Szakáll Sándor szerk.: A Szerencsi-dombság ásványai (Topographia Mineralogica Hungariae 3. Miskolc, 1998)

A megyaszói faopál ásványtani vizsgálata (Papp Gábor, Sámson Margit, Jánosi Melinda & Lovas György)

Topographia Mineralogica Hungáriáé Vol. III. 73-83. Miskolc, 1998 A MEGYASZÓI FAOPAL ÁSVÁNYTANI VIZSGALATA Mineralogical study of wood opal from Megyaszó (NE Hungary) PAPP Gábor, SÁMSON Margit, JÁNOSI Melinda és LOVAS György Abstract: The first reference to the petrified wood from Megyaszó (see footnote 1) can be ascribed to Born (1790). The locality itself was introduced in the geological literature in the 1860s by Austrian and Hungarian geologists (Hauer & Richthofen, 1859; Szabó, 1866; Wolf in Böckh, 1867). The "classical" locality was the Csákó quarry on the NW slope of Nagyrépás Hill; the nearby outcrops Tetlinke ditch and Csordáskút were first described by Hoffer (1937) and Horváth (1954), respectively. The fossil plant remains are found in Lower Pannonian sediments (sandstone, breccia, rhyolite tuff etc.). Early palaeobotanical studies of the silicified wood from Megyaszó were made by Hazslinszky (1866), Stur (1867), and Felix (1887). After a long break Horváth (1954), Kovács (1955), Müller-Stoll & Mädel (1957, 1959, 1960), Horváth (1964) and Greguss (1967, 1969) published palaeoxylotomical observations on materials from Megy aszó. A frequently observed white glass-wool-like weathering crust is the specialty of the wood opal of Megyaszó. It was first described by Hazslinszky (1866). He observed that the white asbestiform threads correspond to tubular wood-fibres. Horváth (1964) assumed that the opalised wood was attacked by subsequent hot fluids. The fluids decomposed mainly the silicified cell walls because of their organic material content so that they disintegrated into cellular fragments. The mineralogical knowledge of the fossil wood of Megyaszó is restricted to the long-standing observation that it is wood opal. For further information, optical, SEM and X-ray diffraction studies were done on three samples. Thin sections and SEM micrographs revealed that the opal forms 10-20 urn thick spherical-botryoidal coatings on the cell walls, the interior of the cells is empty (Figs. 1-2). The opal shows low but distinct birefringence, the refractive index is bigger parallel with the cell walls. The cell walls are discernible in the thin section of the intact part of the wood opal but hardly discernible in the weathered part. Spherical opal precipitations proved to be (presumably opal-CT) lepispheres in SEM micrographs (Fig. 3, cf. Flörke et al., 1975). In the weathered part the cell walls are missing and opal forms a negative cast of the cells (Figs. 4-5), sometimes preserving very fine details of the original structure of the wood (Fig. 6). The sample was kindly determined by dr. Károly Babos (Eötvös L. University, Dept. of Plant Anatomy) as a Taxodiacea. State of preservation doesn't correspond to the intact and weathered state of the specimen as it was influenced mainly by the decay prior to silicification (Fig. 7.) X-ray powder patterns were evaluated on the basis of Graetsch et al. (1987, 1994). The intact parts proved to be of microcrystalline opal and gave opal-CT patterns, whereas the weathered crusts were amorphous and corresponded to opal-A (Fig. 8.). The unusual thickness and abundance of the weathering crusts may be related to the porous structure of the wood opal, but the empty rooms left by the dissolved cell walls obviously played a considerable role in the transport of the fluids as well. The nature of these fluids and the changes in the ordering of the opal during the weathering process needs further clarification. Összefoglalás: A megyaszói faopál az 1860-as évek óta ismert a földtani szakirodalomban. A korábbi munkák, elsősorban ősnövénytani irányultságúak lévén, kevés ásványtani adatot szolgáltattak a famaradványok kovásító anyagára vonatkozóan, beleértve a megyaszói faopálokon észlelhető sajátos fehér, szálas mállási kérget is. Vizsgálataink során megállapítottuk, hogy a faopál ép részében megjelenik a mikrokristályos opál (opálCT), míg a mállási kéregben kizárólag amorf opál (opál-A) mutatható ki. A vizsgált mintákban az opál nem alkot összefüggő kitöltést, hanem csak az egykori sejtfalakat vonja be kb. 20 u.m vastag, gömbösvesés felszínű kéregként. Pásztázó elektronmikroszkópos képek alapján az opálgömbök voltaképpen apró lemezkékből összeálló ún. lepiszférák. A mállási kéreg szálas képletei Hazslinszky (1866) és Horváth (1964) megállapításaival egyezően az egykori sejtek opálkitöltései, amelyek a (Horváth szerint részben már opálosodott) sejtfalak kioldódása után szabaddá váltak. Ezen „opálnegatívokon" helyenként