Juxingite, Bi6Cu140Fe30S125, a new copper sulfide mineral from the Jiama porphyry-skarn Cu-polymetallic deposit, Tibet, China

Abstract Juxingite (IMA2024-011), with an ideal chemical formula of Bi6Cu140Fe30S125 (Z=125), is a new mineral discovered in the skarn-type ore of the Jiama deposit (91°43′06″E, 29°43′53″N) in Tibet, China, located in the contact zone between the Lower Cretaceous Linbuzong Formation and Miocene porphyry. Juxingite occurs as anhedral droplets and veinlets, pale yellow in color with black streak and metallic luster and a few microns to several tens of microns in size, in the matrix of Bi-bearing bornite. It is brittle with uneven fractures with a Vickers microhardness (VHN10) of 122 kg/mm2 (equivalent to Mohs’ hardness of 3) and has a calculated density of 5.048 g/cm3. Electron microprobe analyses give the empirical formula Bi5.52Cu134.10Fe32.02(S124.77Se0.23)Σ125 on the basis of total anions = 125. Juxingite is cubic, space group F4¯3m, with the unit-cell parameters a = 27.3529 (5) Å, V=20464.9 (6) Å3 and Z=4, corresponding to the 5a-5a-5a unit-cell in relation to the cubic 1a-type bornite Cu5FeS4. The structure is composed of alternate S layers and Cu-Fe cation layers in three dimensions. The S layers, with the average interlayer distance of 2.735 Å, can be divided into three types: the pure S layer without Bi atoms; the layer with the addition of a cation site (Bi1) dominated by Bi (occupancy 0.74), and the layer with two cation sites, Bi1 and additional Bi2 (Bi occupancy 0.036). Three types of cation layers are distinguished according to the number of Cu, Fe and vacancies (Cu or Fe occupancy <0.5) in the area a*b: 54Cu+10Fe+36□; 56Cu+16Fe+28□ and 60Cu+8Fe+32□. The Bi atoms are in octahedral coordination with Bi-S distances from 2.786 Å to 2.797 Å (average 2.794 Å) for Bi1 and from 2.54 Å to 3.09 Å (average 2.81 Å) for Bi2, while the Cu and Fe atoms are in tetrahedral coordination with Cu-S distances from 2.261 Å to 2.663 Å (average 2.379 Å) and Fe-S distances from 2.17 Å to 2.44 Å (average 2.305 Å). Juxingite was formed under medium-high temperature conditions and intermediate-sulfidation state, and it is associated with the enrichment of gold.