Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 62. (Budapest 1970)

Szabó, I. ; Ravasz, Cs.: Investigation of the Middle Triassic volcanics of the Transdanubian Central Mountains, Hungary

tais are represented by sanidine, the second essential constituent being biotite. Sanidine is of variable state of preservation: limpid, sub-idiomorphic crystals are just as frequent as cloudy or even altered crystal fragments, containing illite or calcite inclusions, are. Grain size averages 250 u. attaining a maximum of 400 \x. The unaltered specimens of biotite show a yellowish-green to pale-brown pleochroism; the altered ones contain opaque patches due to iron precipitation, being slightly bound for chloritization. Average grain size: 200 The matrix of the slightly rounded trachite lava fragments, 200 to 250 \i in diameter, has been partly devitrified, its magnetite has been limonitized and, in isolated patches, claymineralized, its feldspar microlites also being altered. Making up the matrix of the tuff, illite is of pale greenish-yellow colour, of interwoven, bundled texture in which, beside aggregates of irregular outline, monocrystals of regular rhombohedral shape and 60 to 80 u. in diameter, are also abundant. The clastic mineral constituents of the stratigraphically higher-seated tuffaceous layers of the sequence essentially agree with the above and only slight differences due to the appearance of accessory magmatic quartz and to the alterationdependent variation of the magnetite-Iimonite ratio, can be recognized. Also, the changes in size are very limited. To be more exact: clay mineralization reduces the original size, while a calcareous environment preserves it. As a contribution to the tabulated data of accessory minerals (Table 1), let us add that the chalcedony of sample T-161 indicates silicification, while sample T-165 contains a small amount of cavity-filling collophane. Glassy trachytic lava fragments can be encountered throughout the sequence, its amount, always lower than that of crystal fragments, being variable. The rock material of the tuff layer cut in the lower level of borehole 5 at Pécsely is represented by altered, lithoclastic trachytic tuff with crystal fragments. Fresh trachyte lava fragments, characterized by yellowish-coloured, glassy matrix, by sanidine microlites oriented according to the flow averaging 60 \i in length and by tiny magnetite crystals, are rather scant. What can be encountered more frequently is a redbrown to brown, limonite-stained, feldspar-microlitic fragment, partly recrystallized and altered into illite. Its grain diameter measures, for the most part, 400 m, its shape being angular. The bulk of the crystal fragments is represented by sanidine, being comparatively slightly altered, with an average grain size of 400 \i. The altered specimens contain patches of dioclahedral chlorite? and illite. Biotite and quartz are accessories

. Secondary chert rags can also be recognized. The rock matrix is poor, being constituted by well-crystallized rhombohedral calcite crystals of 100 to 250 u. size. The Upper Anisian tuffs disclosed by exploratory shaft 1 on Ágasmagas Hill at Örvényes represent a slightly altered potash-trachytic crystal tuff (sample T-150). Macroscopically, the rock has a pale-green matrix containing white feldspars and ivy green glauconite grains attaining 3 mm size, as well as limonite patches and streaks, being of medium hardness, unstratified. As viewed in thin sections, the bulk of the rock is made up of calcitized, illitized, and chloritized sanidine crystals. They are nearly idiomorphic, averaging 650 u. and attaining a maximum of 1,200 u, in grain diamètre. Apatite, zircon and altered pyroxene are also contained in the rock. The mafic components have been replaced by secondary minerals. Biotite has been converted into glauconite (pseudohexagonal plates averaging 650 u, in diameter). Pyroxene is represented in a small amount. On the edge and in the interior of its 400 to 500 a crystals, platy and elongated (200 u), columnar, leucoxene-