Crystal structure of nobleite, CaB6O9(OH)2.3H2O, from Jarandol (Serbia)

Abstract

Crystal structure of nobleite from locality Jarandol (Serbia) has been refined using single-crystal data (Mo K α radiation, CCD area detector) to the conventional R 1 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 P 2 1 / a, a = 14.580(1), b =8.0240(7), c =9.8486(8) A, β = 111.801(2) °, V =1069.7(2) A 3 and Z =4. The chemical composition calculated from the chemical analysis corresponds to the formula Ca 0.97 Mg 0.01 Sr 0.01 [B 5.89 O 9 (OH) 2 ]·3H 2 O based on 14 O atoms. Nobleite belongs to M 2+ O·3B 2 O 3 · x H 2 O, i.e. to 1·3· x group of borates, with x = 4 and M = Ca, i.e. its nominal composition is CaO·3B 2 O 3 ·4H 2 O or Ca[B 6 O 9 (OH) 2 ]·3H 2 O. The fundamental building block ( FBB ) of nobleite structure contains three BO 4 -tetrahedra and three BO 3− coordination triangles. The three tetrahedra share one corner denoted by [Φ] in the designation for the FBB. The FBB in nobleite is [Φ] | |, which means that it is composed of three rings. Each six-membered B-O ring is formed by sharing corners among two tetrahedra, 2□, and one triangle, Δ. The FBB s 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 FBB s 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.