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

To evaluate the plagioclase measurements the tables published in the work "Die optische Orientierung der Plagioklase" by C. Burri - R. Parker - E. Wenk (1967) were used. In case of the crystallographic computions the vector geometrical method published by C. Burri in his "Anwendung der Vektorrechnung auf einige häufig auftretende Kristalloptische Probleme" (1950) was applied. Using the U-table all the measurable crystall-morphological and optical directions were determined. Depending on the deviation of the received data every measurement was repeated 5 to 15 times. Out of the morphological directions the (OlO) cleavage and twinning composition plane proved to be satisfactorily measurable, the cleavage on (001) was less measurable. This is caused by the fact that the characteristic form of the twin crystal is question appears in the sections nearly perpendicular to the crystallographic "c" axis, where the normal of the (OOl) plane is nearly perpendicular or as steep as the angle "h" is immeasurable. In one of the twin members of the sample No. R. 9. deriving from the Visegrád Mountains the cleavage according to (HO) proved to be satisfactorily measurable and this plane was parallel with the (OlO) plane of the other complex. In the grain of the sample No. Nm. 28. deriving from the Börzsöny Mountains the (llO) plane was measurable in form of growth line and pinacoid face, and it was parallel with the (010) plane of the other complex. This phenomenon was observed already by M. Sedlacek in the Persian andésite semples. In case of the measurement of the optical main vibration directions the zonality of the plagioclases being a frequent phenomenon in the basic effusive rocks caused heavy difficulties. The inclusions of basic substance of the plagioclases and the patchy extinction also disturbed the measurements. Out of the numerous measurements only those data proved to be applicable where these disturbing phenomena did not cause inaccuracy and where out of the narrow zones the measurement of the zones of equal anortite content proved to be successfull in every twin 20