Origin of low-symmetry growth sectors in edingtonite and yugawaralite, and crystal structure of the k{011} and v{120} sectors of yugawaralite

Abstract

Edingtonite and yugawaralite showing sectoral textures were studied by polarized optical microscopy and X-ray analysis. In edingtonite, the m{110} sector (2Vα = 22°) is optically triclinic and the c{001} sector (2Vα = 52°) is orthorhombic. In yugawaralite, the k{011} sector is optically monoclinic, whereas the v{120} sector is triclinic. Their crystal structures were determined. The results of refinement showed that the space groups of the k{011} (Rw = 4.5%) and v{120} (Rw = 5.1%) growth sectors are monoclinic Pc and triclinic P 1, respectively. In the v{120} sector, several interatomic distances, bond angles and site occupancies are different with respect to a symmetrical plane of the structure, and therefore the monoclinic c glide is extinct. Thus, the X-ray symmetry correlates with the optical one. From the relationship between the surface and internal texture, the symmetry and sector can be explained by cation (Al/Si) ordering during non-equilibrium crystal growth.