Chain multiplicity faults in deformed omphacite from eclogite

Deformed and recovered omphacite from an eclogite sample of the Lower Schist Cover of the Tauern Window was studied by conventional and high resolution transmission electron microscopy. The rock sample, which was used before for petrological investigations (Zimmermann & Franz, 1988) and radiometric age dating (Von Quadt et al., 1997), shows chain multiplicity faults (CMFs) in omphacite which have not yet been described. The CMFs are usually intercalations of one or two double chains parallel to (010) in the single chain omphacite. They are mostly terminated by partial dislocations. Contrast analysis showed that the displacement vector R and the Burgers vector b of the partial dislocations are probably 1/2[011], less likely interpreted as 1/2[101]. It is assumed that the CMFs were formed by stress-induced dissociation of [011] dislocations. A direct proof of CMFs acting as slip planes is given. The frequent occurrence of CMFs in omphacite suggests that 1/2[011](010) is an important slip system which is probably responsible for lattice preferred orientation. The interaction observed between CMFs and antiphase domain boundaries provides a recovery mechanism hitherto not reported from omphacite.