Synthetic 2-line and 6-line ferrihydrite and feroxyhite samples prepared from ferric salt solutions have been investigated by EXAFS spectroscopy. All these materials have been found to be short-range ordered, consisting of Fe octahedra linked by comers, edges, and faces. Their local structures are related to those of well-crystallized (oxyhydr)oxides, and the absence of hkl reflections in some samples is attributed to the small size of coherent scattering domains. The presence of face sharings indicates that these materials have structural similarities with hematite. Based on Fe-Fe distances and the analysis of the static disorder, it has been concluded that the local structure of feroxyhite is close to that of hematite, whereas ferrihydrite has common structural features with both hematite (αFe203) and cdβFeOOFI. The local structure of ferrihydrite thus differs from that of aqueous Fe polymers obtained by the partial hydrolysis of ferric nitrate and chloride solutions. Differences of local structures among hydrous Fe oxides and aqueous polymers have been interpreted on the basis of a room temperature stability phase diagram established for well-crystallized (oxyhydr)oxides.

The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA . . . close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2-96 Å and c = 9-40 Å, and the space group is P1c. (ii) Defective Fh in which Ac1Bc2A and Ab1Cb2A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution with in the hexagonal super-cell with a = 5.26 Å. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 Å. The main structural difference between 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.

Powder X-ray diffraction (XRD) curves were calculated for the different structural models so far proposed for feroxyhite (δFeOOH). The influence on XRD features of different structural parameters, including site occupancy of Fe atoms, atomic coordinates, content and distribution of stacking faults, and dimension of coherent scattering domains, were considered. On the basis of agreement between experimental and simulated curves it is shown that δFeOOH is a mixture of feroxyhite proper and ultradispersed hematite in the 9 : 1 volume ratio. Feroxyhite proper consists of hexagonal close packing of anions containing 5% stacking faults. Iron atoms occupy only octahedral sites and are distributed in such a way that face-sharing filled octahedral pairs regularly alternate with vacant octahedral pairs along the c axis. This distribution of Fe atoms is quite similar to that established by Patrat et al. (1983), but in each pair, Fe atoms are displaced by the same value of 0.3 Å in opposite directions away from the centre of their octahedron. Nearest Fe-Fe distances calculated for the model proposed (2.88, 3.01, 3-39 and 3-73 Å) practically coincide with those found by EXAFS spectroscopy for the same sample (2-91, 3.04, 3.41 and 3.7-3.8 Å).

Dissolution in HCl of hematite particles with different morphologies led to sigmoidal dissolution vs. time curves. The rate of dissolution was directly proportional to the sample surface area and independent of crystal morphology. Hematites produced by heating goethite at 600 or 800~ dissolved more rapidly per unit area than did hematites grown from solution. TEM showed that some platy crystals developed central holes after prolonged acid attack; the hole formation was attributed to enhanced dissolution at screw dislocations present on the (001) faces of the crystals. Except where particularly susceptible regions involving strained areas or dislocations were present, there appeared to be no preferential acid attack at any particular crystal face. The original morphology was usually maintained during the reaction.

In Moroccan calcareous mountains, red soils are distinguishable from brown soils, not only by their colour but also by their microaggregated structure. Clay mineral dispersion studies of these soils were carried out by increasing pH, after different treatments. The formation of the pseudo-particles appears to be dependent on the fixation of amorphous Fe on clay and on the chemical binding of those clays by amorphous Al-hydroxy-polycations. This can be explained by the high isoelectric point of ferric compounds such as ferrihydrite which are soluble in ammonium oxalate. Based on these results and using techniques of granulometric analysis, red clays aggregated in pseudo-particles were separated from less rubified clays. Hematite was identified in pseudoparticles from red soils and an increase of amorphous Fe has been shown in aggregated clays. These microstructural conditions characterizing rubification could also favour it, by creating microsites where water activity decreases and allows the crystallization of hematite by dehydration of ferrihydrite.

The conversion of smectite to illite has been studied in buried bentonites and shales of the East Slovak Neogene Basin, using X-ray diffraction analysis and transmission electron microscopy. A good correlation was observed in both rock types between the content of expandable layers in interstratified illite-smectite (I-S) minerals and burial depth (temperature), with shales from a given depth being markedly more illitic than bentonties. This difference disappeared at a depth of ~3 km, which represents a temperature of ~150°C The diameter of fundamental illite particles increased with decreasing expandability. Potassium fixation together with neoformation appear to be the mechanisms of conversion of smectite to illite.

