WEIRD WULFENITE: AN INVESTIGATION INTO UNUSUAL CROSS-SHAPED CRYSTAL ZONING IN WULFENITE FROM THE 79 MINE, ARIZONA USING SCANNING ELECTRON MICROSCOPY AND ELECTRON DISPERSIVE SPECTROSCOPY

Mineral collectors at the 79 Mine near Hayden, Arizona recently discovered highly unusual wulfenite crystals that exhibit a cross-shaped crystal zonation. From 1919 until the mid 1950s the 79 Mine was operated commercially for Ag/Pb/Cu/Zn, exploiting rich oxidized ores down to a depth of about 450 feet. (1) Since its closure, the mine has been operated in small capacity for mineral specimens, and has produced museum-quality specimens of smithsonite, aurichalcite, hemimorphite, and wulfenite. (2) Recent research has led to the discovery of liudongshengite, a new zinc chromium carbonate mineral. (3)

Most of the wulfenite crystals collected at the 79 Mine form glassy semi-transparent yellow tabular crystals. On rare occasion they will exhibit a red interior zonation with yellow exteriors. Most of the wulfenite extraction occurred on the 400 level of the mine in an oxidized zone next to the fault. Recently, small wulfenite crystals with an unusual thin black cross zonation which starts in the center of the crystals and extends out along the a and b axes perpendicular to the c-axis were extracted from around the 150 level of the mine. This zonation has not been observed in any other examples of wulfenite globally.

We hypothesize that the unusual zonation is thermodynamically controlled with an unknown chromophore ion preferentially bonding at a site as the crystal grows and thereby concentrating along just the cross axis. We aim to capture the crystallography of the crystals using scanning electron microscopy. Then we will utilize electron dispersive spectroscopy to map which ions may be concentrated in the cross. Once the concentrated element is identified, we will gain further understanding into the changing mineralization conditions at the 79 mine, as well as insight into the thermodynamic controls of wulfenite crystal formation.

1. Gibbs, R.B., Micromineraleering in the 79. New Mexico Mineral Symposium Abstracts. November 2018.
2. Keith, S.B., Mineralogy and Paragenesis of the 79 Lead-Zinc-Copper Deposit [Arizona]. The Mineralogical Record: 3(6): 247-264. 1972.
3. Yang, H., Gibbs, R.B., Schwenk, C., Xie, X., Gu, X., Downs., R.T., Evans., S.H., Liudongshengite, Zn₄Cr₂(OH)₁₂(CO₃)ꞏ3H₂O, a new mineral of the hydrotalcite supergroup, from the 79 mine, Gila County, Arizona, USA. The Canadian Mineralogist: 59(4): 763-769. 2021.