On the symmetry and crystal structure of aguilarite, Ag₄SeS

Abstract

An examination of a specimen of aguilarite from the type locality provides new data on the chemistry and structure of this mineral. The chemical formula of the crystal used for the structural study is (Ag3.98 Cu0.02)(Se0.98 S0.84 Te0.18), on the basis of 6 atoms. The mineral was found to be monoclinic, crystallizing in space group P 21/n, with a = 4.2478(2), b = 6.9432(3), c = 8.0042(5) Å , β = 100.103(2)º, V = 232.41(2) Å3 and Z = 4. The crystal structure [refined to R1 = 0.0139 for 958 reflections with I > 2 σ (I)] is topologically identical to that of acanthite, Ag2S. It can be described as a body-centred array of tetrahedrally coordinated X atoms (X = S, Se and Te) with Ag2X3 triangles in planes nearly parallel to (010); the sheets are linked by the Ag1 silver site, which has twofold coordination.Aguilarite is definitively proved to be isostructural with acanthite; it does not have a naumannite-like structure, as previously supposed. Our data support the hypothesis that there are two solid solution series in the system: a monoclinic 'acanthite-like' series (from Ag2S - Ag2S0.4Se0.6), and an orthorhombic 'naumannite-like' series (from Ag2S0.3Se0.7-Ag2Se). This is supported by data gathered on synthetic counterparts. Aguilarite remains as a valid as a mineral species, but it should be described as the Se-analogue of acanthite.In this study we also (1) review the history of the aguilarite; (2) compare properties of synthetic and natural aguilarite; and (3) demonstrate how earlier researchers erred in describing aguilarite as orthorhombic.The Te-bearing composition of the studied aguilarite crystal suggests the possibility of a solid solution with cervelleite (Ag4TeS).

Sulfur–selenium isomorphous substitution and polymorphism in the Ag2(S,Se) series

“…The compositions with 0.6 <х < 0.8 should be additionally analyzed by diffraction techniques since they may contain either or both structures. The recent study of Bindi and Pingitore (2013) proved that aguilarite is isostructural with acanthite. Our data also show that Ag 4 SSe composition has monoclinic (acanthite type) structure.…”

“…The monoclinic Ag 2 S (space group P2 1 /n) [11] and orthorhombic Ag 2 Se (space group P2 1 2 1 2 1 ) [12,13,14] suggest that the continuous isomorphic series of Ag 2 (S,Se) solid solution is impossible. Studies of both natural [9] and synthetic silver sulfoselenides [1][2][3]10] supported the discontinuity of the isomorphic series, however the morphotropic transition range was ambiguously determined by different authors. According to Petruk et al [9] the miscibility limits for Bontschewa-Mladenowa and Zaneva [1], the miscibility break occurs at 50 mol.…”

“…% of Ag 2 S, i.e., Ag 2 S 0.5 Se 0.5 or Ag 4 SSe. However, aguilarite Ag 4 SSe was recently found to be isostructural to acanthite [10], resulting in an expansion of the stability range of the acanthite structure type to selenium content.…”

“…Selenium enters into metal sulfides as an isomorphic impurity to form continuous series of solid solutions such as PbSPbSe, FeS 2 -FeSe 2 , and HgS-HgSe [8]. The possible solid solution between Ag 2 S and Ag 2 Se has been debated for a long time [1][2][3]9,10]. It is obvious that continuous isomorphic series may exist only for the end members having crystal structures of the same type.…”

Probable metal–insulator transition in Ag4SSe

“…Alternatively, it is considered to be an ordinary member of the monoclinic Ag 2 S-type solid solution [3,4]. Status of Ag 4 SSe as a particular mineral species (individual substance) was especially discussed and concluded to be incorrect [15].…”