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

FTIR spectroscopy has been applied to NH 4 + -exchanged dioctahedral clay minerals to determine the molecular environment of NH 4 + and to quantify N concentration. FTIR under vapourpressure control, coupled with heating and freezing treatments has shown that NH 4 + 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 NH 4 + -bending vibration is centred at 1450 and 1425 cm –1 . The Si 4+ -Al 3+ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH 4 + 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 NH 4 + content per half unit cell is provided by n NH4 = k [NH 4 ]/[OH] where [NH4]/[OH] is the band area ratio of the NH 4 + -bending 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 NH 4 + is chosen for the calculation because it is not affected by superimposed contributions.