Abstract
Two kinds of sectoral twinning are observed in epistilbite: one is that of a structure with topologic symmetry C2/m, and the other is due to (Al,Si) ordering. Two-dimensional atomic arrangements are similar on the (001) and (101) surfaces of epistilbite. Of the two kinds of atomic arrangements, the one that is presumably more stable will develop on both crystal surfaces; precession photographs have confirmed that this is the (101) arrangement. The (101) face is normal to the (010) mirror plane in C2/m symmetry, and therefore the two symmetry sites are equivalent on the (101) growth surface, resulting in an (Al,Si)-disordered, monoclinic structure in the {101} sectors. Since the (110) faces are inclined to the mirror plane, the two symmetry sites in the crystal are not equivalent on the surface, resulting in an (Al,Si)-ordered, triclinic structure in the four {110} sectors, which are related by both the (100) and (010) sectoral twin planes.
