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

Published online by Cambridge University Press:  02 July 2018

Maki Hamada ,
Masahide Akasaka  and
Hiroaki Ohfuji
Maki Hamada*
Affiliation:
School of Nature System, College of Science and Engineering, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan
Masahide Akasaka
Affiliation:
Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University, Matsue 690-8504, Japan
Hiroaki Ohfuji
Affiliation:
Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
*
*Author for correspondence: Maki Hamada, Email: hamada-m@se.kanazawa-u.ac.jp
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Abstract

K-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.

Keywords

K-rich nephelineX-ray structural analysistwinning by merohedry
Type
Article
Information
Mineralogical Magazine , Volume 83 , Issue 2 , April 2019 , pp. 239 - 247
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Daniel Atencio

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