Lattice defects in three structurally related minerals: Kyanite,yoderite and staurolite

The structures of kyanite, yoderite and staurolite are all built up of alternating layers one of which is the Al₂SiO₅ layer characteristic of the kyanite structure. All the lattice defects found by transmission electron microscopy in these minerals (dislocations, conservative and nonconservative stacking faults, twins) do not affect these layers. In the case of staurolite two types of (010) nonconservative faults are found. Their displacement vectors are 0 1/3 0] and 1/4 1/n 1/4] with n～10 and they respectively cause local regions to transform from the staurolite matrix to kyanite and spinel microdomains. These faults suggest the occurrence of topotactic transformations relating the corresponding phases.