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 vapour-pressure 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 Td symmetry group.The NH+4-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 NH+4 tetrahedron which acquires a C2v
symmetry. As a consequence, the Td-C2v transition can be used to characterize the smectite-illite transition. Quantification of NH+4 content per half unit cell
is provided by nNH4 = k[NH4]/[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.