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Crystallographic and chemical constraints on the nature of the proustite–pyrargyrite solid-solution series

Abstract The present paper reports a crystal-chemical study of minerals belonging to the proustite-pyrargyrite group by evaluating 32 natural samples of proustite and 27 samples of pyrargyrite from different localities that have variable chemical compositions. The unit-cell parameters were modeled as a function of the Sb contents. The a parameter is strongly influenced by the As ↔ Sb substitution, whereas the influence on the c parameter is very minor (nearly constant trend). The following equations were obtained from the linear fitting of the data: apred = 10.8433(3) + 0.2019(4)Sb (apfu) cpred = 8.7189(6) + 0.0059(9)Sb (apfu) Vpred = 887.77(7) + 34.0(1)Sb (apfu). The crystal structure of the members of the proustite-pyrargyrite solid solution consists of two sets of spiral chains parallel to the c-axis. Each chain contains alternating Ag and S atoms, with each chain being connected by As and Sb atoms, which are the apices of flat pyramidal AsS3/SbS3 groups. Each S atom is part of a different Ag-S chain. The compositional data support the concept that proustite-pyrargyrite solid solutions re-equilibrate and exsolve to near end-member upon cooling. Examples of intermediate compositions are rare in nature and must have quenched above the solvus.