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

Abstract

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 Příbram 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 (hakitesensu 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)∑6.00Cu4.00(Hg1.61Zn0.20Cu0.19Cd0.15Fe0.04)∑2.19(Sb3.85As0.28)∑4.13(Se11.55S1.14)∑12.69. Less common is the Zn-hakite, (Cu5.80Ag0.20)∑6.00Cu4.00(Zn1.33Hg0.42Cd0.22Cu0.18Fe0.01)∑2.16(Sb3.85As0.26)∑4.11(Se10.92S1.81)∑12.73(n= 22), and rare Cd-hakite has an empirical formula (n= 7) of (Cu5.84Ag0.16)∑6.00Cu4.00(Cd1.27Zn0.60Cu0.10Hg0.07Fe0.02)∑2.06(Sb4.00As0.19)∑4.19(Se12.14S0.61)∑12.75. The refined unit cell of Hg-hakite from Příbram, obtained from powder X-ray diffraction data, isa= 10.8783(3) Å withV= 1287.3(1) Å3(Z= 4, for the cubic space groupI4̄3m). 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.

Selenide Mineralization in the Příbram Uranium and Base-Metal District (Czech Republic)

“…In one case, Hg-hakite formed an aggregate of well-formed idiomorphic crystals up to 100 µm [54]. Both of the hakite types occur especially in association with berzelianite, or rarely, with athabascaite.…”

“…However, the dominant C cation in these minerals may be Zn, Fe, Hg, Cu, or Cd, and here we distinguish these cases using the dominant-constituent rule [99]. Hakite has been originally described as an Hg variety, but for the matter of classification, Škácha et al [54] propose the application of the dominant rule (on the atomic basis) for solid-solutions of the members of the tetrahedrite group. That is why we distinguish Zn-, Fe-, Hg-, Cu-, and Cd-members in the following text.…”

“…In contrast to tetrahedrite, hakite is more common in microscopic form, but locally, aggregates up to several mm in size were observed. At present, Škácha et al [54] studied in detail hakite from Příbram. They distinguish three chemical types of hakite: Hg-, Zn-, and Cd-hakite.…”

“…A simplified general formula for minerals of the tetrahedrite group can be expressed according to Moëlo et al [97] as [III] A 6 [IV] (B,C) 6 The classification of the tetrahedrite group of minerals is in need of review and updating, but this has not been done to date [97,98]. Currently, the divalent substituents C in tetrahedral coordination are not generally distinguished in the IMA-approved definitions of the minerals, with the exception of Hg-dominant hakite, although varieties with different dominant C cations are distinguished in the literature [54]. However, the dominant C cation in these minerals may be Zn, Fe, Hg, Cu, or Cd, and here we distinguish these cases using the dominant-constituent rule [99].…”

“…After extending the data set used by Škácha et al [54] by analyses of the new samples, it is possible to define five chemical types of hakite, depending on filling the C position by the respective elements: Hg-, Zn-, Cd-, Fe-, and Cu-hakite.…”