Abstract

The crystal structure of a natural krennerite (Au3AgTe8) from Cripple Creek, Colorado, USA has been refined in space group Pma2 using 3,151 observed unique (acentric) reflections. The final R value for observed reflections was 0.022. Unit cell parameters are a 16.590(3) Å, b 8.867(2) Å, c 4.4886(8) Å, V 660.29(13) Å3, and Z = 2. The precision of the atom position parameters is improved by up to a factor of ten over previous studies. The three Au–Ag sites in the structure are ordered, with Ag = 43% occupancy at the Au1 (2a) position (point symmetry 2) and 59% at the Au2 (2c) position (point symmetry m); the Au3 (4d) position (point symmetry 1) is 100% Au. The observed ordering in the structure appears to be due to avoidance of Ag–Te–Ag bonding. Ordering of Au and Ag in the structure may thus limit the composition range to being between those of calaverite and sylvanite. Detailed petrographic study of the sample in thin section, accompanied by electron microprobe data, demonstrates the paragenesis of the ore that provided the crystal for refinement. The petrographic study provides evidence for sylvanite exsolution from krennerite in natural systems, consistent with the narrow compositional range inferred from the crystallographic study.

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