Structure comparison of co-crystallized 6- and 12-sided large cancrinite crystals

Single crystals of hexagonal and dihexagonal morphology cancrinites, crystallized in the same batch reaction with sodalite, have been characterized by X-ray diffraction analysis. Although the unit cell values are almost identical, Si-O bond lengths are greater for the di-hexagonal form (1.650 Å vs. 1.624 Å) and the Al-O bond lengths are shorter (1.710 Å vs. 1.724 Å), possibly indicating greater T-site Si-Al disorder in the di-hexagonal form. ²⁷Al-MASNMR also indicates that the hexagonal form contains minor occluded Al (OH)₄^- ions (as does the co-crystallized sodalite). In addition to forming homogeneous single crystals, the di-hexagonal form frequently occurs as an overgrowth on hexagonal crystals, indicating that it results from a secondary crystallization, possibly coincident with the dissolution of sodalite coincident with the final phase of fractional crystallization.