The Ca–Na clinopyroxene omphacite and garnet are the defining minerals and main phases of eclogites. The rheological properties of eclogites are dominated by the mechanical behaviour of omphacite. Omphacite can occur in an ordered structure with the space group P2/n and, above 800 °C, in a disordered structure with the space group C2/c. Most eclogites found in the Earth’s crust contain ordered omphacites with the space group P2/n. While there is an increasing number of studies by transmission electron microscopy (TEM) of deformation effects in omphacites from eclogites, corresponding TEM studies of experimentally deformed P2/n-omphacite are lacking. Natural omphacites from Tianshan and Weissenstein with space group P2/n were deformed at temperatures from 500 to 800 °C and strain rates from ca. 10−4 to 10−6 s−1 in a deformation-DIA (D-DIA) apparatus at usually 2 GPa confining pressure. In seven experiments, the specimens were coaxially shortened; one specimen was deformed by simple shear. TEM investigations of the experimentally deformed specimens revealed deformation twin lamellae on (100) and (001), traces of failed twin attempts on (100), dislocations, faults parallel to (010) and {110} and non-crystallographic faults. These crystal defects were not observed in all the specimens; however, a correlation between the deformation effects and the experimental conditions was not apparent. Most of the experimentally produced crystal defects were also found in naturally deformed omphacites. The experiments show that there are sufficient mechanisms for deformation of P2/n-omphacite at lower temperatures, which can compensate for the loss of perfect dislocations with the Burgers vector 1/2〈110〉, which are available only in C2/c-omphacite.

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