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

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).