AbstractTransparent prismatic crystals of Pb₂(AsO₂OH)Cl₂were collected in black metallurgical slag on the coast in the Punta Zeza area, 3 km south of the town of Lavrion, Greece. Analyses by energy-dispersive X-ray spectrometry (EDS) provided the following chemical composition: PbO 73.04, As₂O₃15.97, Cl 11.42, O=Cl –2.58, total 97.85 wt.%. The empirical formula calculated on the basis of As = 1 is H_xPb_2.03(AsO₃)_1.00Cl_1.995(x = 0.96). The infrared spectrum of Pb₂(AsO₂OH)Cl₂has characteristic AsO₃^3–bands at 707 and 594 cm^–1, O–H stretching vibrations at 3310 and 2900 cm_–1and a band at 1107 cm^–1which is assigned to As–O–H bending vibrations. The structure, which is monoclinic P21/m, a = 6.4235(8), b = 5.5399(7), c = 9.321(1) Å, β = 90.767(2)°, V = 331.67(7) Å3, R₁= 0.035, is identical to that of synthetic Pb₂(AsO₂OH)Cl₂and contains two symmetrically unique Pb sites and one As site. The crystal structure is based on [Pb₂AsO₂OH]²⁺double chains interconnected via weak Pb–Cl bonds to produce a three-dimensional framework which is closely related to that of Pb oxysalt minerals containing [O₂Pb₃]²⁺chains of oxocentred [OPb₄]⁶⁺tetrahedra including mendipite, damaraite, rickturnerite and plumboselite.