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=5.3Å, b=9.2Å, c=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.

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Synthetic single crystals of forsterite shocked up to 82.0GPa have been examined by the profile analysis of photo-luminescence spectra. The forsterites give rise eight broad bands in photo-luminescence spectra in contrast to unshocked forsterite having usually faint and obscure luminescence. Their intensity increases linearly with increasing the pressure up to 46.2GPa, while over the pressure the intensity abruptly decreases. The intensity variation also has a close relationship to Raman band shift and broadening, implying the variable degree of lattice deformation in shocked forsterites. The critical shock pressure of 46.2 GPa nearly corresponds to the onset shock pressure of phase transition (50GPa) on the Hugoniot curve of forsterite (Syono et al., 1981). Luminescence mechanism in the shocked forsterite may be the emission transition between the deformed electron levels with vibrational sub-levels in its [SiO₄]⁴⁻ molecular orbitals. This is a new type of photo-luminescence in silicate minerals that can be called as the deformation luminescence.
The correlation between the intensity of photo-luminescence and shock pressure loaded on the forsterites is formulated, and applied to estimate the shock pressure of Dhurmsala LL6 chondrite; it ranges from 26 to 29 GPa.

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