Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
distribution of matrix minerals and in the varying proportions of big phenocrysts. These differences were caused by variations in the circumstances of cooling (5) : however, because of the small number of samples I have treated, I will not go into further details concerning this point. The present purpose was to compare the average grain size distribution with that of the dark grey hyperstheneaugite andésite. The close similarity of the two curves suggests the biolitic pyroxene andésite to be a variety of the dark grey pyroxene andésite (mátraite), forming the main bulk of the Mátra Mountains, solidified at a small depth below surface. According to the amount of glass present, the texture of biotitic pyroxene andésite is partly pilotaxitic (Rosenbusch), or of the fifth degree of crystallinity (Szádeczky-Kardoss, 1956) and partly microholocrystalline-porphyric (Rosenbusch) or of the sixth degree of crystallinity (Szádeczky-Kardoss, 1956), as regards the maximum size and amount of porphyric ingredients. As a matter of fact, the rock is a transition between the two types. The detailed description of the rock-forming minerals, as observed and measured in 16 thin slides, will be given in the following. Most of the porphyric ingredienst consist of feldspar. They may be grouped into four size categories, according to their longest diameters. The maximum diameter in the smallest group is 200 (i ; most of feldspar occurs in the 300 to 500 /i group ; somewhat less than in the second but more than in the first one in the 600 to 900 /u category. The largest size group, with diameters up to 1500 //, contains the smallest amount of feldspar. The constitution of the feldspars is labradoric to acidic bytownitic. It was determined on polylamellar plagioclases by the symmetrical extinction method. In four instances, it was possible to measure combined Karlsbad -)- albite twins.Two of these were bytownites (Ab 28 —An 72 , Ab„ fi —An 7J ), one a labradorite-bytownite (Ab 30 —An 70 ) and the fourth one a labradorite (Ab :j:j —An 67 ). Most frequent is polylamellar twinning according to the albite law, with mostly four, maximally ten intergrown individuals. Karlsbad twins are more scarce. The internal, more acidic parts of zonal plagioclases are surrounded by a more basic shell, in some instances in a rhytmic recurrence. Most of the feldspars are fresh. They contain many inclusions of matrix, less of hypersthene and of glass, disposed in a regular pattern. In some instances, basic labradorite contains chains of even width of matrix inclusions arranged in a wreath-like fashion, and alternating four times with the feldspar. The alteration of feldspar commences along fissures on the grain margins, with the appearance of limonite and chlorite. In such cases the magnetites contained by the matrix inclusions are limonitized as well. Among the dark ingredients, the amount of hypersthene and augite, occurring in varying proportions, is significantly greater (6,1 per cent) than that of biotite (3 per cent). The pyroxene crystals are idiomorphic : each grain is surrounded by an opacitic margin. Hypersthene is generally oblong, prismatic or columnary. It is frequently twinned according to the (011) face. It exhibits a pale green to yellowish green pleochroism. As to its size, it belongs to the 120—300 and 300—420 fi grain size categories. Its maximum size is 1500 //. Of its inclusions, magnetite is the most common, idiomorphic feldspar being rare. Some secondary chloritization and less frequent bastitization is observed. The augites are stockier than the hypersthenes. Their extinction reveals them to be mostly diopsidic. Polylamellar twinning is exceedingly frequent.
