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 vapourpressure control, coupled with heating and freezing treatments has shown that NH4
+ ion symmetry varies with the nature of clay minerals. NH4
+ 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 Td –C2v 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 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.