Dr. Nagy I. Zoltán szerk.: Fragmenta Mineralogica Et Palaentologica 4. 1973. (Budapest, 1973)

Fragin. Min. Pal. 4. 1973. Observation of Twinning Laws According to (110), (110), (130) and (130) in Plagioclase Crystals Lívia ÖRKÉNYI BONDOR — Helga VINCZE SZEBERÉNYI Hungarian Natural History Museum Mineralogical-Petrological Department, Budapest ABSTRACT : In the slides of the andésites deriving from the Visegrád and Börzsöny Mountains authors observed normal plagioclase twinning laws according to (110), (130), (lTO) and (13~0). The anortite content of these plagioclases has been 60 to 85 per cent.The composition of characteristic figure are shown in the photos and drawings. In these compositions the Albite, Carlsbad and Roc Tourné twinning laws always occur. The exactness of the determination of the twinning laws was proved by the vector geometric compution of the measurement data obtained by Utable. Authors observed the above-mentioned twinning laws in two mountains of the Intra-Carpathian volcanic arc. The uncommon but regular intergrowths were observed by Vincze-Szeberényi in 1971 in the course of elaboration of the andésite inclusions of the Visegrád Mountains, and by Örkényi-Bondor in 1972 in the andésite agglomerate of the Southern Börzsöny Mountains. The characteristic twin crystals occurred during the microscopic investigations and they could not be identified with any of the plagioclase twinning laws. Their frequency proved to be much higher than to be reckoned as accidental intergrowth. Since these occurred in the rock samples of both mountains the joint solution of the problem was initiated. As a result of this research the presence of the normal twinning laws according to (110), (110), (130) and (130) was demonstrated which have not been satisfactorily evidenced in case of plagioclases. The law on the face (130) of the orthoclase crystals was published by CF. Naumann (1830). The law according to (110) of