Characterization of smectite and illite by FTIR spectroscopy of interlayer NH+4 cations
Authors: Pironon, J.; Pelletier, M.; de Donato, P.; Mosser-Ruck, R.
Source: Clay Minerals, Volume 38, Number 2, 1 June 2003 , pp. 201-211(11)
Publisher: Mineralogical Society
Abstract: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.
Document Type: Research Article
Publication date: 2003-06-01