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

Abstract

K-rich nepheline with a structural formula of A 2 B 6 T 1 4 T 2 4 T 3 4 T 4 4 O 32 ( 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 T 1– T 4 sites. The chemical formula of a single-crystal sample was (Na 5.437 K 2.248 Mg 0.034 Ca 0.031 ) Σ7.750 (Si 8.332 Al 7.445 Fe 3+ 0.158 Ti 0.009 Cr 0.005 ) Σ15.949 O 32 , which results in 65.2, 27.8, 2.1, 3.2 and 1.6 mol.% NaAlSiO 4 , KAlSiO 4 , NaFe 3+ SiO 4 , □Si 2 O 4 and □ 0.5 (Ca,Mg) 0.5 AlSiO 4 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 P 6 3 . In the structural refinement, the R 1 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) A. The average A –O> and B –O> distances are 3.009 and 2.65 A, respectively. The substitution of K for Na in the B channel results in increased volume and bond-length distortion of the B O 8 polyhedra, which then reduces distortion of the A O 9 polyhedra. The average T –O distances indicate that the T 1 and T 4 sites are essentially filled with Al, whereas the T 2 and T 3 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.