Tyrrellite, Cu(Co0.68Ni0.32)2Se4, isostructural with spinel.

ABSTRACT

Tyrrellite, a naturally occurring Co-Ni-Cu selenide, has been studied by single-crystal X-ray diffraction. It possesses the normal spinel-type structure, with Cu occupying the tetrahedral site and (Co+Ni) the octahedral site. The average Cu-Se distance of 2.3688 (2) A is close to that of 2.3703 (8) A in CuCr(2)Se(4), whereas the average (Co+Ni)-Se distance of 2.3840 (1) A appears to be slightly shorter than most octahedral Co-Se or Ni-Se distances ( approximately 2.40-2.50 A) in other selenides. The refined structure provides a basis for a redefinition of the ideal chemical formula of tyrrellite, which should be Cu(Co,Ni)(2)Se(4), rather than the previously suggested (Cu,Co,Ni)(3)Se(4).