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An in situ high-temperature X-ray powder diffraction study of the thermal behavior of realgar (α-As 4 S 4 ) has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the realgar → β-As 4 S 4  phase transition starts at 558 K and is completed at 573 K due to kinetics. Melting starts at 578 K and is completed at 588 K. Thermal expansion of realgar is significant and fairly isotropic. In fact, the a- and b-parameters expand almost at the same rate, whereas the c-parameter is slightly softer against heating. Moreover, the β-angle contracts as temperature is raised. The geometry of the As 4 S 4  molecule is largely independent from heating. The lengthening of a few As-S and As-As contacts above or near the sum of the As,S van der Waals radii represents the driving force of the phase transition. In addition, the thermal behavior of arsenolite As 2 O 3  and nonstoichiometric As 8 S 8+x  crystals produced from As 4 S 4  melt recrystallization has been investigated. Two members located along the β-As 4 S 4 -alacranite (As 8 S 9 ) series joint were identified at RT: a term close to the β-As 4 S 4  end-member (As 8 S 8+x +x: x = ca. 0.1) and one term of approximate As 8 S 8.3  composition. The thermal expansion of β-As 4 S 4  is significantly anisotropic following the α b  &gt; α a  &gt; α c  relationship. This is clearly the result of the different packing scheme of the As 4 S 4  cages in β-As 4 S 4  with respect to realgar. The dependence of cell parameters and volume of As 8 S 8.3  is more complicated. In fact, a strong discontinuity on the dependence of cell parameters and volume is observed in the 403-443 K thermal range, i.e., that at which As 8 S 8.3  converts partly to realgar. A significant volume expansion is observed as a result of a change of composition to As 8 S 8.7 .

american mineralogist

Thermal behavior of realgar As 4 S 4 , and of arsenolite As 2 O 3  and non-stoichiometric As 8 S 8+x  crystals produced from As 4 S 4  melt recrystallization