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Under hydrothermal conditions, diffusion-driven solid-state reactions can compete with fluid-mediated reaction mechanisms. We have obtained an insight into the complex textures resulting from this competition by studying experimentally the transformation of Au-Ag-telluride sylvanite to Au-Ag alloy under hydrothermal conditions, and exploring the effects of temperature (160-220 °C), pH (2-10), and redox conditions on the sample textures and the reaction kinetics. Sylvanite transformed to Au-Ag alloy over all hydrothermal conditions investigated, but not under dry conditions. The replacement was pseudomorphic, as the Au-Ag alloy preserved the external dimensions of the sylvanite grains. The resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 μm. In addition to Au-Ag alloy, a range of other phases were observed as intermediate products, including petzite (Ag 3 AuTe 2 ), hessite (Ag 2 Te), and two compositions of calaverite: an Ag-rich-Te-depleted composition, (Au 0.78 Ag 0.22 )Te 1.74 , and a normal calaverite, (Au 0.93 Ag 0.07 )Te 2 .

american mineralogist

Dissolution-reprecipitation vs. solid-state diffusion: Mechanism of mineral transformations in sylvanite, (AuAg) 2 Te 4 , under hydrothermal conditions