Gyulai Iván - Szakáll Sándor szerk.: Natura Borsodiensis I. (Miskolc, 1986)

DÓDONY ISTVÁN-TAKÁCS JÓZSEP: A vörösvágási nemesopál színjátéka

PLAY OP COLOURS OP NOBLE OPAL IN VÖRÖSVÁGÁS Abstract Applying the investigation results under electron microscope and polarisation microscope supplied with universal stage of Fedorov, this work reports the structural bases of the play of colour of the noble opal from Vörösvágás /oervenica, E-Slovakia/ and makes comparison with the structures of the milk opal from Vörösvágás, of the Australian opals and of the recently discovered noble opal from Telkibánya /NE-Hungary/. In the noble opal from Vörösvágás, in addition to the X-ray amorphous microstructure, a so-called secondary structure consisting of angular morphological elements fitted periodically in space by triclinic symmetry is developed which can be discovered by etching by HP. The generation of this secondary structure was traced back to the chemical composition of the material filling continuously the space. The periodic inhomogeneity falling to the order of magnitude of the light wavelength and the optical diffraction due to the interaction with the white light a direction-dependent change of colour is produced which is represented by coloured spots of different orientation. Consequently, the noble opal shows anisotropy and measurable 2V in thin section. In the sample from Vörösvágás each colour spot consist of parallel strips similar to the polysynthetic twin lamellae of albite, where every second strip changes its colour simultaneously. The features of the secondary structure of the noble opal from Vörösvágás being different from those known so far provide the possibility to distinguish it from the material of other localities. When having generalized our investigations it has been stated that in all minerals the effect of optical diffraction superponed to the optical parameters should be taken into account, in which periodical inhomogénéities falling to the wavelength range of the light are existing. ,