Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 66. (Budapest 1974)

Örkényi-Bondor, L.: The plagioclase crystals of the andesite agglomerate from the Csattogó Hill in Börzsöny Mountains (Hungary)

even the fine-zoned feldspars do not include groundmass either, but have apatite neddles (cf. specimen 27/1973). Some rocks have a considerable amount of clay minerals. In this case the Porphyrie feldspars and the groundmass has also began to undergo transformation to yield clay minerals. Brown clay minerals can be seen inside of several larger plagioclase crystals. It was observable oriented intergrowth of biotite and intermediate plagioclase. This area differs from the neighbouring areas first of all in the anortite content of the plagioclase. There are also some differences in the distribution of the twin laws. Table 1 shows the mark of the rock specimens, the number of the plagioclase measured by the Fedorow method, the individuals of the twinning, the anortite percentage, and the twin laws. The same individual can form two or three twinning with the neighbouring individuals, but according to the symmetry-law twinning can be formed by individuals, which are not adjoinig. Correlation between the size, the shape, and the anortite content of the plagioclase crystals was found. Figures 1 and 2 show Baveno twinning from the less grain size. The avarage anortite content of this area is 66 percent; the lowest was 47, the highest 92 percent. Both extreme values were found in fine-zoned crystals, but not in the same grain. 92 percent anortite was measured in the inner core of a crystal and 47 percent in the middle zone of an other grain. In the latter case there were further two zones proceeding outward, which have 62 percent anortite like in the core of the crystal. Figure 5 shows a crystal with avarage anortite content twinning by Albite, Carlsbad and Roc-Tourné twin laws. There is a little difference between the avarage anortite content of the various rocks, this value is 65—70 percent. Table 2 shows the avarage anortite content of the specimens. The specimen 25/1973 is much more acidic, contains 59 percent anortite. In this andésite type the anortite content of the larger plagioclase crystals is 50—60 percent, while among the smallest crystals it is about 70 percent. This fact shows inverted crystallization. Inverted zones are also observed at plagioclase 7 of specimen 26/1973. Here the anortite is 48 percent in the core, and 59 percent in one of the external zones. Figure 7 shows the distribution of the grain-size of the porphyric plagioclase crystals. Thus, the anortite content is a function of grain size (Fig. 8). The avarage anortite content of the largest grains is not high, because in their outer part there are zones rich in albite. The anortite content decreases with that of the grain size, but at the smallest grains the anortite content increases again. This observation with the inverted zoning is a volcanological problem, however, further proofs are necessary to assume secondary bulk magma. The distribution of the twin laws of the andésite agglomerate originating from the Csattogó hill is as follows: The Albite, Carlsbad and Roc-Tourné twin laws predominate with 85 percent. The Albite law is 35.6, the Carlsbad law is 26.2 and the Roc-Tourné one is 23.1 percent. The Acline, Baveno and Manebach laws are rare, these are by turns 6.6, 2.4 and 1.7 percent. "Some other" twin laws consist of Ala, Albite-Ala, Börzsöny and Visegrád twin laws. These two latter twinning were reported by ÖRKÉNYI & VINCZE (1973). Börzsöny and Visegrád twinnings are 4.4 percent. Figure 6 shows a rather unusual Manebach and Baveno intergrowth. Figure 9 shows the distribution of the twin laws in the single specimens. No considerable difference exists between the specimens in the distribution of the twin laws, so the genesis of these rocks is similar. The Table 3 shows the relation between grain size and twin laws. At the smallest plagioclase phenocrysts, the Albite twin and the non-twinning crystals prédomina-