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

Örkényi-Bondor, L.: Andesite agglomerate from Zebegény village, Börzsöny Mountains (Hungary)

Sample 2/71 contains many pyroxene phenocrysts. Most of these are hypersthene; some can not be identified withouth further examination. In the same rock tridimite could be recognized in the groundmass, accompanied by limonitic patches. Tridimite is absent in samples 7/71 and 8/71, but there are (up to 10%) in the groundmass spherical, greenish-brown aggregates of clay minerals. These consist of montmorillonite as testified to by the X-ray examinations. In sample 11/75, neither tridimite, nor clay minerals could be observed. However, serpentine deriving from the alteration of pyroxene could be recognized. In the same rock, the two generations of plagioclase differ in twinning. The smaller crystals are polysynthetic twins, the larger ones often single (un-twinned) crystals. Type 3. — A greyish-red, rather massive rock (sample 3/71) represents this type. It is of microholocrystalline texture, but the microlites are very small. Two generations of plagioclase phenocrysts can be recognized. Both of them are finely zoned, with strongly fluctuating An-content. The smaller crystals consists of numerous polysynthetic lamellás, while the larger ones are composed of two or three twin members only. The common large, idiomorphic pyroxene phenocrysts are usually polysynthetic twins. A small amount of amphibole and several quartz crystals are also present. Type 4. — Samples 5/71, 13/71 and 7/75 belong to this group. These are rocks of light grey colour; their texture is microholocrystalline. The phenocrysts are plagioclase, pyroxene and some amphibole. Along with the Albite-Carlsbad-Roc Tourné twin laws in the first two samples there are remarkable Bánát intergrowths, while in the last one Börzsöny and Visegrád twinning could be recognized. (Beside the usual polysynthetic lamellae.) The numerous pyroxenes are fresh in sample 13/71, while in the two others the alteration of pyroxene to serpentine is much advanced. Type 5. —-A yellowish-grey rock, rich in clay minerals, represents this type (sample 9/71). The microholocrystalline groundmass consists of small microlites and clay minerals. Plagioclase and pyroxene constitute the phenocrysts, while amphibole is absent. This rock differs from all the other types in respect to both primary phenocrysts and ulterior alteration. It was in this sample that the most acidic plagioclases have been encountered. The An-content range is narrow, 61-76%. The pyroxenes include hypersthene and augite. Along with great amount of clay minerals (mostly montmorillonite) and serpentine, some amorphous silica and cristobalite are also present. The plagioclase phenocrysts and a considerable part of the groundmass has been altered to clay minerals. In some cases only the external zones are intact. Serpentine developed on the rim and in the cleavages of pyroxene crystals. Type 6. — Sample 10/71 represents this type which occurs but rarely among the bombs and lapillis. It is a greyish-red, hard, massive rock with 1 to 3 mm size amphibole crystals. The texture is trachytic, sometimes fluidal. The porphyric crystals are plagioclase, amphibole and orthorhombic pyroxene. The plagioclase crystals are oriented. Plotted in Stereographic projection, the normals of the faces (010) are located inside of a spherical small circle, 15° in radius. Most of the plagioclases contain 65 to 73 % An, depending on the zoning. Some hypidiomorphic plagioclase phenocrysts have 75-85% An. The Albite- Carlsbad- Roc Tourné laws are overwhelming; the Bánát, Börzsöny and Visegrád twins total only to 2%. Pericline twinning could be suspected in the case of one single grain. Non-twinned plagioclase phenocrysts also can be observed, and such grains, in which 1-2 small twin members are wedged in at some edge of the crystal. There are many inclusions of glass and groundmass in the phenocrysts. Many broken feldspar grains can be observed in contrast to the other rocks. The amphibole crystals are fresh, in some cases the rim is slightly opacitized. On the basis of their optical properties, they are oxyamphiboles. Small amounts of cristobalite and serpentine as well as many opaque grains are characteristic of this rock. Most of the opaque minerals are haematite. Type 7. — Only sample 11/71 belongs to this type. The microholocrystalline groundmass consists of isometric plagioclase grains. Most of these are columnar and polysynthetically twinned. The pyroxene and amphibole phenocrysts, however, differ from those of the other rocks. Their optical properties suggest a composition near to ferrihastingsite. The pyroxene crystals have orthorhombic symmetry and strong pleochroism. They can be identified as Fe-rich hypersthenes. Type 8. — Sample 15:7

1 is a caked tuff with lapillis. This type of rock is common in the agglomarate of Kerekhegy hill. Its examination hurts to a lot of difqculties. The groundmassa is microholocrystalline, consisting of numerous tiny microlities, partly altered to clay minerals. The lapillis are similar to other rock types, the cement, however, contains other minerals as well. The aspect and composition of the polysynthetic plagioclase crystals, amphibole and pyroxene grains with opaque cover do not differ from the average of similar rocks of the bombs. Biotite and garnet crystals, however scarce, occur only in this type. Several generations of pyroxene and amphibole could be recog-