Mambertiite, BiMo5+2.80O8(OH), is a new mineral identified in small vugs of quartz veins from Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as pale yellow {001} tabular crystals, up to 1 mm in length and few µm thick, with adamantine lustre. Mambertiite is brittle, with a conchoidal fracture. It is associated with ferrimolybdite, muscovite, quartz, sardignaite, and wulfenite. Electron microprobe data (wt% – mean of 12 spot analyses) are: Mo2O5 59.59, Bi2O3 36.96, WO3 2.03, H2Ocalc 1.48, sum 100.06. On the basis of 9 O atoms per formula unit, the empirical formula is Bi0.99(Mo5+2.74W0.05)∑2.79O7.97(OH)1.03. Infrared spectra showed absorption bands consistent with the occurrence of OH groups. Mambertiite is triclinic, space group P1̄, with a = 5.854(2), b = 9.050(3), c = 7.637(3) Å, α = 112.85(1), β = 102.58(1), γ = 90.04(1), V = 362.3(2) Å3, Z = 2. The crystal structure of mambertiite was solved and refined down to R1 = 0.050 on the basis of 2019 observed [Fo > 4σ(Fo)] reflections. It is composed by eight-fold coordinated Bi-centred polyhedra and five independent Mo-centred octahedra. Among the latter, two are completely occupied by molybdenum, whereas the remaining three are only partially occupied. Two kinds of (101̄) layers occur in mambertiite, alternating along [101̄]*: one is composed by Bi-centered polyhedra and the two partially occupied Mo4 and Mo5 sites, whereas the other is composed by the zigzag chains, running along c, formed by the fully occupied Mo1 and Mo2 sites, and the partially occupied Mo3 site. Mambertiite is structurally related to gelosaite, BiMo6+2O7(OH) · H2O; their relationships can be conveniently described through the OD theory. Mambertiite is the fourth known mineral with Bi and Mo as essential components. Its name honours the Italian mineral collector Marzio Mamberti (b. 1959) for his contribution to the knowledge of the Sardinian mineralogy. The mineral and its name have been approved by the IMA CNMNC (No. 2013–098).

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