Clinoamphibole formation in naturally deformed augite from a pyroxenite dyke of the Balmuccia peridotite massif (Ivrea Zone, NW-Italy) has been studied by conventional and high resolution transmission electron microscopy. The clinoamphibole lamellae are oriented parallel to (010) and have their crystallographic axes parallel to those of the augite. They originate from dissociation of certain lattice dislocations. The shear produced by glide of 1/2  partial dislocations in (010) combines single to double chains, yielding so-called chain multiplicity faults. The ion exchange necessary for amphibole formation most probably is towards and along the core of the partials. Broadening of the lamellae occurs by lateral motion of partial dislocations along the (010) interface two chains apart. The formation of clinoamphibole is discussed with respect to exsolution and alteration, phase stability and reaction kinetics.