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In-situ X-ray transmission powder diffraction study of the kinetics of the light induced alteration of realgar (-As₄S₄)

Dipartimento di Scienze della Terra, Università di Roma "La Sapienza", P.le A. Moro 5, I-00185 Roma, Italy

^* Corresponding author, e-mail: paolo.ballirano{at}uniroma1.it

An in-situ X-ray transmission powder diffraction study of the kinetics of the light induced alteration of realgar (As₄S₄) has been carried out. Data indicate that the crystalline final products of the process are pararealgar (different As₄S₄ polymorph) and arsenolite (As₂O₃). Because of the chemical unbalance the evolution of SO_x is hypothesized. In fact there are no evidences of the presence of amorphous As-S alloys or amorphous S. The alteration proceeds via the occurrence of an intermediate term along the ß-As₄S₄-alacranite series joint with an estimated As₄S_{4 .2} composition. This phase reach a maximum wt. fraction of ca. 30 % in the early stages of the process and subsequently disappears. From the fraction X(t) of realgar and pararealgar transformed vs. time was determined the kinetic law of the digestion of realgar and formation of pararealgar according to the JMAK model. The refined n parameters indicate that the behavior of the two transformations is intermediate between that of a diffusion-controlled and a first-order model. The increased expansion of the realgar cell during the alteration could be possibly due to the requirement that structural coherency between at least two of the three sulfide phases is preserved. In particular the coherency between realgar and ß-As₄S₄ is expected to exist along a_realgar^*sin ß and c_ß-As4S4, respectively. According to our data we may hypothesize that the light induced alteration of realgar to pararealgar occurs via an intermediate As₄S_4+x product because it is able to sustain the presence of As₄S₅-type cages. Following the model proposed by Kyono et al., (2005) the As₄S₅ cage release the extra S atom to form the As₄S₄ cage of pararealgar-type promoting the development of a new As₄S₅ cage in a cyclic process.

Key-words: sulfide phases, realgar, ß-As₄S₄, pararealgar, arsenolite, alteration, X-ray powder diffraction, Rietveld method.