TEM imaging of polytypism in pseudowollastonite

A study of polytypism in synthetic polycrystalline pseudowollastonite has been made using conventional and high resolution transmission electron microscopy (TEM and HRTEM). Three polytypes of 2, 4 and 6-layers coexist in the one sample in the form of lamellae parallel to (001). From the electron microscopy images the lamellae are interpreted as combinations of the stacking fault vectors: 1/3 [010], 1/6 [310] and 1/6 [3 $bar 1$ 0], which are probably all equivalent at high temperatures. The polytype lamellae are stable even after prolonged annealing at high temperatures greater than 1700K. These observations suggest that the most frequently reported 4-layer polytype is not the only high-temperature stable polytype. No evidence of melt is detected in samples annealed in the regime where premelting behaviour of pseudowollastonite has previously been reported (above 1600 K).