Preisingerite, ideally Bi3O(OH)(AsO4)2, triclinic, P1, a = 9.993(3), b = 7.404(3), c = 6.937(3)Å, α = 87.82(2), β = 115.01(2), γ = 111.07(2)°, Z = 2, was heretofore known as an unnamed species, for which only qualitative chemical composition and unindexed X-ray powder patterns had been reported. The mineral described here occurs in the weathering zones of the Bi–As–Cu deposits of San Francisco de Los Andes and Cerro Negro de la Aguadita, Department of Calingasta, San Juan Province, Argentina, as poorly developed tiny translucent grayish white crystals which are tabular parallel to (010) with a rhomboidal shape. Electron microprobe analysis yields Bi 68%, As 15%, P 0.3% and Pb < 0.5%. The calculated density is 7.24 g/cm3. The crystals are biaxial with α = 2.130, β = 2.16 and γ = 2.195 (λ = 589 nm) derived from reflectivity measurements and from 2V, which is close to 90° as determined with a polarizing microscope.
The structure analysis, which converged to R = 0.046 for 1440 observed reflections, reveals clusters of six Bi atoms which are connected by two oxygens and two OH groups. These Bi6O2(0H)2 groups are linked into a framework via the AsO4 tetrahedra. The three different Bi atoms have one-sided coordinations, each with four oxygens at distances of 2.11–2.46Å (average 2.28Å). These coordinations are supplemented by five additional oxygens at distances of 2.57–3.36Å. Bonds within the AsO4 tetrahedra measure 1.66–1.71Å (average 1.682Å). A hydrogen bond between the OH oxygen and an arsenate oxygen is indicated by O · · · O = 2.72Å.