Abstract:

There occurs falkmanite in the Bairendaba superlarge Ag-Pb-Zn polymetallic deposit in Inner Mongolia, China. It is intimately accompanied by galenite, Ag-tetrahedrite, pyrrhotite, chalcopyrite, and boulangerite. Falkmanite crystals show as needle, pillar, prismatic, hair-like, bunchiness and anhedral grain forms, and its grain sizes commonly vary from 0.05 to 4 mm, even to 12 mm. The mineral is lightly grey-white in colour, metallic in lustur and brashy，93.25 to 127.39 kg/mm², averaging 111.05 kg/mm for 100g load in microhardness and Moh’s=3.06~3.40. Electron microprobe analyses of seven points in the falkmanite gave the compositional results as followings(%)：Pb 52.20～57.80 (54.89 on average)，Sb 22.26～28.13 (26.08 on average)，S 18.65～19.62 (19.01 on average), and minor contents of Fe, Cu, Zn, Ag and As, corresponding to the formula (Pb_4.91,Cu_0.04,Fe_0.03,Zn_0.01)_4.99(Sb_3.97,As_0.04)_4.01S_11.00, and its simplified formula Pb₅Sb₄S₁₁. The crystals are monoclinic, with cell dimensions a 2.156, b 2.349, c 0.810 nm. The formation of falkmanite would be favored by a low temperature, increasing f S₂ and reduced fluid conditions during polymetallic mineralization.

Key words: falkmanite, mineralogy, origin significance, Ag-Pb-Zn polymetallic deposit, Bairendaba, Inner Mongolia