Phacolite from Mitaki, Sendai City, Japan, has a core of penetration-twinned chabazite. The crystal has a convex (0001) face on which many twin individuals are produced by a two-dimensional nucleation mechanism, resulting in complicated twinning during growth. Both the r and the r¯ (101¯1) faces repeat finely during growth because of twinning, resulting in {112¯3} sectors with polysynthetic twinning. Finally, single structures nucleate on the crystal edges [211¯] and develop along the (112¯3) face, penetrating each other near the centre of the face.