Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
The peaks of the grain size distribution occur around 180, 300 fi and 480 //, with maximum diameters reaching 1200 fi. Limonitized magnetite inclusions are common ; feldspar inclusions are also more frequent than in hypersthene. Augites occur often in randomly oriented aggregates of idiomorphic crystals of 60 to 100 fi size. On weathering, the augites undergo chloritization. The biggest of the biotites (400 to 600 //) are platy lamellae, the smaller ones (180 to 300 fi) columnar. The opacitic rim —characteristic of all of the dark ingredients — is very wide, so that no interference colour is seen except around the centers of the hexagonal flakes. Biotite is intensely pleochroic throughout, with yellow to reddish-brown to dark brown transitions. Although its abundance is in general no more than 3 per cent, the rock occurring on and around Mulató Hill deserves special attention, biotite being the predominant dark ingredient there, with some hypersthene and augite occurring sporadically. Most of biotite is digested, with bay-like concavities. On weathering, part of the mineral's iron content is precipitated in the form of limonite. About 50 per cent of the biotite flakes are oriented parallel to the plane of striation. Tridymite, making up about 10 per cent of the rock on the average, is frequent in the light striae. It is present in a number of varieties. The size of tridymite visible to the naked eye is 300 to 350 fi, with the crystals situated generally on the walls of cavities. In the slides, tridymites look like idiomorphic plates with scaly surfaces. Two thirds of the tridymite grains occur as small porphyric ingredients, of a mean diameter of 200 to 250 //, or in aggregates consisting of individuals of 10 to 20 fi grain size. The scaly structure, index of refraction and colour of interference of tridymite make it easily distinguishable from fibrous, tufty chalcedony of concentric structure, and from partly dehydrated opal which has a cracked surface. Chalcedony and opal occur in small amounts, mainly along capillary cracks and on the walls of pores. Feldspar microliths of 12 to 25 fi size form some 40 per cent of the matrix. This abundance decreases to but 25 per cent in the quarry adjacent to the cadaver well, where the matrix contains a swarm of hypersthene needles, coated with limonite, of 12 to 18 and 36 to 60 fi grain size. The feldspar microliths are more acidic than the phenocrysts, namely andesitic to oligoclasic. They exhibit no preferred orientation indicative of a flow direction. Magnetite, as characteristic of oxyvolcanites, is oxidized to limonite. It occurs in two grain size generations, of 8 to 10 and 50 to 60 fi diameter, respectively. Its bulk is found in the dark striae, where its abundance reaches 18 per cent. Beside the listed minerals, the matrix contains hypersthene, some augite and also some scattered biotite, in crystals of 5 to 10 //, size. These ingredients form 13 per cent of the matrix. Field survey and microscopic study have shown that our biotitic pyroxene andésite is a rock of transvaporisatory origin. The simultaneous occurrence of biotite and tridymite, as well as the striation, suggest that the rock was subjected to transvaporisatory effects. The magma mass, having surged at the beginning of the Tortonian stage of the Miocene, has in all probability solidified in the Helvetian sandy clay (Schlier) complex. According to the laws governing transvaporisation, which were recognized of late in their essentials (4), a migration of water vapour and potassium ions has then commenced up the temperature grade. The magma became enriched in volatiles, and the rhytmic precipitation of mutually insoluble phases has begun. The striation is a consequence of rock formation in an alternating sequence of
