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

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

Characterization of the minerals Feldspar — Two different types of occurrence of plagioclase could be distinguished. In rocks of trachytic texture there are plagioclase microlites in form of narrow laths; in the others almost isometric grains are to be found. This difference in shape indicates different rates of cooling. The fluidal nature is characteristic for the trachytic texture of the effusive lava flow. The porphyric plagioclases can be divided into two main groups. 1. Grains of oblong cross section, delimited by cleavage planes with re-entering angles due to polysynthetic twinning. The twins contain less inclusions than usual. These crystals are elongated according to axis ,,c" 2. Idiomorphic crystals, on which beside the (010) face also the (110) and (1Î0) faces are strongly developed. The core of these crystals is of higher An-content. These grains are finely zoned and the shape more varied than in the case of the previous group. In sample 2/75 could be observed an idiomorphic plagioclase crystal the core of which had been slightly elongated according to the normal of the (010) face; later the crystal changed into an almost isometric one. This means increased growth in the direction of axis „c". It is difficult to calculate the average An-content (which, in principle, would be important from both the pétrographie and crystallographic points of view), because of its very large variation range. One has to reconstruct the spatial shape of the grains from a statistical number of measurements made in the plane, in order to calculate the volume from the width of the individual zones. This calculation was performed as an example for two samples only, because in this particular case the calculated average is of little utility in deciphering the genesis of the given rocks. The most basic twin member contained 90-95 An, while the most acidic one only 57%. In all cases from the core upward a steady decrease in the An-content was observed down to 60-72%. Then an inverted zoning followed, resulting in an increase of the An-content by 6-8 %. Such a wide range of data can be connected with rocks ranging from dacite to basalt. Accordingly, in this particular area it is hardly possible to draw any conclusions from the measurement of the An-content of the plagioclases. However, it would be an ulterior task to search for the reason of this extreme variation. In all samples predominate the Albite-Carlsbad-Roc Tourné twin laws. The Albite-AlaAlbite-Ala triple twin law is very rare in spite of the fact that both twin law triples reward the same crystallographic plane (namely the (010) face). Geometrically the probability of occurence would be the same for both twin law triples. It is known from the literature, that the first one is, however, much more common. In this particular case the second twin law triple is extremely subordinate. The Banat type intergrowth of the Baveno twin as well as the Börzsöny and Visegrád twin laws occur in most of the samples in percentages varying from 4 to 11%. Several observations support the earlier theory forwarded by the present author and H. VINCZE in 1973 relating to the origin of twinned plagioclase crystals. Fig. 1. shows the second measured plagioclase phenocryst of sample 1/71 as a fine example of primary intergrowth. The core of the first twin member contains 75% anorthite. The An-content decreases across the zones down to 60% An outward. Then the crystal got intergrown with the second member. The core of the latter had 85% anorthite, while the external zone of the single crystal only 62%. The shape of the first individual changed after intergrowing. Also the shape of the first individual got changed. Beside the intergrowth plane, the (201), and the (110) faces can be measured, delimiting the first single crystal. After the intergrowth, a new face, (001) developed. Later, however, all these faces disappeared and (010) and (001) faces bound the crystal. The (110) face can not be measured. The crystal is probably hypidiomorphic. This is one, but presumably a not very frequent way to originate twinning. There are also arguments in favour of the idea that the polysynthetic plagioclase crystals