A new type of fault, called a chain periodicity fault, has been observed in Funferketten and Siebenerketten pyroxenoids. The faults are mainly parallel to (001) and are due to irregularities in the periodicity along c, the direction of the silicate chains. For example, pyroxmangite (Siebenerketten) contains slabs of rhodonite (Funferketten) one unit cell thick in the c direction. A periodic arrangement of chain periodictiy faults in synthetic iron rhodonite gives rise to a superstructure, where eight Funferketten units along a chain are followed by one Siebenerketten unit. When different pyroxenoids are intergrown, they have a sublattice exactly or nearly in common. Synthesis of pyroxferroite and iron rhodonite (all Mn replaced by Fe) was carried out at low pressure.