Inhomogeneous mica polytypes : 8-layer polytype of the 2M_1 structural series determined by the Periodic Intensity Distribution (PID) analysis of the X-ray diffraction pattern

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Abstract

The stacking sequence of an 8-layer polytype of oxybiotite from Ruiz Peak (New Mexico) has been determined by analyzing the Periodic Intensity Distribution (PID) of the single-crystal X-ray diffraction pattern. The stacking sequence is described by Z symbols (46)<SUB>3</SUB>42 and by RTW symbols (2-2)<SUB>3</SUB>-22. This polytype, represented by Ramsdell symbol 8<I>A</I><SUB>2</SUB>, belongs to the so-called 2<I>M</I><SUB>1</SUB> structural series and consists of a 2-layer 2<I>M</I><SUB>1</SUB> unit repeated three times, with an additional 2-layer unit of the same kind in twin orientation (rotated by 120°). The choice of the proper axial setting has been effective for PID analysis. The cell dimensions, computed on the basis of the refined conventional cell of the 1<I>M</I> polytype from the same sample, are <I>a</I>=5.3Å, <I>b</I>=9.2Å, <I>c</I>=79.6Å, α=90°, β=91.3°, γ=90°. The geometrical analysis of a previous report of the same polytype unambiguously shows that the original diffraction pattern corresponded to a 24-layer polytype and not to a twin of an 8-layer polytype, as previously reported. A plausible formation mechanism, involving lateral attachment of two small crystals, is proposed.

Journal

  • Mineralogical Journal

    Mineralogical Journal 21(3), 103-118, 1999-07-01

    Japan Association of Mineralogical Sciences

References:  69

Codes

  • NII Article ID (NAID)
    10008773494
  • NII NACSIS-CAT ID (NCID)
    AA00739590
  • Text Lang
    ENG
  • Article Type
    ART
  • ISSN
    05442540
  • Data Source
    CJP  J-STAGE 
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