AbstractOptically anisotropic and isotropic haiiyne has been observed in phonolite from Niang- niang Hill, Jiangsu Province, southeastern China. Selected-area electron diffraction (SAED) and transmission electron microscopy (TEM) studies show the anisotropic haiiyne has a one-dimensional superstructure with a period of 6dI10subalong one of the<110) directions of the ideal cubic haiiyne subcell. The superstructure may be considered as a periodic arrangement of (110) layer domains with P23 symmetry along [110]. Neighboring domains are related by a twin relationship. The unit-cell parameters for this haiiyne superstructure are thus metrically orthorhombic, with asup= 2d1̄10sub,bsup= 6d110sub, Csup= csub, but the most likely space group is monoclinic, Pn. SAED patterns and TEM images of two opti­cally isotropic haiiyne samples show they have either a three-dimensional superstructure or no superstructure. The average structure of the isotropic haiiyne with the superstructure is cubic, even though it is composed of twin domains with one-, two-, and three-dimen­sional superstructures. The superstructure may have formed by ordering Of[Na3Ca(SO4)]3+and [(Na,K)4(OH)]3+clusters in the cubo-octahedral cages of the haüyne framework during an arrested phase transition from P4̄3n to P23 symmetry.