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

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

Fig. 1. Plagioclase twins containing two different cores. — Fig. 2. Pyroxene phenocryst with an extinction angle 6° grew as single ones and secondarily became twinned. This way of twinning was assumed after having determined the Börzsöny and Visegrád twin law. A rational plane of composition being supposed, subsequent twinning is the only possible interpretation in the case of the Börzsöny and Visegrád twinning. Namely, one member of both polysynthetic plagioclase crystals which penetrated one another form Börzsöny and Visegrád twins. Between the other members of the two groups no twin laws occur. Consequently, there is no rational compositional plane in spite of the penetrated twinning. It is the same problem with the case of the Banat type of Baveno twin. It was intended to find out empirically whether the polysynthetic twinning took place after crystallization, when a higher symmetry changed to triclinic one. (ÖRKÉNYI & VINCZE). In fact, several observations support the assumption of subsequent twinning. Single crystals can be observed in some cases (at least, no twin members could be detected under the microscope). Other grains manifest one or two narrow and short twin members at one edge of the crystal, testifying to unstable symmetry. A few polysynthetic twin crystals without re-entering angles could be observed, the original shape of the untwinned crystal being conserved. Even the eventual presence of re-entering angles does not refute the working hypothesis, because it may be due to subsequent twinning. E. g., in sample 1/71 the third measured feldspar can be pointed out as subsequently twinned. Another feldspar in the same sample seems to prove, by chance, this hypothesis (Photomicrographs 5 and 6). The plagioclase grain was broken, one part of it toppled, but the original connection is obvious. Re-fitted, the twin members would not be continuous in the two parts. (Taking into consideration that the axis of toppling and the normal of the thin section do not coincide.) This example proves beyond doubt that polysynthetic twinning is posterior to the breaking up of the single crystal. (Given the fact that breaking up must have been caused by some mechanical force, it is possible that this is a case of mechanical twinning.) Microscoping observations of this type may support the hypothesis, but they do not render experimental testing superfluous.