Chemická variabilita vesuvianu ve skarnech z Vápenné (Moravosilezikum)Článek v PDF
Vesuvianite chemical variability in skarns from Vápenná (Moravosilesian Zone)
Vesuvianite is a characteristic and widespread mineral of skarnized marbles, skarnoids and skarns in the area of the Vycpálek quarry near Vápenná, Czech Republic. Mineralogical research including study of its chemical composition demonstrated existence of eight paragenetic types of vesuvianite (VES 1A–4A and alterations VES 1b–3b, 3c with different chemical composition) at two occurrences (SKARN 1 and SKARN 2). The primary vesuvianite VES 1A and its alteration VES1b from SKARN 1 are found in the mineral assemblage (I) Cal + Ves + Grs + Di + Wo ± Ep, with accessory pyrite, titanite and zircon. In contrast, primary VES 2A and its VES 2b alterations appear in the mineral assemblage (II) Hd + Pl + Qtz + Ves + Grs ± Ep. Primary VES 3A, its alteration VES 3b and VES 3c belong to the assemblage (III) with Grs + Hd + Ves ± Ep and primary VES 4A belong to the assemblage (IV) Ves + Grs + Hd ± Ep. Detailed study of chemical composition of the vesuvianite showed considerable chemical variability between the various types of primary and altered vesuvianites; there is a strong predominance of Fe-rich types over Mg-rich vesuvianite. In addition to the main elements, locally elevated contents of Be, Bi, Th, Y, Zr and HREE were found, whereas the content of boron is low. Primary vesuvianite VES1A differs from VES 1b alteration by higher Ti content (2.49–3.81 wt. % vs 0.33–0.56 wt % TiO2) and lower content of F (2.23–2.32 % in VES1b), La, Ce, U and Th; the VES1b is the only type that predominance of Mg (1.347–1.694 apfu) over Fe (1.074–1.159 apfu). Vesuvianites VES 2A and VES2b show slight differences only. The alternation VES2B shows higher contents of Zn, Mn, and Sn, and lower F (1.29–1.98 wt. %), As, Sr and Sb. Compared to the primary vesuvianite VES3a, the VES 3b and VES 3c alterations are characterized by a significantly lower Ti contents (compared to a maximum of 1.46 wt. % in VES3A, the lowest content is 0.10 wt. % TiO2), lower Li (decrease from a maximum of 198 to 4 ppm) and Be (from 1140 to 83 ppm), and increased Zr (from 2–5 to 39 ppm) and Sn (from 28 to 141 ppm). Uranium and thorium are elevated in primary vesuvianites, with Th (801–1904) > U (91–405 ppm), its alterations are depleted in both elements and show low Th/U. Similarly, variable Bi (59–548 ppm) and Zn (376–588 ppm) contents are higher in the primary vesuvianites. The chemical zonation in individual grains and crystals demonstrates changes in the composition of the fluid phase (activity and fugacity of individual components vs. influence of protolith chemistry) over a very short distance (tenths of mm); it is a characteristic feature of vesuvianites of calcareous contact skarns.
Jakub Malý, Department of Geological Sciences, Masaryk University, Brno, Kotlářská 2, 611 37 Brno, Czech Republic; e-mail: email@example.com