The chemistry of smectites from some bentonite deposits derived from intermediate rocks has been examined by electron microprobe methods. A large variation in chemical composition within very short distances, principally controlled by a well-defined negative relationship between Si and A1, and between A1VI and Fe 3+ and A1VI and Mg has been observed. On the other hand, Mg does not vary systematically with either Si or Fe3+. In several bentonites beidellite coexists with montmorillonite and there is a compositional transition between the two smectite minerals, implying the existence of a possible solid-solution series. This transition occurs only when Cheto-type montmorillonites are present, being absent for Wyoming-type montmorillonites. No compositional transition between Wyoming-and Cheto-type montmorillonite was observed. It is believed that the compositional variations reflect initial chemical gradients originated during the devitrification of the volcanic glass, due to the migration of chemical components.

Phyllosilicate associations in hydrothermally altered fluorite ore bodies are: Li-chlorite ± pyrophyllite ± interstratified minerals ± muscovite +± kaolinite. Chlorites, the main alteration minerals, are dioctahedral, d060 = 1.489-1-490/~,, of Ia polytype. The structural formulae indicate substitution of AI for Si from 0.61-0.78 atoms. The total octahedral occupancy ranges from 4.52-4-71 atoms, with 0.49-0-69 Li atoms per half cell unit. This composition indicates that the chlorites are intermediate members of the donbassite-cookeite series proposed by Sudo (1978). The mineralogical associations and textural relations suggest that after intensive silicification which produced alkali alteration under acid conditions, pyrophyllite was produced at the expense of muscovite and then Li-bearing donbassite formed from the pyrophyllite. The Li needed for the formation of the chlorites could be genetically related to granitic batholiths which occur close to the fluorite ores.

The composition of fluvial waters can be used to predict the processes of formation and evolution of secondary minerals in Galician soils. These processes are similar in areas with different rocks, from acid to basic, the only exception being strongly serpentinized materials. In general, the nature of the rock has less influence than the characteristics of the alteration system, which is always open, acid and freely draining. Mineral neoformation is always of the monosialitic type, having different degrees of evolution. The incipient phases tend to form gibbsite as a more stable mineral, and halloysite, allophanes and imogolite as metastable forms. With time, a crystalline kaolinite would be the only stable species. These results agree with the mineralogy and the andic or ferralic properties of the soils.

A moderate to high rate of clay sedimentation is characteristic of the Toarcian sequences in the Umbria-Marche basin. The clay mineral assemblages and geochemical characteristics indicate that the Marne di Monte Serrone Formation and the Rosso Ammonitico Umbro-Marchigiano Unit were deposited in a shallow marine environment. In this general palaeogeographic scheme, a reducing subenvironment must have exsisted in which black shale-type facies were deposited resulting in geochemical anomalies in As, Sb, Zn, Co, Cu, Pb, V, Cr, and Ba, among other elements. The high values of both the detrital index and the Ce/Ce* ratio reveal the influence of proximal emerged reliefs. It is suggested that the palaeosoils that developed on the Liassic carbonate Laziale-Abuzzese platform were the source area of these sediments, following a palaeogeographic scheme analogous to that proposed for the Betic Cordilleras (Spain) during the Middle Domerian to Middle Toarcian. The positive and negative Eu anomalies are due to the decisive influence of the weathering process in which sediments with heterogeneous Eu anomaly size were mixed. The final distribution pattern of REE is the result of the different environments to which the clay minerals were subjected and the differences in intensity of weathering.

Changes caused by pressures between 0 and 20 K bar on two samples of kaolinite have been studied. Increased pressure causes the samples to lose crystallinity, shown by a decrease in the Hinckley, Lietard and reference intensity ratio indices, as well as by the crystallite size measured from the 001 reflection. Results from DTA and solid state NMR show slight or no differences between the compressed and natural samples. Nonetheless, thermogravimetry reveals that the pressure treatment lowers the temperature at which dehydroxylation begins, but, unlike the grinding process, pressure does not alter the OH content of the samples. As a result of the increase in pressure, the IR spectra of the samples show a gradual increase in the intensity of the band at 1104 cm -1 with respect to 1112 cm -1. An increase in intensity can also be observed in the band at 936 cm ~ with respect to the band at 912 cm -1. Examination by electron microscopy reveals the existence of a large number of defects such as fractures, bends and deformations of the sheets, etc.; these are responsible for the decrease in crystallinity.