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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)342 and by RTW symbols (2-2)3-22. This polytype, represented by Ramsdell symbol 8A2, belongs to the so-called 2M1 structural series and consists of a 2-layer 2M1 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 1M polytype from the same sample, are a=5.3A, b=9.2A, c=79.6A, 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.

mineralogical journal

Inhomogeneous mica polytypes. 8-layer polytype of the 2M1 structural series determined by the Periodic Intensity Distribution (PID) analysis of the X-ray diffraction pattern.