Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 72. (Budapest 1980)

Örkényi-Bondor, L.: Andesite agglomerate from Zebegény village, Börzsöny Mountains (Hungary)

Fig. 3. The above pyroxene can be identified as an orthorhombic crystal possible. A monoclinic pyroxene shows symmetrical extinction if the section is made in the zone of axis „b". (In this case the mistake is due to the monoclinic mirror plane.) The measured orthorhombic pyroxene crystals displayed regular orientation. Axis "a" coincides with n^, axis "b" with with n

œ

, and the direction of axis "c" with n

y

. The angle of 2V in some cases was less than 90° (68-90°). The anomalous low values can not be interpreted in terms of chemical composition, because 2Voc would have to be 90° independently of the Fe/Mg ratio. The latter can be deduced from the refraction indices. These were determined by U-stage applying the Nikitin method. The Mg-rich clinohypersthene is mostly hypidiomorphic, while the augites are xenomorphic. This is in harmony with the presupposed sequence of crystallization. Beside the refraction indices the optical orientation in thin section also prove to be useful in identifying the monoclinic pyroxenes. Chemical composition The silica content varies between 51.83 and 57.90%. This range is narrow enough. Accordingly, the Zebegény andésite agglomerates are rather uniform as regards acidity. These data are the nearest to those of the andésites known in the Nagybörzsöny area. They resemble the olivinbronzite andésite (the more acidic samples to a Japanese bronzite-andesite), if compared to the data published by JOHANNSEN. In comparison with further Japanese andésites, they stand close to some olivine-pyroxene andésites. Accordingly the SiO, content of the Zebegény rocks is lower, than that of other andésites which