EXPERIMENTAL EVIDENCE FOR THE PRESENCE OF HYDRONIUM (H3O+) IONS IN NEOFORMED, FIBROUS AND LATH-LIKE ILLITES
Neoformed illites (lengths, ≤ 1.0µm; widths, 0.05 - 0.2µm) which are probably1M polytypes, crystallized in roughly parallel sets at the edges of altered Ms grains. Although the chemical compositions of the illites were reported, the possible relationship between pH and interlayer cation occupancy was not considered.
The interlayer cation-content (K+Na+2Ca) of Ms grains decreased from ~0.97 to ~0.88 during the experiments. Solutions reacted with kaolinite to give neoformed illite plus H+ at 250°C. At lower pH where H3O+ is more abundant, the alkali cation-contents of neoformed illites were also lower, implying that H3O+ substituted in the interlayer sites in neoformed illites and probably in Ms, a hypothesis dating back to Brown and Norrish (1952) and more recently to Nieto et al. (2010).
The interlayer cation-content of neoformed illite appears to be linearly dependent on pH: (K + Na +2Ca) = 0.055pH + 0.527, R2 = 0.96, n = 6. Neutral pH is ~ 5.7 at 250°C. Thus, all experiments were in acid solution. Extrapolation suggests that a pH of 6.38 would be required for neoformed illite to attain end-member composition (~K0.88) at 250°C. The temperature dependence of the linear relationship is unknown.
Pore water pH is 4.5-6.0 in siliceous sedimentary basins (Bjorlykke and Egeberg, 1993). Illite precipitates during diagenesis at depths > 3500 m and temperatures ≥ 120°C (Lanson et al., 2002). Pore water is acid under these conditions. Thus, H3O+ will be available and should enter interlayer sites in neoformed illites and alkali-deficient muscovites. Dehydration results in the loss of H3O+ from interlayer sites, probably leading to partial collapse of the structure to form metastable illite with pyrophyllite domains