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

Noske-Fazekas, G.: Statistical investigations of plagioclase twin laws in lavas erupted between the Danube and the Central Börzsöny Mountains (Hungary)

Fig. 2. Comparison of the frequency distribution of albite and carlsbad twins of the lavarocks from the Börzsöny Mountains. Numbers on the abscissa: 1 = pyroxene andésite, 2 = amphibole — pyroxene andésite, 3 = amphibole andésite, 4 = oxyhornblende andésite, 5 = biotite andésite, 6 = carbonatic amphibole andésite, 7 = inclusion of carbonatic amphibole andésite, 8 = dacite exact determination of the Na/Ca ratio gives a true picture of the basicity of the rock as a whole. Regular variation of plagioclase twin types, a phenomenon insufficiently known until now is equally of genetic significance. The plagioclases in andésites and dacites are present both as phenocrysts and as constituents of the groundmass. The author's universal stage investigations were devoted to porphyric plagioclases that are readily measurable. Notably, as staded by Gorai, there is no substantial difference in composition between the small grondmass plagioclases and the porphyric feldspars. These are almost always twinned, a feature characteristic of plagioclases. Our investigations included both the so-called simple plagioclase grains consisting of one pair of twin and the intergrown penetration complexes. The plagioclases largely vary in freshness both by localities and rock varieties. Micaceous, argillaceous and, less frequently, carbonate decomposition is a common phenomenon which, coupled with the presence of primary inclusions, has rendered the optical observations more difficult. Characteristic of the feldspars of the investigated area is the absence of cleavage, hence the increased importance of twin laminae as crystallographic orientations in universal stage measurements. Zoned plagioclases are not common. The difference in An content between the core and the rim surrounding it may attain an extreme value of 20%. Whiie studying andésite and dacite feldspars, we measured a total of 1027 intergrowths on these, of which in 55 cases the intergrown individuals disproved to be twinned crystals. The distribution of 972 twinned crystals in terms of twin laws is shown in Table 2. It will be enough to cast a glance at it to notice that in andésites and dacites the palgioclases obeying the albite law are most frequent (minimum 30%, maximum 47%). Next to follow in fre-