New results (compositional data and reflectance values) are reported for some rare sulfides enriched in K, Tl and Pb, which are related to djerfisherite, thalfenisite, bartonite, a “Cl-bearing bartonite”, or chlorbartonite, and also for shadlunite, from the Noril'sk and Salmagorsky complexes, Russia. Our observations and comparisons with relevant data in the literature imply that: (1) bartonite is probably a S-dominant (or Cl-free) analogue of djerfisherite; and a “Cl-bearing bartonite” and chlorbartonite are probably compositional variants of the djerfisherite–bartonite series. (2) The most probable formulae of bartonite and djerfisherite are (K,Me2+)6(Fe,Cu,Ni)25–xS26(S,Cl) and (K,Me2+)6(Fe,Cu,Ni)25–xS26(Cl,S), where 0 ≤ x ≤ 5, respectively. (3) Two independent substitution mechanisms probably operate in the natural series. A coupled substitution [Me2+ + S2− ↔ K+ + Cl] is reflected by an observed deficit in K, accompanied by the incorporation of Me2+ (Pb, Fe, or Ni) in the K site. Another mechanism is inferred to be [2Fe3+ + □ ↔ 3Fe2+], which assumes the existence of vacancy-type defects at the Me site. Thus, the second mechanism could possibly control the existing variations of Σ(Fe,Cu,Ni) in the range of ~21 to 25 a.p.f.u., documented in djerfisherite- and bartonite-type minerals. The minerals analysed from Noril'sk, which are free of Cl and related to bartonite and to a Tl-dominant analogue of bartonite (unnamed species), probably crystallized from microvolumes of late fluid rich in K and Tl, under conditions of relatively low oxygen fugacity in the environment. Uniform contentss of Fe and Cu, observed in coexisting phases of normal (Cl-bearing) djerfisherite and bartonite (or Cl-free analogue of djerfisherite) at Salmagorsky imply that they reached equilibrium with regard to the distribution of these elements during crystallization. These phases probably formed as a result of fluctuations in the ratios of sulfur and chlorine fugacity in a fluid at a postmagmatic hydrothermal stage.

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