Characterization of “Sandwich” Wulfenite from the Ojuela Mine, Mapimí, Mapimí Municipality, Durango, Mexico: Evidence of Preferred Secondary Nucleation on Selected Wulfenite Faces

Abstract

This paper describes the formation of “sandwich” wulfenite. Banded wulfenite from the Ojuela Mine, Mapimí, Durango, Mexico, have been found since 2017, but an explanation for the band formation has not been provided. X-ray diffraction (XRD) showed the wulfenite to have a tetragonal unit cell of a = 5.4374(1), c = 12.1123(7) Å. The Raman spectrum was dominated by ν1 (Ag) around 870 cm–1, while the weak shoulder at 859 cm–1 represents the strain activated ν1 (Bu) infrared (IR) band. Two ν2 modes were observed at 318 cm–1 (Ag) and 351 cm–1 (Bg). For the ν3(Eg), the band at 768 cm–1 was assigned to the Bg symmetric mode and the 745 cm–1 band to the Eg vibration. The IR spectrum showed two strong bands at 835 and 779 cm–1 corresponding to v3 modes and a very weak one at 496 cm–1. SEM-EDX (scanning electron microscopy – energy dispersive X-ray analysis) showed that the band does not extend into the main crystal but is limited to the surface as secondary growth. The main thick tabular crystal was dominated by {001}, {110}, and {111}. The {001} surfaces of the wulfenite crystals showed evidence of later dissolution, which may have been part of the source material for this secondary crystallization. The chemical composition of the main wulfenite crystal proved only minor substitution of W for Mo. Contrastingly, a much higher number of substitutions by V, As, and W for Mo – as well as Ca, Fe, Zn, Cu, and Al for Pb – was observed in the secondary band, indicating a change in the fluid composition from which the band crystallized compared to the original wulfenite. Changes in lattice parameters of the secondary wulfenite crystals and the energy involved in nucleation on the surface of the original wulfenite as well as the different bonds exposed on different faces favored the formation of the band on only {110}.