The importance of precipitation rate as an effective control on illite and kaolanite formation during diagenesis has been examined by measuring precipitation rates, from Al fluid concentration, in a Dickson fluid-sampling vessel at 160°-250° and 500 bars (50 MPa). These experiments are considered to be analogues of the precipitation of clays in sandstones from porewaters containing dissolved carboxylic acids, which have a transient stability and may influence aluminosilicate solubility. Precipitated illite had a lath-shaped morphology and its composition was consistent with authigenic illite in sandstones. Kaolinite formed tabular rather than vermicular shaped crystals. Kaolinite precipitation rate was two orders of magnitude faster than illite precipitation and was rate-limited by the decomposition of oxalate; kaolinite formation should be equilibrium-controlled at virtually all stages of burial. Extrapolation of illite precipitation rate to burial temperatures indicates that the first appearance of illite in a burial sequence may be kinetically controlled. A model of illite precipitation based on these experimental results has been used to predict the time required to precipitate illite during burial of a sandstone, taking into account temperature changes during burial. For northern North Sea examples, a predicted illitization threshold of -60°C occurring at 60–80 Ma corresponds to the observed initiation of authigenic illite precipitation. Times of around 2–5 Ma would be required to reach a 98% approach to equilibrium at this threshold. The main phase of illite precipitation in the northern North Sea basin is a later, hydrologically controlled event (30-50 Ma). Equilibrium would be approached in around 0.1 Ma during this phase, which is consistent with the narrow illite K/Ar age range (1-5 Ma) recorded for some sequences.

Diagenetic kaolin minerals are very common in the Permo-Triassic succession from the SE Iberian Range, Spain. The morphology and crystal structure of kaolin minerals has been examined in four size fractions (<1 µm, <2 µm, <6.3 µm and <20 µm) of sandstone samples by means of scanning electron microscopy, X-ray diffraction, infrared spectroscopy, differential thermal analysis and thermogravimetry. Experimental data reveal that dickite is the dominant kaolin-type mineral in the entire range of size fractions, whereas small amounts of kaolinite coexists with dickite in all size fractions. Dickite appears typically as booklets of pseudo-hexagonal plates with blocky habit. The increase in size fraction is concomitant with the increase in the amount of dickite and the progressive improvement of its structural order. The extensive dickitization is attributed to the high paleogeothermal gradient recorded in the studied area and the increase in H+, presumably resulting from the flux of organic acids derived from the underlying Carboniferous rocks and/or the late Permian succession. These conditions are more likely to be associated with the late Cretaceous post-rift thermal stage of the eastern Iberian Basin. Lately, during the maximum burial depth, the fine crystalline kaolin minerals were slightly illitized. Given the very small feldspar content in the studied sequence, the results reflect the important contribution of mica alteration to the early diagenetic formation of kaolinite as well as the late conversion to dickite.

Fluvial and aeolian sandstones of the Sergi Formation are the most important reservoirs of the Recôncavo Basin, Brazil. Optical and scanning electron microscopy, X-ray diffraction and infrared spectroscopy revealed the occurrence of dickite, a clay mineral indicative of deep burial conditions (T >100ºC), in the shallow Buracica (630–870 m) and Água Grande (1300–1530 m) oilfields. Vermicular dickite replaces K-feldspar and plagioclase grains, and fills intra- and inter-granular pores. Its vermicular habit is a product of pseudomorphic kaolinite transformation during burial. The presence of dickite is in accordance with the intensity of compaction, post-compactional quartz cementation and δ18O values of calcite cements (T up to 109ºC). These petrological features of deep burial, as well as apatite fission-track analyses, indicate that uplift and erosion of at least 1 km, and probably >1500 m, affected the central part of the Recôncavo Basin and possibly the entire region. This uplift has not been detected previously by conventional structural and stratigraphic models.

The diagenetic evolution of kaolin and illitic minerals in sandstones is described here. The structural characterization of these minerals, the possible reaction pathways leading to their crystallization, and the origin of the fluids involved are discussed specifically.

While early precipitation of kaolinite is in general related to flushing by meteoric waters, subsequent diagenetic kaolinite-to-dickite transformation probably results from invasion by acidic fluids of organic origin. Dickite is the stable polytype in most sandstone formations and the kaoliniteto- dickite conversion is kinetically controlled.

The conventional model of kaolin illitization, assuming a thermodynamic control in a closed system, is discussed and compared to an alternative model in which illitization of kaolin is not coupled to dissolution of K-feldspar (Berger et al., 1997). In the latter model, illite crystallization at the expense of kaolin implies that an energy barrier is overcome either by an increased K+/H+ activity ratio in solution or by a considerable temperature increase.

Hydrocarbons (bitumens sensu lato) in sandstones have been recorded in several instances to have partially replaced their host rock, including quartz grains. Many replacive hydrocarbons are uranium-rich: associated non-uraniferous hydrocarbons are not replacive. Uranium is transported as carbonate complexes, which may be decomposed by organic acids to yield UO22+ ions and CO2. The UO22+ will be absorbed onto hydrocarbons before reduction to a mineral phase, generally uraninite; and the CO2 may be aggressive towards the silicate grains of the host sandstone. Accretionary nodules of replacive uraniferous hydrocarbon in red beds (e.g. at the cores of reduction spots) can provide valuable information about petroleum migration. They occur particularly in the vicinity of faults, and may record the interactions between metal-rich groundwaters and hydrocarbons leaking along a fault from an underlying reservoir. A trial study in Devonian sandstones of Easter Ross successfully traced a hydrocarbon-bearing sandstone reservoir from an occurrence of uraniferous hydrocarbon nodules.

