The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

Densities of nucleation sites of clinopyroxene, plagioclase and spherulite were measured and their morphologies were observed from the core to the outer parts of a pillow lava of tholeiitic basalt collected at Awa-Kamogawa, Boso Peninsula. Systematic changes of nucleation densities and morphologies were observed, which were interpreted in terms of the sample's cooling history. It was concluded that the lava had solidified within an hour, and that changes in nucleation densities and morphology represented the events of this short period.

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Phlogopites in phlogopite-diopside pegmatoid (KS) and apatite-phlogopite carbonatite (TT) from Palabora Igneous Complex, South Africa show reverse pleochroism: i.e., in KS, X=pale yellow-orange>Y=Z=colourless to very pale yellow; in TT, X=deep brown>Y=Z=pale brown to yellow brown. Polytype structures of both phlogopite samples are of 1M in majority. These phlogopites are chemically characterized by the deficiency of TiO₂ and Al₂O₃, and by the entrance of Fe³⁺ in the tetrahedral site. The ⁵⁷Fe Mössbauer spectra of these phlogopites have been measured on oriented flakes and powdered samples at room temperature. Those spectra are resolved into two quadrupole splitting doublets of Fe²⁺ and Fe³⁺ using a technique of the Lorentzian curve fittings. It is found that Fe²⁺ occupies preferentially the octahedrally coordinated M2 site, whereas Fe³⁺ occupies preferentially the tetrahedrally coordinated T site.
The hyperfine parameters of those ions are nearly identical with the values already reported. Asymmetric quadrupole splittings, however, were clearly observed in the doublet of Fe³⁺. In oriented KS flake, cleavage plane normal to incident gamma-ray, the area of lower velocity peak is 1.88 times as much as the area of higher velocity peak, whereas this relation is quite reverse in oriented TT flake. Therefore, it is possible to estimate that the sign of electric field graident at Fe³⁺ nucleus in KS is negative, while its sign is positive in TT sample. On the other hand, for Fe²⁺ spectra, the area of each component in the doublet is nearly constant. Asymmetry in the line width is also observed in both iron spectra. The reverse pleochroism may be related to these asymmetries, which are caused by the localized distribution of electron.
The Fe³⁺/Fe²⁺ ratios derived from both the Mossbauer absorption areas and the chemical analyses are not necessarily accordant to each other, because of inevitable preferred orientation in the absorber of phlogopite.

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