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Characterization of smectite and illite by FTIR spectroscopy of interlayer NH₄⁺ cations

¹ UMR G2R, Université Henri Poincaré, BP 239, 54506 Vandæuvre-lès-Nancy, France, and ² Laboratoire Environnement et Minéralurgie, INPL-CNRS UMR 7569, BP 40, 54501 Vanduvre-lès-Nancy, France

^* E-mail: jacques.pironon{at}g2r.uhp-nancy.fr

(Received 10 October 2001; revised 18 November 2002)

FTIR spectroscopy has been applied to NH₄⁺-exchanged dioctahedral clay minerals to determine the molecular environment of NH₄⁺ and to quantify N concentration. FTIR under vapour-pressure control, coupled with heating and freezing treatments has shown that NH₄⁺ ion symmetry varies with the nature of clay minerals. NH⁺ 4 has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the T_d symmetry group. The NH₄⁺-bending vibration is centred at 1450 and 1425 cm^–1.

The Si⁴⁺-Al³⁺ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH₄⁺ tetrahedron which acquires a C_2v symmetry. As a consequence, the T_d–C_2v transition can be used to characterize the smectite–illite transition. Quantification of NH₄⁺ content per half unit cell is provided by n_NH4= k[NH₄]/[OH] where [NH₄]/[OH] is the band area ratio of the NH₄-bending NH₄ vibration to the OH-stretching vibration. k = 1.1 for hydrated smectite, 0.9 for dehydrated smectite and 0.8 for illite or tobelite. The bending vibration of NH₄⁺ is chosen for the calculation because it is not affected by superimposed contributions.

KEYWORDS: ammonium, smectite, illite, tobelite, IR spectroscopy, hydration

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