Crystals of merwinite were prepared at 1550 °C from chemical reagents, and their intracrystalline microstructures were examined by the combined use of x-ray diffraction and optical microscopy. The crystals were composed of pseudomerohedral twins. The adjacent twin domains were related by the pseudosymmetry two-fold axis parallel to ⟨011⟩with the composition surface {811} The overall twin structure was constructed by introducing the pseudo-symmetry three-fold axis normal to (100), which must originally be a symmetry element of the former high-symmetry phase. The transition from the primitive trigonal (point group 3m) to the primitive monoclinic (space group P21/a) was accompanied by the combination of reducing the order of the point group and the change in the size of the unit cell. The order of the point group was reduced from 12 to 4, resulting in three twin domains with six different interfaces. This accounted for the experimentally observed microstructure consisting of repeated lamella twins in several orientations. Because the unit lattice translation would be lost during the transition, the formation of antiphase domains was expected. The lost translation vectors were 1/2[011], 1/2[100], and 1/2[111] resulting in four antiphase domains. As a result, the total number of domains possible in the transition was 3 × 4 = 12.