Original paper

Polytypic and polymorphic relations between sazhinite and isochemical alkali-REE layer silicates

Cadoni, Marcella; Ferraris, Giovanni

Abstract

An analysis of the crystal structures of natural and synthetic phases with ideal chemical formula A3REE[Si6O15]·nH2O(A = 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 + k = 2n + 1 in the diffraction patterns of synthetic phases (space group Cmm2 and b = 31 Å) shows an isoriented presence of a minor sazhinite-type structure (space group Pmm2 and b = 15.5 Å).

Keywords

sazhinite-(ce)sazhinite-(la)sazhinite polytypesazhinite polymorphmodular structurephyllosilicates