High-resolution transmission electron microscope studies reveal the origin and nature of polytypism in baumhauerite. The polltypes are stacking variants of the basic baumhauerite structure, a = 22.80, b = 8.36, c = 7.89 Å, β = 97.27°, and involve inversion of the Pb and As distribution about b and lead to multiplicity along a. A new polytype of baumhauerite is reported: baumhauerite-ΨO3abc, a = 68.3, b = 8.4, c = 7.9 Å. The mineral baumhauerite-2a is also considered to be a member of this polytypic family. Stacking disorder was observed in crystals of the longer period polytypes. The long-period polytypes baumhauerite-2a and baumhauerite-ΨO3abc contain up to one atom of Ag pfu, and it appears that Ag may play a role in stabilizing the longer period polytypes.

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