The new mineral cuprozheshengite, Pb4CuZn2(AsO4)2(PO4)2(OH)2, from Yunnan, China, with site-selective As-P substitution

Abstract

Abstract Cuprozheshengite, Pb4CuZn2(AsO4)2(PO4)2(OH)2, is a new mineral species from Yunnan, China. It occurs as sub-millimeter greenish-blue hemispherical aggregates of microscopic blade-like crystals on hemimorphite and is closely associated with veszelyite and galena. Cuprozheshengite is brittle with irregular fracture and has a Mohs hardness of 2½–3 and perfect cleavages on {011}. The calculated density is 5.91 g/cm3. The empirical chemical formula of the holotype is (Pb3.97Na0.04Ca0.01)Σ4.02Cu1.06 Zn2.09(AsO4)2[(P0.84As0.12Si0.01)Σ0.97O4]2(OH)2 based on 18 O atoms per formula unit. Cuprozheshengite is triclinic, space group P1, with unit-cell parameters a = 4.7977(8), b = 8.5789(8), c = 10.3855(9) Å, α = 97.270(8)°, β = 101.902(12)°, γ = 91.495(11)°, V = 414.30(9) Å3, and Z = 1. Cuprozheshengite is a member of dongchuanite group, whose general formula is A4VIBIVB2(X1O4)2(X2O4)2(OH)2, where A is an interlayer cation with Pb being dominant; B are transition metals with two crystallographic positions, IVB has tetrahedral coordination and is fully occupied by Zn, while VIB has octahedral coordination and is dominated by Zn or Cu; X1 and X2 are cations with tetrahedral coordination, occupied by As and P. Like other dongchuanite group minerals, the structural framework of cuprozheshengite is composed of two heteropolyhedral columns along [100]. Type 1 columns comprise corner-linked [IVBO4] and [X2O4] tetrahedra. Each tetrahedron is connected with three other tetrahedra in the columns. Type 2 columns have alternating [VIBO4(OH)2] octahedra with pairs of corner-connected [X1O4] tetrahedra. These two columns are connected by corner-sharing between [IVBO4] and [X1O4] tetrahedra to form layers parallel to (011). Pb atoms occupy two independent sites between the layers. Cuprozheshengite is named as the copper analog of zheshengite. Single-crystal X-ray diffraction reveals that As and P order over the X1 and X2 sites, with As tending to occupy X1. Density functional theory (DFT) calculations confirm the occupancy propensity of As benefiting structural stability. The structural and stability studies of cuprozheshengite may have implications for local environmental governance. As a stable mineral in the water and elemental cycles after weathering, cuprozheshengite still has the potential to continually crystallize, fixing As into a stable crystalline waste form.