Crystal structure of uchucchacuaite, AgMnPb (sub 3) Sb (sub 5) S (sub 12) , and its relationship with ramdohrite and fizelyite

Uchucchacuaite, ideally AgMnPb (sub 3) Sb (sub 5) S (sub 12) , was originally reported as orthorhombic, with possible space group Pmmm, P222, or Pmm2, and unit-cell parameters a=12.67, b=19.32, and c=4.38 Aa obtained from powder X-ray diffraction data (Moelo et al. 1984a). Using single-crystal X-ray diffraction, we examined two uchucchacuaite samples, one from the type locality, Uchucchacua, Peru, and the other from Hokkaido, Japan (designated as R100213 and R070760, respectively). Our results show that uchucchacuaite is isostructural with ramdohrite and fizelyite, with monoclinic symmetry (P2 (sub 1) /n) and the unit-cell parameters a=19.3645(11), b=12.7287(8), c=8.7571(6) Aa, beta =90.059(3) degrees for R100213 and a=19.3462(7), b=12.7251(5), c=8.7472(3) Aa, beta =90.017(2) degrees for R070760. Both samples are pervasively twinned and the twin refinements yielded the final R (sub 1) factors of 0.037 and 0.031 for R100213 and R070760, respectively. The chemical compositions determined from electron microprobe analysis are Ag (sub 0.99) (Mn (sub 0.92) Pb (sub 0.03) Sb (sub 0.02) Bi (sub 0.01) ) (sub Sigma =0.98) Pb (sub 3.00) Sb (sub 5.00) S (sub 12.00) for R100213 and Ag (sub 1.00) (Mn (sub 0.82) Sb (sub 0.11) Ag (sub 0.04) Bi (sub 0.02) ) (sub Sigma =0.99) Pb (sub 2.98) Sb (sub 5.00) S (sub 12.00) for R070760. The key structural difference among uchucchacuaite, ramdohrite, and fizelyite lies in the cations occupying the M2 site, which can be expressed with a general structural formula as Ag(M (super 2+) (sub 2y) Ag (sub 1/2-y) Sb (sub 1/2-y) )Pb (sub 3) Sb (sub 5) S (sub 12) , where M (super 2+) represents divalent cations with 0< or =y< or =1/2. From the current list of IMA-defined minerals, we consider M=Cd with y=0.125 for ramdohrite, M=Pb with y=0.25 for fizelyite, and M=Mn with y=0.5 for uchucchacuaite. Associated with the variation in the average M2 cation size from fizelyite (1.078 Aa) to ramdohrite (0.955 Aa) and uchucchacuaite (0.83 Aa) is the significant decrease in the average M2-S bond distance from 2.917 to 2.834, and 2.654 Aa, respectively, as well as corresponding variations in the unit-cell b dimension from approximately 13.23 to 13.06 and 12.73 Aa.