- journal_title：American Mineralogist
- Contributor：Luca Bindi ; Silvio Menchetti
- Publisher：Mineralogical Society of America
- Date：2011-
- Format：text/html
- Language：en
- Identifier：10.2138/am.2011.3736
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：96
- issue：5-6
- firstpage：792
- section：Articles
The mineral fettelite, [Ag6As2S7][Ag10HgAs2S8], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag6As2S7]2− and a second module layer (labeled B) with composition [Ag10HgAs2S8]2+. Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chañarcillo, Copiapó Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P3̅m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.