CONTINUOUS SOLID-SOLUTION BETWEEN MERCURIAN GIRAUDITE AND HAKITE

Abstract

Among the minerals of the selenide assemblage at the Niederschlema-Alberoda uranium deposit, Erzgebirge, Germany, members of the mercurian giraudite-hakite solid solution intergrown with berzelianite and galena have been identified as rare and previously unknown phases. They form complexly zoned, anhedral, minute (<350 mu m) grains embedded in a dolomite matrix. Compositional variability is expressed by the following crystallochemical formula (calculated on the basis of 29 atoms per formula unit): (Cu-9.92-Ag0.01-0.08)(Sigma 10.00) (Hg0.92-1.81Cu0.06-1.12Zn0.05-0.10Fe0.00-0.15)(Sigma 1.98-2.06) (As0.69-3.98Sb0.02-3.29)(Sigma 3.91-4.08) (Se10.47-11.53S1.47-2.61)(Sigma 12.90-13.09). The solid solutions span the range from gir(99.5)hak(0.5) to gir(16.2)hak(83.8), suggesting complete miscibility between mercurian giraudite and mercurian hakite in nature, equivalent to what already has been established for their S-bearing analogues, tennantite and tetrahedrite. The lack of thermodynamic data for both Se-rich species limits reliable inferences on the P-T-X conditions that prevailed during their formation. The assemblage mercurian giraudite-hakite + berzelianite + galena may represent a short-term equilibrium paragenesis of Jurassic age, formed at temperatures between 110 and 150 degrees C under the conditions of low Se and S activities (i.e., -26 < logf(Se-2) < -31 and -24 < logf(S-2) < -28 at similar to 110 degrees C), before the bulk of the selenide minerals crystallized. A second, less likely hypothesis calls upon the formation of the mercurian giraudite-hakite solid solutions during an early Cretaceous event, when pre-existing selenide minerals (berzelianite) were partially attacked by infiltrating fluids that introduced the major portion of the As and Sb into the system.