Osumüite-(Mg) in a buchite xenolith from Badacsony — a new mineral in Hungary

Abstract

This paper reports on the first finding in Hungary of osumilite-(Mg), an important high temperature
mineral in Hungary. The osumilite-(Mg)-bearing buchite xenolith was found in the basáit quarry located
north of Badacsonytördemic. The xenolith is macroscopically very similar to those described earlier from
Szigliget (TÖRÖK 2002). It consists mainly of quartz with subordinate feldspars and magnetite. The
xenolith is cut by abundant dark grey to black melt veins. Several mineral parageneses were observed in
the melt veins of the xenolith during microscopy and scanning electron microscopy (clinopyroxene +
plagioclase + magnetite ± orthopyroxene ± K-feldspar ± ternary feldspar, olivine + anorthite +
magnetite, cordierite + K-feldspar ± orthopyroxene ± hercynitic spinéi, and osumilite-(Mg) ± cordierite
± magnetite ± orthopyroxene). This study deals in detail only with the osumilite-(Mg)-bearing
parageneses in detail.
The osumilite-(Mg) and the the osumilite-(Mg)-bearing parageneses were förmed from a quartz-rich,
muscovite-bearing assemblage which may have contained biotite under relatively oxydizing
environment (within the stability field of magnetite) characterized with low water activity. Based on the
mineral assemblages and the structure of the crust beneath the Balaton Highlands (SZAFIÁN et al. 1999)
the xenolith originates from a maximum depth of about 15 km (p < 400 MPa) and underwent a heating
of up to 950-1100 °C.