SUBSTITUTION LIMITS IN NEW MINERAL STAROČESKÉITE Ag_0.7Pb_1.6(Bi_1.35Sb_1.35)_2.70S₆

R. Pažout

Institute of Chemical Technology, Technická 5, Praha 6,166 28, Czech Republic

richard.pazout@vscht.cz

A new mineral, staročeskéite, a sulfide of Ag. Pb, Bi and Sb, was found in Kutná Hora ore district, Czech Republic, chemical composition established, structure solved, proposal for a new mineral submitted to the Commission for new Mineral of the International Mineralogical Association and mineral approved.  The simplified empirical formula based on electron probe microanalysis is Ag_0.70Pb_1.60(Bi_1.35Sb_1.35)_Σ2.70S₆. The mineral belongs to the family of lilllianite homologous series with N = 4, with a general formula Ag⁺_X Pb²⁺_3-2X Bi³⁺_Y S₆.The N value represents a number of octahedra  in two neighbouring blocks in the structure (sites M1 and M2, Fig 1), where two main substitutions take place: 1. Ag⁺ + Bi³⁺,Sb³⁺ ↔ 2 Pb²⁺and 2. Bi³⁺ ↔ Sb³.

Staročeskéite is orthorhombic, space group Cmcm, with a = 4.2539(8), b = 13.3094(8), c = 19.625(1) Å, V = 1111.1(2) ų, Z = 4.  The structure of staročeskéite contains four sulfur sites and three metal sites: one pure Pb site M3 in trigobal prismatic coordination and two mixed octahedral sites - each with three atom species: M1 (0.52Bi + 0.356Ag + 0.124Sb) and M2 (0.601Sb + 0.259Pb + 0.14 Bi).  The found mineral is characterized by Bi:Sb ratio 1:1 (Bi/(Bi + Sb) = 0.50) and the Ag⁺ + Bi³⁺,Sb³⁺ ↔ 2 Pb²⁺ substitution (L%) equal to 70 %.

Because there are several minerals of a similar and very close composition in the lillianite series of sulfosalts (Fig 2), it is necessary to determine the ranges of substitution (substitution limits) within which the mineral is defined. Empirically it was determined that the name staročeskéite would be valid for a lillianite structure with composition Ag_xPb_3-2xBi_ySb_2+x-yS₆ with the boundaries ½ ≤x≤ 0.8, and 1-½ x ≤y≤ 2, where the parameter x = Ag content = L% and y = total Bi content.  Thus we concluded that for staročeskéite to exist, there must between 20 to 50 % occupation of M2 site by Pb, apart from fully occupied M3 site. Other Pb concentrations in M2 site lead to different minerals.

Figure 1. Crystal structure of staročeskéite

Figure 2. Diversity of mineral species from the series of lillianite homologues with N = 4: Star - staročeskéite, Gus - gustavite, UN - undersubstituted gustavite, Ter - terrywallaceite, Fiz – fizelyite,  Ram – ramdohrite, And IV – andorite IV, Nak – nakaseite (Cu-rich andorite VI).

1.   R.Pažout, M. Dušek (2010) Crystal structure of natural orthorhombic  Ag_0.71Pb_1.52Bi_1.32Sb_1.45S₆, a lillianite homologue with N = 4; comparison  with gustavite. European Journal of  Mineralogy, 22,741-750.

2.    R. Pažout, J. Sejkora J (2018) Staročeskéite, Ag_0.70Pb_1.60(Bi_1.35Sb_1.35)Σ_2.70S₆,  from Kutná Hora, Czech Republic, a new member of lillianite  homologous series. Mineralogical Magazine, 1-26. doi:10.1180/minmag.2017