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

orthoclase was proved by K. Haushoffer (1879), H. Klockmann (1882), H. Laspeyres (1877), and recently by J. Drugman (1927, 1938). The two laws of the monoclinlc feldspars were collectively designated to "prism" law. In case of tri clinic symmetry right and left laws should be distinguished according to both forms and the name "prism" is irrelevant. In case of plagioclases the intergrowth, where the groups twinned according to the Albite and Carlsbad law include an angle of about 60°, was supposed to be a twin crystal on the face (130). Such intergrowth was observed by M. Belowsky (1892), A. Klautzsch (1893) in the rocks of Equador, by P. Heineck (1903) in the diabase of Hessen and by M. Sedlacek (1940) in Persian rocks. On the basis of U-table measurements A. Köhler (1950) concluded to the presence of plagioclase twin on (I30). As far as we know the normal twinning laws of triclinic feldspars according to (llO) and (IÎO) have not been observed yet (C. Burri - R. Parker - E. Wenk: Die optische Orientierung der Plagioklase, 1967, p. 69.). Characterization of the twin crystals The plagioclase crystals intergrown both according to the (110) and (130), and according to their left laws are characterized by the same occurrence. Microscopically, these occur among the rock-formig plagioclases as porphyric components of 200 to 1555 (j. of the intermediary volcanic rocks. The groups consisting of twin members intergrown on the (OlO) plane include an angle of about 60° with each other and they are mostly similar to the Greek letter A* Rarely, the two groups being of oblique crossshape totally penetrate each other and they form penetration twins. The fact that the twinning laws mentioned above would exist alone never occurred. Any one of the groups including an angle