Crystal chemistry of K-rich nepheline in nephelinite from Hamada, Shimane Prefecture, Japan. (2nd July 2018)

• Record Type: Journal Article
• Authors: Hamada, Maki; Akasaka, Masahide; Ohfuji, Hiroaki
• Is Part Of: Mineralogical magazine. Volume 83:Number 2(2019)
• Page Start: 239
• Page End: 247
• Publication Date: 2018-07-02
• Subjects: K-rich nepheline, -- X-ray structural analysis, -- twinning by merohedry; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2018.133
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: K-rich nepheline with a structural formula of A 2 B 6 T 14 T 24 T 34 T 44 O32 ( 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 (Na5.437 K2.248 Mg0.034 Ca0.031 )Σ7.750 (Si8.332 Al7.445 Fe 3+ 0.158 Ti0.009 Cr0.005 )Σ15.949 O32, which results in 65.2, 27.8, 2.1, 3.2 and 1.6 mol.% NaAlSiO4, KAlSiO4, NaFe 3+ SiO4, □Si2 O4 and □0.5 (Ca, Mg)0.5 AlSiO4 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 63 . 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) 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 B O8 polyhedra, which then reduces distortion of the A O9 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.