ABSTRACT
The recent description of nafertisite (Na,K,☐)4(Fe2+,Fe3+,☐)10[Ti2O3Si12O34](O,OH)6 (a = 5.353, b = 16.176, c = 21.95 Å, β = 94.6°) and a review of the titanosilicates from the Khibina-Lovosero complex (Kola Peninsula) show a key role of the polysomatic phenomena in (i) modelling unknown crystal structures; (ii) correlating phase transformations; (iii) explaining mineral associations. The structure of nafertisite has been derived by comparison with bafertisite {a = 5.36, b = 6.80, c = 10.98 Å, β = 94°; Ba2(Fe,Mn)4[Ti2O4Si4O14](O,OH)2} and astrophyllite {a = 5.36, b = 11.63, c = 11.76 A, α = 112.1, β = 103.1, γ = 94.6°; (K,Na)3(Fe,Mn)7[Ti2O3SigO24](O.OH)4}. These titanosilicates have layer structures: two (001) sheets (H), based on [100] rows of Ti octahedra alternated with different strips of Si tetrahedra, sandwich an octahedral sheet (O); two such HOH layers clamp the large cations. The heterophyllosilicates polysomatic series BmMn has been defined (B bafertisite-like and M mica-like modules cut across HOH). Bafertisite is not reported in the Khibina-Lovosero complex, but several minerals based on its H sheet are widely present. The dimensions of this H sheet are fixed by its periodicities along ( ≈ 5.4 Å) and across ( ≈ 7 Å) the Ti rows. The corresponding HOH layer occurs in a group of bafertisite-like structures (seidozerite derivatives) which differ only in their interlayer contents. The inclusion among these derivatives of some poorly described species is discussed. Four members of the group (sobolevite, lomonosovite, quadruphite and polyphite) have been observed together in oriented associations indicating close genetic relationships. Corrugated bafertisite-like H sheets are tridimensionally connected in the Zr-di-orthosilicates parakeldyshite, keldyshite, an unnamed species and khibinskite (Zr plays the role of Ti): the first three minerals form, in the order, an evolutionary series where parakeldyshite is the primary phase and the others are its products of alteration. Other (de)hydration/(de)cationization reactions are discussed as well.
