Single crystal electron diffraction patterns of an omphacite (CaNa, Mg2+, Fe2+, Al3+, Fe3+)Si2O6 from a glaucophane schist facies (type C) eclogite, Tiburon Peninsula, California show weak reflections violating the C2/c space group. Transmission electron microscopy with dark field imaging, using h+k = even reflections reveal no special features, while dark field viewing through certain superstructure reflections violating the C2/c space group (h k l, with h+k = odd and h 0 l with h and/or l odd) reveal anti-phase domains, 100 to 3000 A in width. Diffraction contrast experiments show that these domains are related by a displacement vector R=1/2[110]. This displacement vector explains the origin of the domains in terms of ordering of various cations in the octahedral (M1 type) and polyhedral (M2 type) sites without any changes in the tetrahedral silicate chains.