The “modified chlorite structure” forms by the dehydroxylation of the interlayer octahedral sheet of magnesian chlorite at around 500°C and results in a structure with a basal spacing near 27 Â (Brindley and Chang 1974). This process involves drastic textural modifications as indicated by gas adsorption experiments which reveal the formation of structural micropores. Infrared spectroscopy as well as thermogravimetry and mass spectrometric analysis show that these micropores are filled with molecular atmospheric water, carbon dioxide, nitrogen, argon and hydrocarbons which condense once the samples cool down. A high temperature treatment is needed in order to release the different phases. A heterogeneous dehydroxylation mechanism is proposed in which micropores are formed in donor regions and magnesium and oxygen are concentrated in acceptor regions. This leads to a 27 Å structure with micropore zones and enriched interlayer oxide zones alternating along the z-axis of the mineral.

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.

Recent evidence of bluetongue (BT) virus infection of livestock in scattered localities in the neotropics prompted a serologic survey of cattle in Colombia and Costa Rica. In Costa Rica 48·1% of 1435 bovine animals had BT virus antibody in the agar gel precipitation test (AGPT). In Colombia 51·8% of 635 cattle were AGPT-positive for BT virus. Antibody prevalence ranged from over 50% in the lowlands to 0% in Costa Rica and 19% in Colombian cattle above 2000 m altitude. Neutralization tests indicated that Costa Rican cattle had been exposed to BT virus types 6, 12, 14 and 17.