Szakáll Sándor - Fehér Béla: A polgárdi Szár-hegy ásványai (Topographia Mineralogica Hungariae 8. Miskolc, 2003)

Vezuviánok és gránátok a polgárdi Szár-hegyről (Dódony István, Németh Péter és Elena Belluso)

Topographia Mineralogica Hungáriáé Vol. VIII. 115-134. Miskolc, 2003 Vezuviánok és gránátok a polgárdi Szár-hegyről Vesuvianiíes and garnets from the Szár Hill, Polgárdi, Hungary DÓDONY István

1

'*, NÉMETH Péter

1 és BELLUSO, Elena

2 1 Eötvös Loránd Tudományegyetem, Ásványtani Tanszék, 1117 Budapest, Pázmány P. sétány 1/C. 2 Dipartimento di Scienze Mineralogiche e Petrologiche, Université degli Studi di Torino Via Valperga Caluso 35,1-10125 Torino, Italy. * E-mail: dodony@ulixes.geobio.elte.hu Abstract Vesuvianite and garnet are rock-forming minerals of the Szár Hill skarn, formed at the contact of Devonian Polgárdi Limestone and Mesozoic andésites. Vesuvianite and garnet crystals were investigated by optical and electron microscopy, X-ray diffraction and energy dispersive X-ray spectrometry. On the basis of morphology and colour there are two types of vesuvianite: (i) brown, idiomorphic, elongated prisms with bipyramidal terminations; (ii) yellow, anhedral, isometric grains, forming granular aggregates. The latter are richer in Mg and its back scattered electron image reveals zones with higher Fe content. Pétrographie microscopy showed brownish isotropic crystals (garnet and vesuvianite) in calcite and clay matrix in the samples from the transition zone, i.e. the boundary zone between andésite and skarn. In thin sections from the vesuvianite skarn despite the low birefringence, yellow, brownish yellow and lilac anomalous interference colour vesuvianite crystals can be observed with diopside and calcite. Zoned crystals appeared in the thin sections of yellow vesuvianite. The c

0 lattice constant and the 001 projection of the unit cell of vesuvianite is quite similar to that of garnets. The main difference between them is the presence of structural channels in vesuvianite. These channels are along the two four-fold axes in the unit cell. The possible ordering of the five-fold and the eight-fold co-ordination polyhedra as well as an oxygen position in channels leads to PAIn and PAnc symmetry. PAInnc symmetry can be considered as the statistical arrangement of the PAIn and PAnc type ordering (Giuseppetti & Mazzi, 1983). Allen & Burnham (1992) suggested 400-800 °C for the formation of the PAInnc vesuvianite. Vehlen & Weichmann (1991) determined 780-800 °C as the transition temperature between the high symmetry (PAInnc) and low symmetry (PAIn) vesuvianite. 400-800 °C temperature interval can be applied for the formation of vesuvianite from Polgárdi due to the only presence of PAInnc vesuvianite. SAED and X-ray powder diffraction patterns of vesuvianites from Polgárdi revealed the PAInnc symmetry. According to the HRTEM images the studied vesuvianites have homogeneous structure. X-ray single-crystal refinement of a brown vesuvianite crystal showed statistical arrangement of the five-fold and the eight-fold co-ordinated cations along the c axis. These observations confirmed that there are PAInnc vesuvianite and garnet in the skarn. The yellow and brown vesuvianites have different chemical composition. According to these data, Polgárdi vesuvianites must have formed between 400 and 800 °C by a volcanic body enriched in volatiles. The yellow vesuvianites were formed by fluids richer in Mg and Fe as compared to the brown ones.