The Cerro Prieto geothermal system provides a unique opportunity for the detailed study of calc-silicate mineral transitions between the diagenetic clay-carbonate and greenschist facies within the terrigenous sediments of the Colorado River delta. In this system, progressive devolatization reactions within carbonate-cemented, quartzofeldspathic sediment have produced a distinct hydrothermal mineral zonation at temperatures between 200–370°C and fluid pressures below 0.3 kbar. Descriptive and compositional data are presented for these minerals which include wairakite, epidote, prehnite, actinolite, clinopyroxene, garnet, sphene, biotite, microcline, and calcite. Partitioning of octahedral Fe, Mg, and Al between coexisting authigenic silicates is comparable with data from higher temperature metamorphic rocks and demonstrates an approach to local equilibrium within this system. Calculated fugacities of oxygen at temperatures above 300°C are (with rare exception) more reducing than that defined by the quartz-fayalite-magnetite buffer, a result consistent with the scarcity of hematite and grandite and the ubiquitous presence of organic material in Cerro Prieto sandstones.

The concentrations of Br in potable groundwaters in the United Kingdom range from 60 to 340 µg 1-1. The occurrence of Br is described in terms of the Br/Cl weight ratio which enables small changes in bromide concentrations to be assessed in terms of salinity. Median values of Br/Cl in groundwaters range from 2.60 to 5.15 × 10−3 compared with a sea water ratio of 3.47× 10−3. In recent shallow groundwaters the Br/Cl ratio is rather variable in response to a range of natural and anthropogenic inputs (marine and industrial aerosols, industrial and agricultural chemicals including road salt). Some slight enrichment in Br/Cl also occurs naturally during infiltration as a result of biogeochemical processes.

Evolution of Br/Cl along groundwater flow lines reflects the sources of increasing salinity; either the influence of marine sedimentary formations or evaporites. The groundwaters in the Triassic sandstones of the English Midlands show significant Br depletion due to the evaporite source, in contrast to groundwaters in Cumbria. Br/Cl ratios in the Sherwood Sandstone of the East Midlands mainly reflect the natural input sources and can be used to help understand the palaeohydrology.

Elemental compositions of the <2mm fraction of three sediments sampled along the Kupa River are presented. On plots of log(SiO2/Al2O3) vs. log(CaO+Na2O)/K2O, the sediments plot closest to average sandstone compositions. On plots of weathering indices A vs. B (Kronberg and Nesbitt, 1981) the sediments plot close to basalt, terrestrial crust and granite. X-ray diffraction of the <63 μm fraction suggests chlorite, vermiculite and mica are present as weathering products in the upper, middle and lower stream sediments. Depletion of some elements in the sediments suggests loss of elements from the sediments due to dissolution of minerals.

Tosudite, a regularly interstratified chlorite-smectite, crystallizes as an alteration mineral of several preexisting Al-bearing silicates (feldspars, kaolin minerals, chlorites) present in arkosic sandstones hosted in uranium deposits in Niger. X-ray diffraction patterns show a sharp superstructure at 29–29.6 Å for an air-dried state and a peak at 30.8–31.6 Å following ethylene glycol solvation. The 060 reflection at 1.507–1.509 Å indicates an overall dioctahedral character, and the very low coefficient of variation of the d00l reflections for the solvated mineral (0.03–0.13) permits validation of the regular interstratification justifying its identification as tosudite. Microprobe analysis allowed specification of the component layers of this mixed-layer mineral. The chlorite is a di-trioctahedral type analogous to sudoite (Si3Al4Mg2(OH)8), and the smectite component is a low-charge montmorillonite type Tosudite is characterized by large Al2O3 and MgO contents and small Fe content; its composition corresponds approximately to the formula where octahedral occupancy is ∼7. Scanning electron microscope (SEM) observations show that tosudite is closely associated with some uranium minerals: tosudite crystallization occurred during a late alteration event which post-dates burial diagenesis and during which uranium was remobilized by Mg-rich oxidizing fluids.

Analysis of conglomerates and sandstones of the Sperm Bluff Formation at the base of the Taylor Group (Devonian) between the Mackay Glacier and Bull Pass provides new insights into the nature of initial coarse-grained deposition on basement along the northern side of the McMurdo sedimentary basin. Six lithofacies are recognized in the Sperm Bluff Formation: conglomerate lithofacies, pebbly sandstone lithofacies, cross-bedded sandstone lithofacies, low-angle cross-stratified sandstone lithofacies, bioturbated sandstone lithofacies and interbedded siltstone/sandstone lithofacies. Sedimentary environments ranged from wave-dominated delta, estuary or lagoon to shoreface and inner shelf. The assemblage is thought to reflect changes in sea level. Rhyolite is the most abundant clast type in the coarse lithofacies, but is unknown in outcrop in southern Victoria Land. The rhyolites correlate in age and geochemistry with Cambrian granites in the basement. Coarse beds also contain numerous quartzite clasts, probably derived from the late Precambrian Skelton Group. Palaeocurrents on Mount Suess indicate a strong unimodal flow to the west, but other sites show polymodal palaeoflow. The Sperm Bluff Formation is correlated with Terra Cotta Siltstone, New Mountain Sandstone and Altar Mountain formations based on the lithology of sandstones and their ichnology. A northward onlap during the Early Devonian is indicated.