Structure of synthetic Li₂(Mg,Cu)Cu₂([Si₂O₆]₂; a unique chain silicate related to pyroxene

A unique Cu-bearing chain silicate, Li2(Mg,Cu)Cuz[Si206b was synthesized, and the structure was determined by single-cry~tal X-ray diffraction techniques. The structure was found to be triclinic, spacoe group PI, with unit-cell parameters a = 5.7068(7), b = 7.4784(9), c = 5.2193(3) A, ex = 99.911(8), J3 = 97.436(8), 'Y = 84.52(1)°, and Z = 1. The arrangement of zweier single chains, [Siz06], differs significantly from chain arrangements in the pyroxene and pyroxenoid structures, and the "I-beam" description of the pyroxene structure is not applicable. The structure may be classified as a new derivative type of the pyroxene structure, with an "oblique I-beam". Cu atoms are coordinated by four ° atoms in a square-planar arrangement with 1.94-2.00 A for Cu-O and two ° atoms with longer Cu-O distances of 2.41-2.92 A, consistent with the crystal-field stabilization ~f the d9.electroni~ structure of Cu2+. The square-planar Cu04 units form a [CunOZn+2] nbbon with n = 3 ill the structure, which is also found in Cu-bearing chain silicates such as shattuckite and plancheite with n > 3. Mg is octahedrally coordinated by ° atoms, but the configuration is affected by the partial replacement by Cu.