Polysomatic structures

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Leonyuk, L., Babonas, G.-J., Maltsev, V., and Ribakov, V. (1999). Polysomatic series in the structures of complex cuprates. Acta Cryst. A55, 628-34. 

Following the first case treating the polysomatic series of biopyriboles [4], structures of several groups have been described as members of the same polysomatic series (see below). 

In the survey of modularity reported in Chapter 1 of [2], a general description of the modular interpretation of crystal structures has been extensively discussed. In this chapter we consider only planar modules, and the examples are devoted to show the polysomatic, merotype and plesiotype aspects (see below) of some series that are important either methodologically or for their technological and mineralogical relevance. The possibility of exploiting the modularity to model unknown crystal structures is illustrated with some examples. 

The members of a polysomatic (signifying many bodies [4]) series are crystal structures based on the varied stacking of the same modules A, B, C, With reference... 

The raw structure model of nafertisite was obtained on applying the principles of the polysomatic series and modifying the structures of bafertisite and astrophyllite after noting the following. 

The successful modelling of the crystal structure of nafertisite, according to the principles of modular crystallography, indicated [25, 31, 32] to correlate via a polysomatic series the group of titanium silicates of which the stmctures are based... 

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]... 

MICA STRUCTURE AND POLYSOMATIC SERIES Layer silicates as members of modular series ... 

The M mica module occurs not only in biopyriboles, chlorites and interstratified clay minerals as mentioned above, but also in some other polysomatic series. Because these series represent different possibilities for the presence of mica-like structures in minerals, it seems useful to shortly describe some of them. 

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 ... 

Take uchi Y, Sadanaga R (1959)The crystal structure of xanthophylhte. Acta Crystallogr 12 945-946 Thompson JB Jr (1978) Biopyriboles and polysomatic series. Am Mineral 63 239... 

Examples of such materials have been known for a number of years in the mineralogical field, where the terms intergrowth phases , polysynthetic twinned phases , polysomatic phases and tropochemical cell twinning have been applied to describe the structures.In recent years, the application of electron microscopy has revealed the defect structures of vast numbers of phases containing extended defects. In this section, some well-characterized examples chosen are chosen for illustration. 

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