The new mineral tavagnascoite, Bi4O4(SO4)(OH)2, was discovered in the Pb-Bi-Zn-As-Fe-Cu ore district of Tavagnasco, Turin, Piedmont, Italy. It occurs as blocky, colourless crystals, up to 40 μm in size, with a silky lustre. In the specimen studied, tavagnascoite is associated with other uncharacterized secondary Bi-minerals originating fromthe alteration of a bismuthinite ± Bi-sulfosalt assemblage. Electronmicroprobe analyses gave (average of three spot analyses, wt.%) Bi2O3 85.32, Sb2O3 0.58, PbO 2.18, SO3 8.46,H2Ocalc 1.77, sum 98.31. On the basis of 10 O apfu, the chemical formula is (Bi3.74Pb0.10Sb0.04)Σ = 3.88O3.68(SO4)1.08(OH)2, with rounding errors. Main calculated diffraction lines are [d in A (relative intensity) hkl] 6.39 (29) 012, 4.95 (19) 111, 4.019 (32) 121, 3.604 (28) 014 and 3.213 (100) 123. Unit-cell parameters are a = 5.831(1), b = 11.925(2), c = 15.123(1) A, V = 1051.6(3) A3, Z = 4, space group Pca21. The crystal structure was solved and refined from single-crystal X-ray diffraction data to R1 = 0.037 on the basis of 1269 observed reflections. It consists of Bi-O polyhedra and SO4 tetrahedra. Bismuth polyhedra are connected each to other to form Bi-O sheets parallel to (001). Successive sheets are linked together by SO4 groups and hydrogen bonds. Tavagnascoite is the Bi-Analogue of klebelsbergite, Sb4O4(SO4)(OH)2, and it is the fifth natural known bismuth sulfate without additional cations. The mineral and its name have been approved by the IMA CNMNC (2014-099)
