The crystal-structure of synthetic NaNa2Mg5Si8O21(OH)3, a triclinic C¯1 amphibole with a triple-cell and excess hydrogen.

Synthetic NaNa2Mg5Si8O21(OH)3 is the fi rst triclinic member of the amphibole group, and has a tripling of the unit cell in the b direction. The space group is C1 . and the triple-b repeat gives Z = 6. The unit-cell parameters are: a = 9.883(2), b = 54.082(9), and c = 5.277(1) A, ?¿ = 90.045(4)<, ?À = 103.068(3)<, ?Á = 89.960(4)<, and V = 2748(1) A3. The crystal structure has been refi ned to R1 = 7.6 and wR2 = 16.7% for the 1835 refl ections with Fo > 4?ÐF and for 4832 supercell refl ections in the 2?Æ range 2.25<, respectively. The structure is pseudo-monoclinic, but both the intensity distribution and refi ned model indicate space group C1 . . Compared with the common C2/m amphibole structure, the two halves of an I-beam unit are no longer mirror-related, and the overall structure can be rationalized in terms of two different types of I-beam occurring in the unit cell. The fi rst (with multiplicity 2) is centrosymmetric, and the second (with multiplicity 4) is non-centrosymmetric. There are also signifi - cant displacements of the cations, especially at the M4 sites, from their corresponding locations in the C2/m structure. The correlated displacements of Na atoms at the M4 sites permit incorporation of excess protons in pseudo-tetrahedral cavities between two adjacent chains of tetrahedra belonging to different I-beams. Bond-valence calculations and crystal-chemical analysis suggest that excess protons are bonded to O atoms at the O4 sites, and are hydrogen bonded to O atoms at adjacent O2 sites. The infrared spectrum of the amphibole in the principal OH-stretching region has a triplet of sharp bands at 3740, 3727, and 3711 cm.1, which are assigned to the three independent gnormalh O3-H groups in the triclinic structure. There is an additional intense and very broad absorption at 3430 cm.1 that is resolved only when adsorbed moisture is removed. This band is assigned to the extra H in the structure. All the details provided by structure refi nement and the proposed location of the excess H atoms is in accord with previous HRTEM and 29Si- and 1H MAS NMR studies of this amphibole.