Mimetic twinning

Synthetic zeolite NaPl (or Linde B) is apparently an isotype of gismondine (Figure 14) according to a recent study by Baerlocher and Meier (4). The cubic structure which was proposed earlier has been ruled out in this work. The symmetry of the NaPl framework is noncubic (despite the fact that the unit cell is compatible with cubic symmetry), and the phase appears to consist of mimetic twins. 

FIGURE 14.23 Mimetic twinning in aragonite. This gives rise to trilling twins when the operation is repeated. 

A different interpretation of alite has been given by Fundal (1982), who stated that alite crystals are twinned individuals of a lower symmetry which imitate a higher symmetry (a mimetic relationship). The monoclinic form of alite was said to have an index of refraction of 1.72 and a birefringence of approximately 0.0050. Tri clinic alite was reported to be polysynthetically twinned in a fused clinker and have a birefringence of 0.0020. Alite birefringence, however, exhibits two average val ues in Fundal s Table 1 (1982) 0.0043 (monoclinic) and... 

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