Quadratite, AgCdAsS₃: Chemical composition, crystal structure, and OD character

Abstract

A re-investigation of the crystal structure of quadratite, ideally AgCdAsS₃, was undertaken using a single crystal from the type locality, Lengenbach, Binntal, Switzerland. The average of five electron microprobe analyses led to the empirical formula (Ag_0.994Cd_0.738Pb_0.231Cu_0.006Tl_0.005Mn_0.003Fe_0.004Zn_0.002 Cr_0.001)_Σ=1.984(As_0.955Sb_0.003)_Σ=0.958S_3.058. A single-crystal structure refinement (R1 = 4.84% for 558 observed reflections) shows that quadratite crystallizes in the space group P4₃22 and exhibits an atomic arrangement similar to that of the recently approved new mineral manganoquadratite, AgMnAsS₃. Like manganoquadratite, quadratite adopts a galena-derivative framework, with metal atoms occupying all the available octahedral interstices, although only M1 and M2 cations, occupied mainly by Cd, adopt a fairly regular octahedral coordination; the M3 cation, occupied by Ag, is located outside the center cavity in a square-pyramidal coordination, whereas Pb at the split position M3′ coordinates six S atoms. Arsenic also adopts a 3 + 3 asymmetrical coordination, thus forming the AsS₃ pyramidal groups that typically occur in sulfosalts.

The structure can be also described as a stacking of BAB slabs [A: (Cd,Ag)CdS₂ atomic plane; B: (Ag,Pb)AsS₂ atomic plane] along [001]. The rectangular unit cell of these slabs is oriented diagonally to the a axes of quadratite and consecutive slabs are related via interlayer twofold rotation operations parallel either to [100] or to [010]. This ambiguity leads to an OD structure with various possible stacking sequences, from which the tetragonal space group P4₃22 was observed.