Crystal structure of nobleite from locality Jarandol (Serbia) has been refined using single-crystal data (MoKα radiation, CCD area detector) to the conventional R1 factor 0.0324 for 2432 reflections having I > 2σ(I) and 0.0540 for all 3383 data. Hydrogen atoms were found in difference Fourier maps and were refined isotropically with no constraints. Nobleite is monoclinic, space group P21/a, a = 14.580(1), b=8.0240(7), c=9.8486(8) Å, β = 111.801(2) °, V=1069.7(2) Å3 and Z=4. The chemical composition calculated from the chemical analysis corresponds to the formula Ca0.97Mg0.01Sr0.01[B5.89O9(OH)2]·3H2O based on 14 O atoms. Nobleite belongs to M2+O·3B2O3·xH2O, i.e. to 1·3·x group of borates, with x = 4 and M = Ca, i.e. its nominal composition is CaO·3B2O3·4H2O or Ca[B6O9(OH)2]·3H2O. The fundamental building block (FBB) of nobleite structure contains three BO4-tetrahedra and three BO3− coordination triangles. The three tetrahedra share one corner denoted by [Φ] in the designation for the FBB. The FBB in nobleite is [Φ]<Δ2□>|<Δd2□|<Δ2□>|, which means that it is composed of three <Δ2□> rings. Each six-membered B-O ring is formed by sharing corners among two tetrahedra, 2□, and one triangle, Δ. The FBBs are further polymerized into sheets, parallel to the 100 plane, with Ca cations in the holes of the sheets and water molecules between them. The Ca atom is coordinated by six oxygens from FBBs and three water molecules. The Ca-polyhedron with coordination number 9 can be described as a distorted hexagonal bipyramid with one vertex split. All hydroxyl groups and water molecules form hydrogen bonds located mainly between sheets. Present results show that nobleite is not structurally identical to tunellite.

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