www.jgeosci.org Journal of Geosciences, 63 (2018), 215–226 DOI: 10.3190/jgeosci.266 Original paper Compositional, structural and vibrational spectroscopic characteristics of feldspar megacrysts in alkali basalts from southern Slovakia Monika HURAIOVá 1 , Christian L. LENGAUER 2 , Rainer ABART 3 , Vratislav HURAI 4 * 1 Comenius University, Department of Mineralogy and Petrology, Ilkovičova 6, 842 15 Bratislava, Slovakia 2 University of Vienna, Institute for Mineralogy and Crystallography, Althanstrasse 14 (UZA II), 1090 Vienna, Austria 3 University of Vienna, Department of Lithospheric Research, Althanstrasse 14 (UZA II), 1090 Vienna, Austria 4 Slovak Academy of Sciences, Institute of Earth Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia; vratislav.hurai@savba.sk * Corresponding author Feldspar megacrysts in Late Miocene–Pliocene maars, diatremes and basaltic lava fows in the northern part of the Pan nonian Basin span the compositional range from Nasanidine, through anorthoclase to oligoclase and andesine. Newly formed An 82 plagioclase (bytownite) crystallized within composite melt inclusions hosted by the oligoclase. Powder Xray difraction data indicate strong structural disorder diagnostic of the high albite–high sanidine series typical of magmatic feldspars. The magmatic origin is also corroborated by chemical compositions, which plot along the 700 ± 50 °C solvus, with the exception of the most sodic Ab 79 plagioclase megacryst projecting along the 600 °C isotherm. Vibrational spectroscopic records document that the basic groups of magmatic feldspars can be identifed by the specifc pattern of the group I and IV bands in the 350–600 cm –1 range. The IVa band at 560–570 cm –1 is diagnostic for triclinic feldspars. The anorthite and orthoclase contents can be inferred from the 506–515 cm –1 (Ia) and 473–486 cm –1 (Ib) peak separation values combined with the Ia bandwidth. Keywords: feldspar, megacryst, alkali basalt, Pannonian Basin, Slovakia, Western Carpathians Received: 4 January 2018; accepted: 15 August 2018; handling editor: R. Skála There are several studies focused on the Raman spectral characteristics of feldspargroup minerals (e.g., Mernagh 1991; Bendel and Schmidt 2008; Freeman et al. 2008; Bersani et al. 2018). However, comparison of results and identifcation of feldspar with lowresolution portable and feld spectrometers are still hindered by the varying degree of accuracy, random sample orientation and the need for a precisely calibrated spectrometer. Here we demonstrate that the compositional and structural characteristics of magmatic feldspars can be inferred solely from a Raman spectral record obtained without a precisely calibrated highresolution spectrometer and special sample preparation. 2. Geological setting The Late Miocene–Pleistocene SouthSlovakian Vol canic Field (SSVF) extends over an area of ~300 km 2 along the Hungarian–Slovakian border. The SSVF belongs to the alkali basalt volcanic province of the intraCarpathian backarc basin (Pannonian Basin), which is interpreted to result from a withinplate, post collisional volcanic activity triggered by the decom pression melting of depleted asthenosphere (Dobosi 1. Introduction Feldspar megacrysts up to 12 cm in size occur in conti nental alkali basalts (e.g., Bahat 1979; Aspen et al. 1990; Upton et al. 1999, 2009). Their compositions comprise sanidine, anorthoclase, potassic oligoclase and albite (Chapman 1976; Chapman and Powell 1976; Ulrych et al. 1998; Upton et al. 1999; Gernon et al. 2016), and typi cal orthoclase contents range between 10 and 30 mol. % (Aspen et al. 1990; Guo et al. 1992). A high degree of disorder and structural states corresponding to the high albite–high sanidine series (Kroll and Ribbe 1983) are diagnostic of a magmatic origin (Hofer and Hofer 1973; Chapman 1976). This paper provides frst information on alkali feldspar and plagioclase megacrysts from Late Miocene–Pleis tocene maars, diatremes, and lava fows in the northern promontory of the Pannonian Basin. The investigated feldspars represent a unique set of rapidly quenched and chemically homogeneous minerals devoid of exsolu tions, which had not undergone lowtemperature re equilibration and hydrothermal alteration. As such they are particularly suitable for a Raman spectrometric study supplemented by powder Xray difraction (XRD) and electronprobe microanalysis (EPMA).