Standard molar enthalpies of formation of sulfosalts in the Ag-As-S system and thermochemistry of the sulfosalts of Ag with As,Sb,and Bi

High‐temperature direct‐synthesis calorimetry of the silver thioarsenite minerals smithite (AgAsS₂), trechmannite (AgAsS₂), and proustite (Ag₃AsS₃) at 793 K yielded mean values for the standard enthalpy of formation from the elements of −59.1 ± 2.3,−74.77 ± 2.9, and −111.3 ± 3.4 kJ·mol⁻¹, respectively.

The enthalpy of transformation of the low‐temperature polymorph of AgAsS₂(trechmannite) to the high-temperature polymorph (smithite) was determined to be 15.65 ± 1.7kJ·mol⁻¹. At the equilibrium temperature of this transformation, 593 K, this yields an entropy of transformation of 26.4 ± 3.0 J·⁻¹mol⁻¹.

A systematic study of the thermochemistry of the sulfosalts of Ag with the Group V elements As, Sb, and Bi shows that the enthalpies of formation for the low-temperature polymorphs become progressively more exothermic in the order Bi →Sb→ As. The enthalpies of transformation from the low‐ to high‐temperature polymorphs of AgBiS₂ (β → α matildite), AgSbS₂ (α → β miargyrite), and AgAsS₂ (trechmannite → smithite) become more endothermic in the order Bi≈Sb→ As, whereas the corresponding entropies of transformation of these minerals (i.e., ΔS_trans) increases in the order Sb → Bi → As. For the transformation of α− to β−miargyrite, ΔS_trans 2R In 2 and may be attributed to the complete disordering of Ag and Sb atoms in the miargyrite structure. This approximate equality is in marked contrast to both AgAsS₂ and AgBiS₂, for which ΔS_trans ΔS_trans≫2R ln 2.

The molar volume differences (i.e., ΔV_trans) for these polymorphic transformations are negative and decrease in magnitude in the order AgAsS₂→AgSbS₂→AgBiS₂. Each of the polymorphic transformations therefore involves (at 298 K) a decrease in molar volume and a positive entropy change with no apparent correlation between ΔV_trans and ΔS_trans.