Stacking faults, twinning, and polytypism in wollastonite

Stacking faults are a-boundaries for which a = 2xg R. (0gi-0g2) is zero for all g. In some structures, stacking faults and twins are closely related, and different regular sequences of these defects produce various polytypes. Wollastonite is a relatively simple example of such a structure, for which the stacking faults have been studied in some detail by TEM, both by their a-fringe contrast and in two-dimensional high-resolution lattice images. 

Wollastonite is common in thermally metamorphosed impure limestone and can occur in contact-altered calcareous sediments. In most occurrences, it has been formed as a result of the reaction 

Because of their structural relationship, wollastonite and pturawolla-stonite are better described as polytypes rather than as polymorphs. Other polytypes are possible (Deer, Howie, and Zussraan 1963). For example, a new polytype would be formed if regular stacking faults occurred in every third triclinic unit cell, producing the packing sequence 

7(b) and (c) shows that the monoclinic unit cell produced by this twin operation is identical to that produced by a single stacking fault. A wide twin lamella can be produced by stacking faults in every adjacent triclinic unit cell  

Note also that the twin composition planes on each side of the twin lamella are in the center of parawollastonite cells. 

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