“…For texture symmetry groups which do not belong to the 32 crystallographic point groups, one of the Curie groups oo oo m, oo oo, oo/mm, oo m, oo/m, oo 2 or oo is more appropriate. This can be illustrated by considering polar glass ceramics with conical symmetry (Gardopee, Newnham & Bhalla, 1981). A glass can be crystallized under a strong temperature gradient such that polar crystals grow like icicles into the interior from the surface.…”