Aragonite twinning

Figure 7.3. Structure of aragonite twin [4]. Triangles denote CO3, and the solid and open circles represent Ca.

Pseudo-hexagonal Ostensibly hexagonal because of cyclic twinning Aragonite... 

Figure 107 Twinning in an orthorhombic crystal. Hie twiiming of aragonite (CaC03) is shown. The intersection of crystal I with crystal II occurs in the section labeled I,II, where coordination of the ions differ only slightly from those in the individual crystals. (Reproduced by permission from Bloss, F. D., Crystallography and Crystal Chemistry, Mineralogical Society of America, Washington, DC, 1994, Figure 10-25C, p. 329.)...

An example of how twins can form in a more complex system is shown schematically in Figure 14.23 for aragonite. This figure shows the COs anions as triangles (as we saw in calcite) while the closed circles represent Ca ions. (The numbers give heights of ions in the cells and serve to emphasize the twin symmetry.)... 

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

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