Zinconigerite-2N1S ZnSn2Al12O22(OH)2and zinconigerite-6N6S Zn3Sn2Al16O30(OH)2, two new minerals of the nolanite-spinel polysomatic series from the Xianghualing skarn, Hunan Province, China

Zinconigerite-2N1S ZnSn2Al12O22(OH)2 and zinconigerite-6N6S Zn3Sn2Al16O30(OH)2 are two new minerals with different numbers and ratios of nolanite (N) and spinel (S) modules. Both phases have been discovered in the Xianghualing skarn, Hunan Province, China. Zinconigerite-2N1S (zn-2N1S) and zinconigerite-6N6S (zn-6N6S) are named for their chemical composition, number, and ratios of N-S modules, according to the nomenclature of the nolanite-spinel polysomatic series of Armbruster (2002). Both phases occur as aggregates, sub-to-euhedral crystals, with maximal dimensions up to 100 μm, within fluorite aggregates, and they are closely associated with phlogopite, chrysoberyl, magnetite, cassiterite, margarite, and nigerite-taaffeite group minerals. They do not show fluorescence in long- or short-wave ultraviolet light. The calculated densities are 4.456 g/cm3 for zn-2N1S and 4.438 g/cm3 for zn-6N6S. Optically, zn-2N1S is uniaxial (+) with ω = 1.83(1), ϵ = 1.84(2); zn-6N6S is uniaxial (+) with ω = 1.85(1), ϵ = 1.87(2) (λ = 589 nm). Their chemical compositions by electron-microprobe analyses give the empirical formulas (Zn0.734Mn0.204Na0.122Ca0.063Mg0.044)ς1.166(Sn1.941Zn0.053Ti0.007)ς2 A l 11.018 F e 0.690 3 + Z n 0.200 S i 0.092 12 O 22 (O H) 2 $\left({Al}{11.018} {Fe}{0.690}{3+} {Zn}{0.200} {Si}{0.092}\right) {12} {O}{22}({OH})2$for zn-2N1S and Z n 1.689 M n 0.576 M g 0.328 F e 0.407 3 + ς 3 S n 1.882 Z n 0.047 $\left({Zn}{1.689} {Mn}{0.576} {Mg}{0.328} {Fe}{0.407}{3+}\right) {\Sigma 3}\left({Sn}{1.882} {Zn}{0.047}\right.$ T i 0.071 ς 22 A l 14.675 F e 1.088 3 + N a 0.13 C a 0.086 S i 0.017 ς 15.996 O 30 (O H) 2 $ \left.{Ti}{0.071}\right) {\Sigma 22}\left({Al}{14.675} {Fe}{1.088}{3+} {Na}{0.13} {Ca}{0.086} {Si}{0.017}\right) {\Sigma 15.996} {O}{30}({OH})2$for zn-6N6S. Both phases have trigonal symmetry; the unit-cell parameters of zn-2N1S (P3m1) and zn-6N6S (R3m), refined from single-crystal Xray diffraction data, are, a = 5.7191(2) and 5.7241(2) Å, c = 13.8380(6) and 55.5393(16) Å, V = 391.98(3) and 1575.96(12) Å3, and Z = 1 and 3, respectively. The structure of zn-2N1S is characterized by the alternating O-T1-O-T2-O-T1 layers stacked along the c-axis, showing the connectivity of N-S-N. The polyhedral stacking sequence of zn-6N6S is 3 × (O-T1-O-T2-O-T2-O-T1), reflecting a N-S-S-N-N-S-S-N-N-S-S-N connectivity of the polysomatic structure. By contrast, the structure of zn-2N1S shows the elemental replacements of Al → Sn and Al → Zn, suggesting the substitution mechanism of 2Al → Zn + Sn. The complex substitution of Zn by multiple elements (Al, Fe3+, Mn, Mg) in the structure of zn-6N6S, is coupled with the low occupancy of Al5-octahedra. Fe3+ → Al substitution occurs in Al1-tetrahedra of both zn-2N1S and zn-6N6S. The new polysomes, zn-2N1S and zn-6N6S, likely crystallized under F-rich conditions during the late stages of the Xianghualing skarn formation. The discovery of zn-2N1S and zn-6N6S provides new insights into the crystal chemistry of the N-S polysomatic series and its origin.