Characterisation of tobelite (NH_4)Al_2[AlSi_3O_(10)](OH)_2 and ND_4-tobelite (ND_4)Al_2[AlSi_3O_(10)](OD)_2 using IR spectroscopy and Rietveld refinement of XRD spectra

摘要

Tobelite (NH_4) Al_2 [AlSi_3O_(10)] (OH)_2, the ammonium analogue of muscovite, and its deuterated form ND_4-tobelite (ND_4) Al_2 [AlSi_3O_(10)] (OD)_2 have been synthesised at 600 ℃ and 200 and 500 Mpa using a well homogenised, stoichiometric SiO_2-Al_2O_3 oxide mix with Al_2O_3 in excess of 5 mol% and a 25% NH_3 solution whose relative abundance was such that the amount of NH_4~+ stoichiometrically available was in excess of 50%. Characterisation of both tobelite and ND_4-tobelite using IR-spectroscopy, Rietveld refinement of X-ray powder diffraction data, and electron microprobe analysis indicate that, similar to K~+ in muscovite, the NH_4~+ or ND_4~+ molecule occupies the interlayer site. IR absorption bands caused by NH_4~+ and ND_4~+ can be explained, to a very good approximation, on the basis of T_d symmetry. Nevertheless, substantial line broadening and the occurrence of shoulders indicate a deviation from ideal T_d symmetry. However, even at 77 K, no discrete splitting of the degenerate states could be confirmed. The OH stretching frequencies observed for synthetic tobelite are quite similar to those for muscovite, indicating that the replacement of K~+ by NH_4~+ has no effect. The low FWHH of the OH bands indicate that the hydroxyl groups are well ordered within the structure. Rietveld refinement of tobelite and ND_4-tobelite indicates that all samples synthesised consist of the 3 different mica polytypes which are typical of muscovite-namely 1M (C2/m), 2M_1 (C2/c) and 2M_2 (C2/c). Tobelite and ND_4-tobelite synthesised at 500 Mpa principally contain the 1M polytype, whereas the principle polytype for ND_4-tobelite synthesised at 200 Mpa, is 2M_2. Rietveld refinement of X-ray diffraction spectra for tobelite synthesised at 200 Mpa was problematic due to the very broad FWHH of the X-ray peaks indicating poor crystallinity. In comparision to synthetic muscovite, the cell dimensions observed for tobelite and its deuterated analogue are quite similar except for the lattice constant c. Due to the larger radius of NH_4~+ or ND_4~+ compared to K~+ cation, the c-direction is expanded form 10.275 A in muscovite to approximately 10.540 A in tobelite and ND_4-tobelite.