Dr. Nagy I. Zoltán szerk.: Fragmenta Mineralogica Et Palaentologica 4. 1973. (Budapest, 1973)
member. On the basis of the Burri - Parker - Wenk tables (1967) the anortite percentage from the measurement data as well as the twinning laws within each complex were determined. The determination of the fact that either Albite or Manebach twinning laws can be found in the complex twinned according to the normal law was ambiguous only in case of 75 to 85 per cent anortite content. Thus no data are published about the grains in case of which the possibility has arisen that the (001) face was explained as a (010) one. Consequently, from this range of anortite content only those plagioclase crystals will be introduced in case of which either the Carlsbad or Roc Tourné twinning laws occur at both complexes, or in addition to the composition plane other crystal-morphological direction proved to be measurable . When designing and computing the Carlsbad twinning axis there arose some difficulties, i.e. in case of 80 to 85 per cent anortite content the direction of the [riß] main refraction index of the two components is close to each other and this resulted in an elongated triangle of error. This problem of the plagioclases is well-known from the literature. Thus, the bisectrix unit vector of the adequate bisectrix of|n a J andjjXyTJ directions was taken into account and this was considered to [P0l3 zone axis when being to 90° from the acute bisectrix of thejiiß^j main vibration direction within a limit of error of 0.5 per cent. In case of the same anortite content the £npj and JriyJ main vibration directions of the two members lie on the same zonesphere; by only designing resp. computing of the bisectrix the position of the twinning axis cannot be determined. When the Roc Tourné twinning law also occurs in the complex, the proper bisectrix is choosen on the basis of the knowledge of the [Oön zone sphere. In lack of the Roc Tourné twinning law the determination of the Euler angles makes definite the position of the [boil zone axis. Having explained the twinning laws within the complex the fact should be determined that anyone of the members of one complex lies in twin position with anyone of the other or not, and in the former case what crystallographic direction coincides with the twinning axis. We tried to verify
