Mixed-layer clays of variable composition and structure occur in core samples from two drillholes (WK207 and WK210) drilled into the Te Mihi sector of the Wairakei geothermal field. These were identified by X-ray diffraction analysis of glycolated and oriented sample fractions at less than 2 μm and less than 0.2 μm.

Low permeability lacustrine sediments encountered by drillhole WK207 contain a well-developed sequence of mixed-layer clays. The shallowest downhole appearance of mixed-layered illite/smectite (I0.6/Sm) occurs at 146 m depth where temperature is only 100°C. Discrete illite is present only below 297 m (200°C) in the finer size fraction (less than 0.2 ¼m). Chlorite first appears downhole, in association with illite-smectite, at 177 m depth (110°C).

Drillhole WK210 encountered predominantly ignimbrites and rhyolites, and fluid flow here is mainly in channels. Within these rocks, a sequence of interlayered clays is poorly developed. Discrete illite and chlorite are present in core from only 244 m (180°), but the measured temperatures where interlayer clays occur ranges from 140 to 209°C.

Differences in the identity of clay minerals present in the Wairakei reservoir, where conditions are otherwise the same, demonstrate the strong control that the type of fluid flow has on their formation. In poorly-permeable sediments, where diffuse fluid flow prevails, a clearly-defined sequence of mixed-layer clays occurs. These are absent where channel flow dominates, the discrete chlorite and illite deposit directly from solution.

The results of a new technique for the measurement of the thickness distribution of fundamental particles are reported. The technique is based on high-resolution transmission electron microscopy (HRTEM) of Na-saturated mixed-layer illite-smectite dispersed in polyvinylpyrrolidone (PVP-10). Intercalation of PVP-10 increases the spacing of expandable interlayers and changes the arrangement of particles so that the number of layers per fundamental particle can be counted easily on HRTEM images. The data obtained by HRTEM on PVP-10-intercalated illite-smectite of hydrothermal origin are compared with data from the Pt-shadowing technique. A good agreement between the two methods for the measured thickness distributions, mean thickness and expandability confirms the reliability of the new technique. The same technique is applied to a set of four sedimentary samples with different expandabilities (83-18%). The thickness of illite particles from shales and claystones has a lognormal distribution. Detrital anddiscrete illite particles can be distinguished from the thickness distribution of authigenic illite.

The Çankırı Basin is one of the largest Cenozoic basins in Central Anatolia, Turkey and contains possible economic hydrocarbon and evaporite reserves. Gypsum is the dominant mineral in the evaporite-bearing Pliocene deposits of the Çankırı Basin. In claystones, the abundance of smectite, dolomite, illite/mica and chlorite in association with minor amounts of mixed-layer chlorite-smectite, mica-vermiculite, amphibole, serpentine, quartz and feldspar together indicate an alkaline environment. Minor kaolinite is also present in some clay samples. Smectite is both detrital and authigenic. Palynological analysis revealed the existence of a mixed forest (Pinus, Cathaya, Tsuga, Cedrus, Abies, Quercus, Ulmus, Juglans, Pterocarya, Acer, Carya, Carpinus, Fagus) dominated by Pinus with a widespread herbaceous understory (Poaceae) interspersed sparsely with open areas occupied by Asteraceae. This flora reflects warm-temperate and humid climatic condition. δ13C analyses have shown that the vegetation was dominated by C3 plants.