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A synthesis technique is described which results in > 99 % pure NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and its deuterium analogue ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2. Both phases are characterised via IR spectroscopy at 298 and 77 K and Rietveld refinement of their X-ray powder diffraction pattern. Both the NH4+ and ND4+ molecules are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either the NH4+ or the ND4+ ions can be explained to a good approximation using Td symmetry as a basis. Substantial broadening in the absorption bands, accompanied by the occurrence of shoulders, indicates a deviation from ideal Td symmetry. However, IR absorption spectra taken at 77 K indicate no discrete splitting in the degenerate states. Down to 77 K, the frequencies of the N-H stretching vibrations indicate that there is either no or only weak hydrogen bonding to the surrounding oxygens. The low FWHH for the OH bands indicates that the structure is well ordered. The OH stretching frequency in synthetic phlogopite is lowered by about 15 cm-1 when K+ is replaced by NH4+. This indicates that the OH groups have a similar behaviour in K-, NH4- and ND4-phlogopite. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M1) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3112 were only found in the synthesis run for the ND4-phlogopite. Electron microprobe analyses indicate that both the NH4- and ND4-phlogopites deviate from the ideal phlogopite composition both with respect to variable Si/Al and Mg/Al on the tetrahedral and octahedral sites, respectively due to the "Tschermaks"-substitution. Mg2+VI + Si4+IV <—> Al3+VI + Al3+IV and with respect to vacancies on the interlayer site due to the exchange vector NH4+XII + Al3+IV <—> qXII + Si4+IV. This deviation is supported by the presence of an additional infrared band in the OH-stretching region for both the NH4- or ND4-phlogopites at 3675 and 2710 cm-1, respectively. In comparison to K-phlogopite, the unit-cell parameters for NH4- and ND4-phlogopite are larger due to the substitution of the NH4+ molecule for the smaller K+ cation. This increase is especially evident with respect to the considerably larger value for the c lattice-constant.

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Harlov, D. E.; Andrut, M.; Melzer, S. (2001): Characterisation of NH4-phlogopite (NH4)(Mg3)[AlSi3O10](OH)2 and ND4-phlogopite (ND4)(Mg3)[AlSi3O10](OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra. Physics and Chemistry of Minerals, 28, 2, 77-86.