the crystal structures of natural and synthetic phases with ideal chemical formula A3REE[Si6O15]nH2O (A ¼ usually alkalis, n 2) shows that they are either 2O2 polytypes or polymorphs of sazhinite, which represents the 1O1 polytype. The polytypic and polymorphic relations are essentially determined by the topology of the [Si6O15]6 anion which is a corrugated sheet and varies according to the type of coalescence of the basic xonotlite-like module. The sheet can consist of: five- and eight-rings (5281 ring symbol); four-, six- and eight rings (416181); four-, five, six- and eight-rings (41516282). The occurrence of weak reflections with h þ k ¼ 2n þ 1 in the diffraction patterns of synthetic phases (space group Cmm2 and b¼31A ° ) shows an isoriented presence of a minor sazhinite-type structure (space group Pmm2 and b ¼ 15.5 A ° ).
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