Compositional, structural and vibrational spectroscopic characteristics of feldspar megacrysts in alkali basalts from southern Slovakia

Abstract

Feldspar megacrysts in Late Miocene–Pliocene maars, diatremes and basaltic lava flows in the northern part of the Pannonian Basin span the compositional range from Na-sanidine, through anorthoclase to oligoclase and andesine. Newly formed An82 plagioclase (bytownite) crystallized within composite melt inclusions hosted by the oligoclase. Powder X-ray diffraction 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 Ab79 plagioclase megacryst projecting along the 600 °C isotherm. Vibrational spectroscopic records document that the basic groups of magmatic feldspars can be identified by the specific 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.