Crystal structure and chemistry of natural kutinaite from Černý Důl, Krkonoše, Czech Republic

Abstract

We have characterized the crystal structure of natural kutinaite, a rare mineral from the ores of Černý Důl, Czech Republic, by single-crystal X-ray diffraction and chemical analysis. We found that the structure of natural kutinaite is not identical to that of synthetic Cu14Ag6As7, previously reported to be cubic, space group Pm3m. Although topologically similar, the structure of natural kutinaite is indeed tetragonal, space group P4/mmm, with cell parameters: a = 11.789(2), c = 11.766(2) Å, V = 1635.5(4) Å3 and Z = 4. Electron microprobe analyses pointed to the (K,Tl)0.25Cu14Ag6As6.75 stoichiometry (Z = 4), or (K, Tl)Cu56Ag24As27 with Z = 16. The crystal structure of an untwinned crystal has been refined to R1 = 2.61%. It consists of clusters of eight edge-sharing tetrahedra of Cu, which alternate in a 3D chess-board manner with octahedral clusters of six Ag atoms. The latter are surrounded by triangularly coordinated copper in eight faces of a cuboctahedron. The last structure components are large cavities containing partly occupied (K,Tl) sites, coordinated by 18 Ag and As ligands. The structure is full of direct metal-metal contacts although As plays the role of anion, associating especially with copper.

Modular Crystal Chemistry of Thallium Sulfosalts

“…Copper interacting with LEP of As at the distance of 2.64 Å in spaltiite is in agreement with the As-Cu bond lengths in natural kutinaite [76] (which is arsenide of Cu-Ag with minor Tl) which lie between 2.280 and 2.578 Å, though mostly above the 2.46 Å threshold. The short Ag-As distance of 3.049-3.060 Å, observed in raberite, agrees with the set of As-Ag bond lengths in natural kutinaite [76], which span 2.926-3.091 Å.…”

“…The cation-pnictogen interaction observed in the present structure for Cu1-As1, equal to 2.65 Å, is an unusual feature of this structure (Figure 65). The distance is at the upper range of distances observed in copper arsenides, which are 2.41-2.60 Å in low domeykite (Iglesias and Nowacki [100]) and 2.28-2.58 Å in natural kutinaite [76]. If it was a strong single bond in a tetrahedral configuration, it should be shorter.…”

“…One more remarkable cage structure with thallium is that of natural kutinaite, (K,Tl) 0.25 Cu 14 Ag 6 As 6.75 , from Krkonoše, Czech Republic [76]. This is an alloy full of metal-metal bonds; however, As still plays a role of anion in it.…”