Intriguing minerals: photoinduced solid-state transition of realgar to pararealgar—direct atomic scale observation and visualization

Abstract

This lecture text is aimed at teaching some insight into phase transitions of minerals. It summarizes the results of a detailed study of the reaction mechanism of photoinduced solid-state transformation of the mineral realgar (α-As4S4) to its distinct polymorph pararealgar by a combination of in situ single-crystal X-ray photodiffraction, Fourier transform infrared spectroscopy, and micro-Raman spectroscopy. The process of transformation takes place in four steps. The initiating photoreaction step requires oxygen and thereby the intermediate uzonite (As4S5) and arsenolite (As2O3) are obtained (step 1). The process continues through a set of cyclic reactions in which the sulfur atom released by the decomposition of As4S5 (step 2) reacts with a molecule of realgar to produce a molecule of pararealgar (step 3), whereupon a sulfur atom is released which continues the process (step 4). The photodiffraction technique provides direct atomic resolution evidence of formation of intermediate As4S5 phase in which half of the realgar molecule retains its envelope-type conformation, while the geometry of the other half is transformed by effective switching of positions of one sulfur and one arsenic atom. The migration (hopping) of sulfur atoms between the molecules of the single crystal of realgar is observed and visualized.