Crystal chemistry of K-rich nepheline in nephelinite from Hamada, Shimane Prefecture, Japan

Abstract

AbstractK-rich nepheline with a structural formula of A2B6T14T24T34T44O32 (Z = 1) within melilite–olivine nephelinite from Hamada, Shimane Prefecture, Japan, was investigated to clarify its crystal structure and to determine cation distributions in the A and B structural positions of structural channels and tetrahedral T1–T4 sites. The chemical formula of a single-crystal sample was (Na5.437K2.248Mg0.034Ca0.031)Σ7.750(Si8.332Al7.445Fe3+0.158Ti0.009Cr0.005)Σ15.949O32, which results in 65.2, 27.8, 2.1, 3.2 and 1.6 mol.% NaAlSiO4, KAlSiO4, NaFe3+SiO4, □Si2O4 and □0.5(Ca,Mg)0.5AlSiO4 end-member components, respectively, where □ is a vacancy. X-ray diffraction data of a single crystal with dimensions of 0.28 mm × 0.15 mm × 0.05 mm measured at 296 K indicate the space group P63. In the structural refinement, the R1 factor was reduced to 3.69% by taking twinning by merohedry into the refinement. The refinement accounted for 77.7% of the absolute structure and 22.3% of the a and b axes reversed absolute structure. The atomic populations determined in the A and B positions were 1.834 K + 0.166 □ and 5.705 Na + 0.198 K + 0.031 Ca + 0.034 Mg, respectively, implying the substitution of K for Na in the B position. The a and c dimensions are a = 10.0270(3) and c = 8.4027(3) Å. The average <A–O> and <B–O> distances are 3.009 and 2.65 Å, respectively. The substitution of K for Na in the B channel results in increased volume and bond-length distortion of the BO8 polyhedra, which then reduces distortion of the AO9 polyhedra. The average T–O distances indicate that the T1 and T4 sites are essentially filled with Al, whereas the T2 and T3 are filled with Si. Despite the deviation of the O1 oxygen from the triad axis and the combination of K+ ions and vacancies in the hexagonal channels, an incommensurate structure was not observed in the X-ray diffraction data or using the electron diffraction technique.