Ultrahigh-temperature sphalerite from zn-cd-se-rich combustion metamorphic marbles, daba complex, central Jordan: Paragenesis, chemistry, and structure

Minerals of the Zn-Cd-S-Se system that formed by moderately reduced ~800–850 °C combustion metamorphic (CM) alteration of marly sediments were found in marbles from central Jordan. Their precursor sediments contain Se-and Ni-enriched authigenic pyrite and ZnS modifications with high Cd enrichment (up to ~10 wt%) and elevated concentrations of Cu, Sb, Ag, Mo, and Pb. The marbles are composed of calcite, carbonate-fluorapatite, spurrite, and brownmillerite and characterized by high P, Zn, Cd, U, and elevated Se, Ni, V, and Mo contents. Main accessories are either Zn-bearing oxides or sphalerite, greenockite, and Ca-Fe-Ni-Cu-O-S-Se oxychalcogenides. CM alteration lead to compositional homogenization of metamorphic sphalerite, for which trace-element suites become less diverse than in the authigenic ZnS. The CM sphalerites contain up to ~14 wt% Cd and ~6.7 wt% Se but are poor in Fe (means 1.4–2.2 wt%), and bear 100– 250 ppm Co, Ni, and Hg. Sphalerite (Zn,Cd,Fe)(S,O,Se)_cub is a homogeneous solid solution with a unit cell smaller than in ZnS_cub as a result of S²⁻ → O²⁻ substitution (a = 5.40852(12) Å, V = 158.211(6) ų). The amount of lattice-bound oxygen in the CM sphalerite is within the range for synthetic ZnS_1−xO_x crystals (0 < x ≤ 0.05) growing at 900 °C.