ペリステライトのイリデッセンス : ペリステリズム

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Iridescence observed in peristerite stone, i. e., peristerism, has been studied by theoretical calculations on the basis of experimental results. From spectral distributions of peristerism and electron microscopical observations, it is found that peristerite from pegmatite, plutonic and metamorphic rocks exhibits exsolution lamellae structure producing iridescence with compositions from An_5 to An_13. Characteristic colour changes from violet to red as anorthite (An) content decreases, whereas whitish colour is produced with compositions from An_7 to An_10. From microanalysis of lamellae with ion microprobe mass analyzer (IMA-2) confirmed that the mean chemical compositions of An-poor and An-rich lamellae are An_<2(2)> Or_<(1)> and An_<21(2)> Or_ <2(1)>, respectively. The mean composition difference between adjacent lamellae is 19 percent An and 1 percent Or. As the boundary roughness of lamellae increases, more diffuse reflection is produced resulting in whitish colour near composition An_<10>, where spinodal exsolution-structure cannot coarsen and "tweed textures"are existed. The results of calculations based on the two models which have already discussed in labradorescence agree with the experimental results. The calculated shift of peak maximum of intensity reflectance, 1.2 (nm/degree), is consistent with the experimental value. 0. 8-1. 0 (nm/degree). Therefore, peristerism is the interference phenomenon of ref1ected waves by exsolution lamellae which consist of two periodically stacked multilayer. It can be understood that whitish colour of peristerism is due to the irregularity of lamellae-periodicity and wavy boundary of lamellae.
1978-06-15

ペリステライトのイリデッセンス : ペリステリズム

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