The crystal-structure of synthetic NaNa2Mg5Si8O21(OH)3, a triclinic C1̅ amphibole with a triple-cell and excess hydrogen

Synthetic NaNa₂Mg₅Si₈O₂₁(OH)₃ is the first triclinic member of the amphibole group, and has a tripling of the unit cell in the b direction. The space group is C 1̅ 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) Å, α = 90.045(4)°, β = 103.068(3)°, γ = 89.960(4)°, and V = 2748(1) ų. The crystal structure has been refined to R₁ = 7.6 and wR₂ = 16.7% for the 1835 reflections with F_o > 4σ_F and for 4832 supercell reflections in the 2𝛉 range 2–25°, respectively. The structure is pseudo-monoclinic, but both the intensity distribution and refined model indicate space group C 1̅. 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 first (with multiplicity 2) is centrosymmetric, and the second (with multiplicity 4) is non-centrosymmetric. There are also significant 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 “normal” 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 refinement 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.