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

Summary : The twinning axis to be determined has coincided with the face normal of the (110) and (130) faces, resp.with their left forms within a limit of error of 3° in general, but mostly within 1°. In spite of neglecting the measurement of the composition plane the conclusion can be drawn that normal twins were observed according to these faces. Irregular composition occurs in case of other types of normal twins, too, e.g. in case of the Baveno twins of Bánát-type, or sometimes in that of the Albite twins. Having performed the transformation of co-ordinates the Euler angles proved to be easily controlled. On this basis the position of the optical main vibration directions excellently approached the standard values given to the adequate anortite content . In case of all the twins mentioned above the [ooi] zone axis of the first member coincides, of course, with the [OQlj zone axis of the other twin member. In case of 80 per cent anortite content the twins according to (110) and (130) are easily recognizable about the fact that the n^vibration direction of the one member nearly coincides with (OlO) face normal of the other. In case of the same anortite content it is characteristic of the twins according to (130) and (1Ï0) that the twojvv^jvibration directions are only to several degrees from each other. Among the twinning laws according to (llO) and (13O), resp. according to their left forms such reguliarities were observed the evidence of which is now under preparation, thus it will be published later. The above mentioned twin laws have not special names yet, the designation "prizm" would not be correct for triclinic feldspars. The twins of typical occurrence and considerable quantity deriving from the investigated area are worthy to be termed.