The palysepiole polysomatic series

The palysepiole polysomatic series PpSs (Ferraris et al. 1998) includes minerals whose structures contain one or both the types of TOT ribbons (modules) which are present in palygorskite and sepiolite these ribbons are reminiscent of the TOT modules occurring in amphiboles (Fig. 19). The two modules are  

Guggenheim and Eggleton (1987, 1988) described some modular 2 1 layer silicates in terms of fragments of the M (TOT) mica module, with or without interlayer cations. The modularity of these silicates originates by the inversion of part of the tetrahedral linkage. On the basis of the inversion fragments, the basic TOT layers may form either islands (e.g., stilpnomelane and zussmanite) or strips (where the octahedral sheets remains continuous as in ganophyllite and minnesotaite, or discontinuous as in the above mentioned palysepioles). 

TOT modules occur in different mineral structures and the following examples have been discussed in this Appendix. 

It seems reasonable to connect variety and frequency of occurrence with structural stability. The wide range of conditions under which the TOT layer is stable on its own occurs in talc and pyrophyllite, built up by this layer only. The TOT layers are even able to survive through reactions generating other minerals [cf Baronnet (1997) and Buseck (1992) and references therein] including other micas (cf Ferraris et al. 2001a). 

Probably features other than crystal chemistry concur to explain the wide distribution and persistence of variously sliced mica modules. The high symmetry of the mica modules could be a key feature, in the sense that it favors different stacking and connections with other modules both of the same kind and different nature. Mica polytypes and twins are clear examples of symmetry-assisted structures. The flexibility of 

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Because the sepiolite framework is substantially modified, intersilite is in a plesiotype relationship with the palysepiole polysomatic series. 

A description of the palysepiole polysomatic series and related structures offers the opportunity to mention the main members of the lintisite series of which the crystal stmctures are based on three building modules. One of these modules is a cut of sili-naite [57] (NaLiSi205.2H20 C2/c, a = 14.383, b = 8.334, c = 5.061 A, 13 = 96.6°), a crystal structure that shows a chess-board arrangement of channels and ribbons comparable with that of palysepioles. Modules of silinaite (S) have been used [58] as one of three modules needed to describe the modularity in the lintisite group (Figure 17-14). The other two modules are slabs (L) of the structures of lorenzenite [59]... 

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