Chukochenite, (Li0.5Al0.5)Al2O4 , a new lithium oxyspinel mineral from the Xianghualing skarn, Hunan Province, China

Chukochenite, (Li 0.5 Al 0.5)Al 2 O 4 , is a new mineral species from the Xianghualing skarn, Hunan Province, southern China. It occurs as subhedral to euhedral crystals up to 200 μm across in the green rock of Xianghualing skarn, closely associated with fluorite, phlogopite, chrysoberyl, margarite, chlorite, ferronigerite-2N1S, and zinconigerite-2N1S. The crystals are colorless and transparent with a vitreous luster. Chukochenite is brittle with irregular fracture, has a Mohs hardness of 8, and shows light red fluorescence under 253.7 nm UV radiation and light green fluorescence under 365 nm UV radiation. The calculated density is 3.771 g/cm 3. Chukochenite is optically biaxial (-) with α = 1.79(2), β = 1.82(2), and γ = 1.83(2) (589 nm). The calculated 2V is 60°, with the optical orientations X, Y, and Z parallel to the crystallographic a, b, and c, respectively. Electron microprobe analysis (Li by LA-ICP-MS) yielded in wt% Al 2 O 3 80.70, Fe 2 O 3 8.16, Li 2 O 3.68, ZnO 3.25, MnO 2.49, MgO 1.70, Na 2 O 0.11, CaO 0.08, TiO 2 0.02, K 2 O 0.01, and Cr 2 O 3 0.01 (total 100.24 wt%), giving an empirical formula [(Li 0.355 Al 0.138 Na 0.005 Ca 0.002) Σ0.5 (Al 0.145 Fe +3 0.147 Mg 0.061 Zn 0.058 Mn 0.051 Si 0.001) Σ0.463 ]Al 2 O 4 on a basis of 4 O atoms per formula unit. Chukochenite is orthorhombic, Imma, a = 5.659 (1), b = 16.898 (1), c = 7.994 (1) Å, V = 764.46 (8) Å 3 , and Z = 12. The nine strongest lines of powder XRD [d in Å (I) (hkl)] are: 2.405 (53) (231); 1.996 (29) (260); 1.535 (77) (303); 1.413 (100) (264); 1.260 (52) (2 12 0); 1.068 (36) (1 13 4); 1.039 (61) (503); 0.999 (59) (008); and 0.942 (35) (3 13 4). Chukochenite has a framework structure of spinel with low symmetry (orthorhombic Imma) due to the ordering of Li cations over octahedrally coordinated sites, which has not been previously reported for synthetic (Li 0.5 Al 0.5) Al 2 O 4. This structure type is based on a framework of AlO 4 tetrahedra, AlO 6 , and LiO 6 octahedra. AlO 6 edge-sharing octahedra form chains along the a axis. AlO 6 octahedra and LiO 6 octahedra in a 2:1 ratio share edges, forming octahedral chains along b. These octahedral chains are connected by AlO 4 tetrahedra and each corner of an AlO 4 tetrahedron shares with three AlO 6 octahedra or two AlO 6 + one LiO 6 octahedra. The discovery of chukochenite adds a new perspective on the cation ordering and the mechanism of luminescence and magnetism in (Li 0.5 Al 0.5)Al 2 O 4 .