A new mineral species, franksousaite (IMA2021-096), ideally PbCu(Se6+O4)(OH)2, has been found from the El Dragón mine, Antonio Quijarro Province, Potosí Department, Bolivia. It occurs as short prismatic crystals included in colourless anglesite. Associated minerals are Co-bearing krut'aite–penroseite, chalcomenite, schmiederite, olsacherite, phosgenite, anglesite and cerussite. Franksousaite is blue in transmitted light, transparent with very pale blue streak and has a vitreous lustre. It is brittle and has a Mohs hardness of 2–2½. Cleavage is perfect on {100}. The calculated density is 5.64 g/cm3. An electron microprobe analysis yielded an empirical formula (based on 6 O apfu) of Pb1.02Cu0.98[(Se0.84S0.17)Σ1.01O4)](OH)2, which is simplified to PbCu[(Se,S)O4)](OH)2.

Franksousaite is the Se6+ analogue of linarite, PbCu(SO4)(OH)2. It is monoclinic, with space group P21/m and unit-cell parameters a = 9.8208(3), b = 5.7340(2), c = 4.74980(10) Å, β = 102.683(2)°, V = 260.947(13) Å3 and Z = 2. The crystal structure of franksousaite consists of Jahn-Teller-distorted Cu2+O6 square bipyramids, which form chains along b by sharing trans edges across their square planes. The chains are decorated by SeO4 tetrahedra and linked to one another by hydrogen bonds to form layers parallel to (100). These layers are bound together by double layers of PbO8 and SeO4 polyhedra. The PbO8 polyhedron exhibits one-sided coordination typical of Pb2+ with a stereochemically active 6s2 lone-electron-pair. The major structural difference between franksousaite and linarite lies in the <Se–O> vs. <S–O> bond distances (1.615 vs. 1.482 Å), accounting for the larger unit-cell volume for franksousaite. A comparison of Raman spectra for the four minerals in the linarite group (linarite, franksousaite, munakataite and schmiederite) reveals the obvious differences among them, especially in the range from 700 to 1000 cm–1, indicative of the presence or absence of (Se4+O3)2–, (Se6+O4)2–, and/or (SO4)2– groups.