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., ΔStrans) increases in the order Sb → Bi → As. For the transformation of α− to β−miargyrite, ΔStrans 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 ΔStrans ΔStrans≫2R ln 2.
The molar volume differences (i.e., ΔVtrans) 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 ΔVtrans and ΔStrans.