Hakite from Pribram, Czech Republic: compositional variability, crystal structure and the role in Se mineralization

摘要

Hakite, ideally Cu10Hg2Sb4Se13, is a Se-dominant member of the tetrahedrite group occurring at only a few localities in the World. A new occurrence of this mineral in the Pribram uranium and base-metal ore district, Central Bohemia, Czech Republic, is reported in this paper. Hakite was found to be locally abundant and was identified in several samples with Se mineralization. Three chemically distinct types of hakite were distinguished based on electron microprobe study, Hg-rich hakite (hakite sensu stricto), Zn-rich hakite and Cd-rich hakite. Hg-hakite dominates among the samples studied. Its average empirical formula based on 29 apfu (n = 54) is (Cu5.61Ag0.39)(Sigma 6.00)Cu-4.00(Hg1.61Zn0.20Cu0.19Cd0.15Fe0.04)(Sigma 2.19) (Sb3.85As0.28)(Sigma 4.13)(Se11.55S1.14)(Sigma 12.69). Less common is the Zn-hakite, (Cu-5.80 Ag-0.20)(Sigma 6.00)Cu-4.00(Zn1.33Hg0.42Cd0.22Cu0.18Fe0.01)(Sigma 2.16)(Sb-3.85 As-0.26)(Sigma 4.11)(Se10.92S1.81)(Sigma 12.73) (n = 22), and rare Cd-hakite has an empirical formula (n = 7) of (Cu-5.84 Ag-0.16)(Sigma 6.00)Cu-4.00(Cd1.27Zn0.60Cu0.10Hg0.07Fe0.02)(Sigma 2.06)(Sb4.00As0.19)(Sigma 4.19)(Se12.14S0.61)(Sigma 12.75). The refined unit cell of Hg-hakite from Pribram, obtained from powder X-ray diffraction data, is a = 10.8783(3) angstrom with V = 1287.3(1)angstrom(3) (Z = 4, for the cubic space group I(4) over bar 3m). Structure refinement from the precession electron diffraction data collected on the transmission electron microscope (R = 24.4% for 424 observed reflections), confirmed that hakite is isostructural with tetrahedrite. The evolution of hydrothermal fluids, from which Se mineralization formed, suggests a distinct enrichment in sulfur and depletion in selenium over the time span of crystallization.