Sursassite: Hydrogen bonding, cation order, and pumpellyite intergrowth

The crystal chemistry of sursassite, simplified formula Mn(2)(2+)Al(3)Si(3)O(11)(OH)(3), from six different localities [(1) Falotta, Switzerland, (2) Woodstock, New Brunswick, Canada, (3) Kamisugai, Japan, (4) Kamogawa, Japan, (5) Molinello, Italy, and (6) Gambatesa, Italy] was studied using electron microprobe analysis (EMPA), Fourier transform infrared spectroscopy (FTIR), and single-crystal X-ray diffraction methods. The structure has two symmetry independent Mn sites. The Mn I site is seven coordinated by 0 and hosts, in addition to Mn(2+), up to 20% Ca, whereas Mn2 has octahedral coordination and is strongly selective for Mn(2+). In the simplified formula, three smaller octahedral M sites are occupied by Al. However, M1 also accepts significant amounts of divalent cations, such as Cu, Mg, Fe, and Mn, whereas M2 is occupied exclusively by Al. The unit-cell parameters of sursassite are a = 8.698-8.728, b = 5.789-5.807, c = 9.778-9.812 angstrom, beta = 108.879-109.060 degrees, V = 465.7-470.0 angstrom(3), the space group is P2(1)/m. Structure refinements converged to R(1) values of 2.15-6.62%. In agreement with bond-valence analyses, at least three OH groups, depending on the concentration of divalent cations at M 1, are found at the 06, 07, and 0 11 positions. However, the bond-valence sum at 0 10 is always low, thus partial hydroxylation is assumed at 010 to maintain charge balance. Owing to the influence of divalent cations at M1 in sursassite the hydrogen-bond systems in sursassite and isostructural macfallite are different. The FTIR spectrum in the region of OH-stretching vibrations is characterized by three strong bands at 35 11, 3262, and around 2950 cm(-1), the latter being broad. The band at 2950 cm(-1) is assigned to strong hydrogen bonds between O6 and O10 (O6 center dot center dot center dot O10 = 2.66 angstrom). Residual difference-Fourier peaks in the refinement of the Kamogawa and Molinello (specimen 1) crystals indicated less than 5% pumpellyite intergrowth.