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Synthetic nagyagite was grown from a melt as part of a search for materials with high-temperature superconductivity. Electron microprobe analyses of synthetic nagyagite and of nagyagite from the type locality Nagyag, Transylvania (now Sacarimb, Romania) agree with data from literature. The crystal chemical formula [Pb(Pb,Sb)[S.sup.2]][(Au,Te)] was derived from crystal structure investigations. Nagyagite is monoclinic pseudotetragonal. The average crystal structure was determined from both synthetic and natural samples and was refined from the synthetic material to R = 0.045 for 657 single-crystal X-ray data: space group P[2.sub.1]/m, a = 4.220(1) [Angstrom], b = 4.176(1) [Angstrom], c = 15.119(3) [Angstrom], [Beta] = 95.42(3) [degrees], and Z = 2. Nagyagite features a pronounced layer structure: slices of a two slabs thick SnS-archetype with formula Pb(Pb,Sb)[S.sub.2] parallel to (001) have a thickness of 9.15 [Angstrom]. Te and Au form a planar pseudo-square net that is sandwiched between the SnS-archetype layers; it is assumed that planar [Au.sup.[4Te]][Te.sub.4] configurations are edge connected to chains and that Te atoms are in a zigzag arrangement. Ordering within the SnS-archetype and gold-tellurium layers, intense twinning and/or stacking variants are responsible for the often observed superstructure reflections. For buckhornite, [[([Pb.sub.2]Bi).sub.[Sigma]3][S.sub.3]][[(AuT[e.sub.2]).sub.[Sigma]3]], a structure model is proposed considering a homologous series with nagyagite, [[([Pb.sub.3](Pb,Sb).sub.3])[Sigma]6][S.sub.6]][[(Au,Te).sub.3]].

Suredaite, ideally PbSn[S.sub.3], is a new mineral species from the Pirquitas Ag-Sn deposit (Province Jujuy, NW-Argentina). It was observed in symmetrically banded veins in the Oploca district, and is associated with sphalerite, arsenopyrite, pyrite-marcasite, cassiterite, cylindrite, franckeite, hocartite, rhodostannite, and various Ag-Sb and Ag-Bi sulfosalts in minor amounts. Suredaite occurs in layers up to 1 cm in thickness as aggregates of radially arranged tabular-prismatic (single) crystals, has a metallic lustre, and a dark grey streak. [VHN.sub.50] ranges between 18.2 and 20.6 (mean 19.6) GPa, the Mohs hardness is 2.5-3. It has perfect cleavages parallel to {001}, {101}, and {100}. The measured density varies between 5.54 and 5.88 g/[cm.sup.3], [D.sub.x] was determined to be 5.615 g/[cm.sup.3]. In reflected plane-polarised light, it is white and is not perceptibly bireflectant or pleochroic. It lacks internal reflections and is weakly anisotropic with metallic blue, mauve to brown rotation tints. Specular reflectance percentages in air and in oil are tabulated from 400 to 700 nm and compared graphically with those for the type specimen of teallite, PbSn[S.sub.2]. Electron microprobe analyses showed suredaite to be chemically inhomogeneous with respect to the compositional variations (in wt%): Pb 42.3-48.5, Ag 0.3-1.1, Fe 0.3-1.0, As 0.2-2.1, Sn 27.7-30.2, S 23.1-24.7. The crystal structure determined from single-crystal X-ray diffraction data revealed orthorhombic symmetry [space group Pnma, Z = 4, a = 8.8221 (3), b = 3.7728(3), c = 14.0076(3) [Angstrom]; V = 466.23(4) [[Angstrom].sup.3]. The atomic arrangement is isostructural to the [NH.sub.4]Cd[Cl.sub.3] structure type which exists in a series of isotypic sulfides and selenide compounds. The suredaite structure, which is the natural analogue of synthetic PbSn[S.sub.3], consists of columns of double-edge sharing octahedra running parallel to the b axis, which house the Sn atoms. These columns are linked by rods of eightfold-coordinated Pb atoms. On the basis of the structure determination, the empirically determined idealized formula follows a [sup.[8]](Pb, As,Ag, Sn) [sup.[6]](Sn,Fe)[S.sub.3] stoichiometry. Crystalchemical arguments suggest Ag possibly to occupy interstitial sites according to the alternative formula [sup.[14]]([],Ag) [sup.[8]](Pb, As, Sn) [sup.[6]](Sn,Fe) [S.sub.3]. The name of this new mineral species is in honor of R.J. Sureda Leston, head of the Department of Mineralogy and Economic Geology, University of Salta, Argentina.