Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 73. (Budapest 1981)
Embey-Isztin, A. ; Noske-Fazekas, G.: On the chemistry of the large phenocrysts in the tuff of Godóvár (Börzsöny Mts., Hungary)
Results of the chemical analyses Several traverses to earn the knowledge of changes in chemical composition of the zoned phenocrysts have been made by the aid of an ARL-SEMQ automated microprobe in the Naturhistorisches Museum (Vienna, Austria) by courtesy of G. KURAT. Separated amphibole and pyroxene grains were also analysed by the wet analytical method (GY. PITTER, Hungarian Natural History Museum, Budapest). Representative electron microprobe analyses of zoned pyroxene crystals and chemical analyses of separated pyroxene grains are presented in Table 1. Taking a glance at the data, we can see that the span between the outer and innermost zones is surprisingly important. In the core the composition reveal a rather primitive pyroxene with a content of Cr 2 0 3 up to 0.34 per cent* (Fig. 1), NiO up to 0.1 and with FeO (total Fe) as low as 4.2 per cent, as well as mg-value as high as 87.6 (anal. No. 8 in Fig. 2) which proovs to be a diopside (Mg 47 . 7 Fe 68 Ca 455 ) in the triangle of Mg-Fe-Ca (Fig. 1). In contrast, the outermost rim (anal. No. 1 in Fig. 2) has a FeO content as high as 9.8 per cent, mg-value as low as 71.2 with only traces of Ni and Cr, if any. In the triangle of Poldervaart and Hess, it proves to be an augite (Mg 42 x Fe 170 Ca 40 9 ). Between the outermost rim and the primitive core there are zones of intermediate compositions. Analysis No. 3 in Fig. 2a-b represents well such a composition with a mg-value of 76.5 and with 8.2 per cent FeO. It is also an augite (Mg 44 5 Fe 13 . 7 Ca 418 ). Besides compositonal changes already mentioned, Ca increases proceeding from the border zones to the core and so does Na, while there is a slight increase in Si in the outermost zones. In contrast, Al and Ti vary very irregularly but in the outermost zones they tend to show lower contcentrations. What is remarkable, that is the highly sympathetic variation of Ti and Al and in a less pronounced manner that of the Na, while Si seems to vary probably in the opposite direction. Speaking in terms of end-molecules, this may signify varying degrees of minor substitutions of hypothetical molecules such as CaTiAl 2 0 6 (Ti-augite), NaAlSi 2 0 6 (jadeite), and CaAl 2 Si0 6 (Ca-Tschermak's molecule). In addition, some Fe 3+ must also be present judging from teh excess of ÜFe in Fig. 3, especially in the outer zones, probably rather in the CaFe| + Si0 2 (Ca-ferri-Tschemak's molecule) thean in acmite (NaFe 3 + Si 2 O fi ), Fig. 3. Mg versus Fe in clinopyroxenes. Circles: core; crosses: rim pyroxenes * Cr 2 03 as high as 0.42 (Table 1, analysis No. 25) has also been determined in another pyroxene crystal.
