The structures of bustamite and the poly-types of wollastonite are shown to be derived from variations in the stacking sequence of a single type of structural unit. This structural unit is a column of unit cells of wollastonite along the b-axis and is bounded by (100) and (001) planes (P1¯ cell); these units may be stacked together along [100] and [001] with either zero displacement between successive units, or a relative displacement of 12[010]. Regular stacking sequences give rise to the ordered structures of wollastonite polytypes and of bustamite, while the transformations between these structures proceed by the propagation of line defects with Burgers vectors of 12[010] which thus change the stacking sequence of the structural units.