Dislocation-mechanism of clinoamphibole formation in deformed augite

Clinoamphibole formation in naturally deformed augite from a pyroxenite dyk e of the Balmuccia peridotite massif (Ivrea Zone, NW-Italy) has been studie d by conventional and high resolution transmission electron microscopy. The clinoamphibole lamellae are oriented parallel to (010) and have their crys tallographic axes-parallel to those of the augite. They originate from diss ociation of certain lattice dislocations. The shear produced by glide of 1/ 2 [101] partial dislocations in (010) combines single to double chains, yie lding so-called chain multiplicity faults. The ion exchange necessary for a mphibole formation most probably is towards and along the core of the parti als. Broadening of the lamellae occurs by lateral motion of partial disloca tions along the (010) interface two chains apart. The formation of clinoamp hibole is discussed with respect to exsolution and alteration, phase Stabil ity and reaction kinetics.