15. EXPERIMENTAL AND THEORETICAL APPROACHES TO IRIDESCENT LABRADORITE :

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  • MIURA,YASUNORI
    Department of Mineralogical Sciences and Geology, Faculty of Literature and Science, Yamaguchi University
  • TOMISAKA,TAKESHI
    Department of Mineralogical Sciences and Geology, Faculty of Literature and Science, Yamaguchi University
  • KATO,TOSHIO
    Department of Mineralogical Sciences and Geology, Faculty of Literature and Science, Yamaguchi University

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Abstract

By applying optical and electromagnetic theories, the theoretical approaches to the optical interference in the lamellar structure of labradorite are made on the basis of experimental results. The results of calculation by the computer simulation model of labradorescence are consistent with the experimental results. They are also consistent with BφGGILD's rule and the observation by NISSEN (1971). The regression equation for lamellar thicknessd (Å) and wavelength λ(Å), where maximum of reflected intensity is taken when the angle of incidence θ_m changes from 20° to 90°, is obtained as follows: λ=3.128・(d)-47.798 (standard deviation:σ=0.999). This equation suggests, together with other regression equations, that the kink point at An_<35> and the concentrated point at An_<56> may be explained as submicroscopically homogeneous point highly ordered with respect to Al-Si arrangement. The other kink point at An_<50> may be explained as the reverse point of alternate lamellar thicknesses. The relation between color and lamellar thickness in peristerite, cryptoperthite and opal may be also explained by this regression equations.

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