Monoclinic sanidine is the high-temperature modification of potassium rich alkalifeldspars. The crystal structure consists of a framework of corner-sharing TO4 tetrahedra (T = Si, Al) with a temperature dependent disorder of Si and Al on the tetrahedral sites. Among the sanidines those from the volcanic fields of the Eifel play an exceptional role as they show unique optical anomalies at temperatures above about 750 °C. Further, ultrasonic investigations revealed strong evidence for the onset of dynamic processes between 900 °C and 1000 °C on laboratory time-scale. The proposed in-situ high-temperature neutron diffraction experiments on a sanidine megacrystal from Volkesfeld, Eifel, will provide insight into the role of Al/Si mobility and disorder for the anomalies observed.