X-ray study of potassium feldspars from different granitoid types and gneisses of Papuk Mt. (Slavonia, Croatia)

Abstract

Potassium feldspars from different granitoids and gneisses of Papuk Mt. (Slavonia, Croatia) have been investigated by X-ray powder diffraction. Diffraction patterns classically observed as well as patterns calculated by Rietveld refinement were compared and discussed. Triclinicity was calculated according to GOLDSCHMIDT & LAVES (1954) while the structural state of the feldspars was determined using the methods of KROLL & RIBBE (1983) and GODINHO & JALECO (1973). Results showed that the type of potassium feldspar depend on the investigated host-rock, indicating variation in the structural state from orthoclase, intermediate microcline to highly ordered microcline. Potassium feldspar megacrysts in biotite-granodiorites and monzogranites are intermediate microcline or orthoclase, while two-mica monzogranites contain low microcline. Gneisses contain low microcline and orthoclase in Brzaja Creek and low microcline in Djedovica Quarry. Classically observed and digital diffraction patterns calculated by the Rietveld refinement method produced comparable results and provided a very good correlation of the results obtained by different methods. High triclinicity values of feldspars from investigated granitoid and gneiss samples from Papuk Mt. (Slavonia, Croatia) are in accordance with a high Al content in the T1o site and their fully ordered state indicates a slow(er) cooling-rate. Low triclinicity values, an Al content in T1o site around 0.60 and ordering index smaller than 0.80 can be interpreted as a result of relatively fast(er) cooling which allowed lower ordering of the potassium feldspar.