Crystal Chemistry Research of Minerals of Nigerite Group and Its Significance

There is a type of complex oxide mineral, composed of many elements such as Sn, Mg, Fe, Zn, Ti, Mn, Al, etc., in the areas of Anhua, Linwu and Shizhuyuan of Hunan Province. These minerals belong to nigerite, Mg-nigerite (pengzhizhongite), etc.. According to the principle of closest packing, the crystal chemical properties of nigerite (brown), pengzhizhongite (buff), Zn-nigerite (fawn), Fe-nigerite, Mn-nigerite (greenish-black) etc. have been analyzed. Their crystal structures may be characterized by O (the layers of cation octahedral coordinations), T₁ (the mixed layers of cation octahedral coordinations and cation tetrahedral coordinations in same directions), T₂ (the mixed layers of cation octahedral coordinations and cation tetrahedral coordination in different directions). The position of layer-O and layer-T is alternate permutation. The crystal structure of pengzhizhongite (6H) may be expressed by …OT₂OT₁OT₁…, taaffeite (8H), …OT₂OT₁OT₂OT₁…, and nigerite (24R), … OT₁OT₂OT₂OT₁ …×3 etc.. In their structure, there are not only the crystal structure units of spinel … OT₂ OT₂ … but also the units of nolanite … OT₁ OT₁ …. The research of these minerals has important theoretic and practical significance in the fields of minerals, gemology, material science etc..