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

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

Mechanical twinning may have taken place in some cases, but it can not be common. Most of the plagioclase crystals contained by the andésite rocks are twinned. In this case of young volcanics, this general phenomenon can not be attributed to mechanical stresses provoked by tectonism. The importance of the Börzsöny and Visegrád twin laws lies in the circumstance that these penetrating twins gave rise to the idea that monoclinic plagioclase structure may exist in nature. This is supposed to exist in the case of basic, or neutral plagioclases in a very narrow range of high temperature. With cooling, it would turn unstable and change into triclinic symmetry, by means of twinning. By producing experimentally this hypothetic monoclinic phase, it would provide an exact thermometer for the genesis of andésites which contain Börzsöny, Visegrád and Baveno-Bánát type twin laws. It is hardly a mere chance, that all these have been described from andésites of the Carpathian Basin. The plagioclase phenocrysts are but rarely altered. In some samples they have been transformed into clay minerals ; in two samples numerous columns of zoizite can be observed around the decayed feldspars. In one rock subsequent silcification made the feldspar unrecognizable. Amphibole — The amphibole crystals occurring in the samples from Kerekhegy turned out to be basaltic hornblende and (subordinate) common hornblende, on the basis of their optical characteristics. In one sample ferrihastingsit could be inferred. In all samples of Vizesárok valley fresh grains of hornblende have been found. In contrast to the other andésite agglomerate occurrences of the Börzsöny Mountains the twin on (100) is represented very scarcely, in form of a narrow twin member in the middle of the grain. Other types of intergrowth are more common; however, most of the grains are single crystals. Three types of intergrowth have been observed which are not known in the literature. These will be described in another paper in the journal Annales Universitatis budapestinensis de R. Eötvös nom., sect. geol. It is much more difficult to measure and to evaluate amphiboles by means of U-stage than it is in the case of plagioclase phenocrysts. The margin of error is much larger in measuring the cleavage planes, an average can be calculated from 15-20 measurements at least. Beside the good clevage planes there are also others (not necessarily in the zone of axis „c") which make difficult to determine the crystallographic orientation. The inner conic refraction disturbs considerably the measurement of r\ß and of the optical axes. The own colour of amphibole modifies the interference colours. Accordingly, a great many measurements on numerous thin sections are required for obtaining statistically évaluable data sets. Because of the multiple variations in composition, such statistical optical measurements are worth to be carried out only if accompanied by electron microprobe test. In the amphibole crystals collected from Kerekhegy the angle between the ny and axis "c" varies from 2° to 5° from 12° to 15°, while in the case of the samples collected in the Vizesárok valley the same angle ranges 5-6°. Fresh amphibole occurs exclusively in the Vizesárok samples (and in sample 10/71). In the others the amphibole crystals are more or less altered. In some cases magnetite, haematite, maghemite, small pyroxene and plagioclase crystals together make up pseudomorphs after amphibole. Pyroxene — Pyroxenes occur in form of idiomorphic, hypidiomorphic or xenomorphic crystals. The pyroxene crystals richer in iron are longer columns than the others and are mostly idiomorphic. The Mg-richer pyroxenes are often hypidiomorphic grains. In the latter case the symmetry can be established only by means of U-stage measurements. In the literature abound data obtained from microscopical examination without U-stage. In order to check the reliability of such data, I have examined 15 pyroxene grains relying upon the symmetrical extinction. Out of the 15 grains of oblique and asymmetrical extinction, 14 turned out during the U-stage studies to be of orthorhombic symmetry. (One of them could not be evaluated due to very unfavourable orientation.) Figs. 2 and 3 show one of these crystals. An opposite error is also