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amphibole with a triple-cell and excess hydrogen
1 CNR-Istituto di Geoscienze e Georisorse, sezione di Pavia, via Ferrata 1, I-27100 Pavia, Italy
2 Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
3 Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
4 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
Correspondence: * E-mail: camara{at}crystal.unipv.it
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 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, 
= 90.045(4)°, ß = 103.068(3)°, 
= 89.960(4)°, and V = 2748(1) Å3. The crystal structure has been refined to R1 = 7.6 and wR2 = 16.7% for the 1835 reflections with Fo > 4
F and for 4832 supercell reflections in the 2
range 2–25°, respectively. The structure is pseudo-monoclinic, but both the intensity distribution and refined model indicate space group C . 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.
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