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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
ISSN
1945-3027
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Volume 100, Issue 5-6

Issues

An insight into the inverse transformation of realgar altered by light

Giovanni Pratesi
  • Museo di Storia Naturale, Sezione Mineralogia e Litologia, Università di Firenze, via La Pira 4, I-50121 Firenze, Italy
  • Dipartimento di Scienze della Terra, Università di Firenze, via La Pira 4, I-50121 Firenze, Italy
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/ Matteo Zoppi
  • Corresponding author
  • Museo di Storia Naturale, Sezione Mineralogia e Litologia, Università di Firenze, via La Pira 4, I-50121 Firenze, Italy
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Published Online: 2015-05-12 | DOI: https://doi.org/10.2138/am-2015-5045

Abstract

The light-induced alteration of realgar and β-As4S4 is a well-known phenomenon but still displays interesting aspects not completely explained. In the transformation from realgar to pararealgar the molecule As4S4 undergoes a structural modification, and ever since the initial studies two important issues have been highlighted: the influence of the oxygen and the reversibility of the process. The previous study on the reversibility of the altered β-As4S4 points out that this polymorph exhibits a dual behavior. When the light-induced alteration occurs with the presence of the air, pararealgar and arsenolite, along with amorphous material, are the products, while if the air is not present β-As4S4 turns completely into pararealgar. Moreover, when annealing the altered material in the realgar stability fields (220 °C), in the first case pararealgar and amorphous material turn into stoichiometric alacranite, while in the second case the alteration is completely reversible. Similarly, the present study focuses the attention on the question if realgar, when altered by means of the light and when annealed, might behave as β-As4S4 does. These results display that the phenomenon is more complex. The alteration of realgar with the presence of the air yields pararealgar along with arsenolite, a small quantity of uzonite and amorphous material, and when the air is not present pararealgar is the only product. In the first case, when annealing the products of alteration at 220 °C, alacranite, β-As4S4, realgar, and little amorphous material occur along with arsenolite. In the second case at first β-As4S4 crystallizes, then it turns into realgar, but this process yields orpiment and amorphous material as it moves forward, showing that from the products of alteration of realgar it is not possible to obtain the starting material. Examination of the stoichiometry of the products let to infer that the amorphous material occurring in the two cases has very different content of arsenic and sulfur.

Keywords: Realgar; light; heat; pararealgar; alacranite; uzonite; Rietveld

About the article

Received: 2014-05-09

Accepted: 2014-11-30

Published Online: 2015-05-12

Published in Print: 2015-05-01


Citation Information: American Mineralogist, Volume 100, Issue 5-6, Pages 1222–1229, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5045.

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© 2015 by Walter de Gruyter Berlin/Boston.

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