Polysynthetic twinned phases

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. 

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 stmctures. In recent years, the application of electron microscopy has revealed the defect stmctnres of vast numbers of phases containing extended defects. In this section, some well-characterized examples chosen are chosen for illustration. 

A projection of Py2AsF6 is shown in Figure 9. It consists of two independent stacks with different setting angles but identical overlap (type 1). Because the crystal exhibits polysynthetic twinning, a full crystal structural analysis has not been undertaken. Preliminary data indicate that in a phase transition, both stacks become identical that is, the structure is face-centered in the low-temperature phase. This conclusion is supported by electron spin resonance (ESR) data. 

Different 38 phases, in the case of low content of belite in clinker, is most convenient to identify using light microscopy. Among others, Metzger [161] applying this method found the presence of phase a. The striations of belite crystals always observed in industrial ehnkers due to polysynthetic twinning, are distinctly differentiate this phase from alite crystals and are formed in the polymorphic transformation process. 

Keiviite-(Yb) and -(Y) were described as new minerals by Voloshin et al. (1983a, 1985). Yakubovich et al. (1986) determined the crystal structure of keiviite-(Yb) using a polysynthetically twinned sample, which consisted of keiviite-(Yb) and another phase of R2Si2 07, i.e., Er2Si2 07 having a different structure from that of keiviite-(Yb). The crystal structure of keiviite-(Yb) (fig. 37) is isostructural with that of thortveitite (fig. 36). 

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