Solid State Studies on Synthetic and Natural Crystalline Arsenic(III) Sulfide, As2S3 (Orpiment):  New Data for an Old Compound

Synthetic monoclinic orpiment was recrystallized by annealing the glass at 523 K for 1 year. The apparent heat of fusion of orpiment at T fus = 601 K (onset) was found to depend linearly on the ratio V/m (V = inner volume of the sample pan, m = in-pan sample mass) because of partial sublimation. This gives Δfus H = 131 J g-1 if V/m = 0 and the specific volume of the vapor phase v(v) = 190 cm3 g-1 if Δfus H = 0 at the conditions of the orpiment−liquid−vapor triple point. Lattice parameters of orpiment and liquid density were measured in the 283−393 K and 750−1250 K ranges, respectively. Subsequent specific volume values were used to calculate the slope dp/dT (≈53 MPa K-1) for the orpiment−liquid equilibrium curve of the As2S3 p−T diagram.