Uchucchacuaite, ideally AgMnPb3Sb5S12, was originally reported as orthorhombic, with possible space group Pmmm , P 222, or Pmm 2, and unit-cell parameters a = 12.67, b = 19.32, and c = 4.38 A 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][1] and [R070760][2], respectively). Our results show that uchucchacuaite is isostructural with ramdohrite and fizelyite, with monoclinic symmetry ( P 21/ n ) and the unit-cell parameters a = 19.3645(11), b = 12.7287(8), c = 8.7571(6) A, β = 90.059(3)° for [R100213][1] and a = 19.3462(7), b = 12.7251(5), c = 8.7472(3) A, β = 90.017(2)° for [R070760][2]. Both samples are pervasively twinned and the twin refinements yielded the final R 1 factors of 0.037 and 0.031 for [R100213][1] and [R070760][2], respectively. The chemical compositions determined from electron microprobe analysis are Ag0.99(Mn0.92Pb0.03Sb0.02Bi0.01)∑=0.98Pb3.00Sb5.00S12.00 for [R100213][1] and Ag1.00(Mn0.82Sb0.11Ag0.04Bi0.02)∑=0.99Pb2.98Sb5.00S12.00 for [R070760][2]. 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(M2+2 y Ag½– y Sb½– y )Pb3Sb5S12, where M2+ represents divalent cations with 0 ≤ y ≤ ½. 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 A) to ramdohrite (0.955 A) and uchucchacuaite (0.83 A) is the significant decrease in the average M2-S bond distance from 2.917 to 2.834, and 2.654 A, respectively, as well as corresponding variations in the unit-cell b dimension from ~13.23 to 13.06 and 12.73 A. [1]: /lookup/external-ref?link_type=GEN&access_num=R100213&atom=%2Fgsammin%2F96%2F7%2F1186.atom [2]: /lookup/external-ref?link_type=GEN&access_num=R070760&atom=%2Fgsammin%2F96%2F7%2F1186.atom