Order-disorder transitions convergent

Omphacite is a clinopyroxene that can be disordered, but is rarely disordered in nature because it typically occurs in eglocite facies (metamorphic) rocks as a main phase with garnet. Mg and Al cations order convergently on the Ml positions of the monoclinic structure, leading to omphacite with the space group P2/n. The [110] translational symmetry is lost in the ordering transition so that antiphase domains (APDs) with the displacement vector R = j[l 10] can form [323,324]. 

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