UV-visible light absorption spectra of dilute suspensions of standard and commercial montmorillonites (bentonites) and of nontronite, hectorite and pyrophyllite, were studied.

All montmorillonites exhibit either an absorption band or a shoulder at a wavelength of 245 nm. The absorption peak for nontronite is shifted to 260 nm and absorption intensity is very high. Hectorite exhibits a very slight shoulder whereas pyrophyllite shows almost none. Absorption intensity in the UV range was found to be in direct correlation to structural octahedral Fe(III) contents. Direct evidence for the role of Fe(III) in the specific UV absorption of smectites was obtained by the fact that reduction with hydrazine fumes resulted in the disappearance of the peak and reoxidation with H2O2 caused its reappearance with even stronger intensity than originally. Mössbauer spectra showed that the Fe(III) disappeared almost completely in the reduced clay.

From equilibrium silica concentrations over imogolite and boehmite at 100–155°C, the heat and free energy of the reaction (HO)3Al2O3SiOH + H2O ⇌ 2AlOOH + Si(OH)4 have been obtained: ΔH°r (298·15 K)=38·6 ± 4·1, ΔG°r (298·15 K) = 26.8 ± 1·1 kJ mol−1, and hence the heat and free energy of formation of imogolite: ΔH°f (298·15 K) = −3189·6 ± 4·1, ΔG°f (298·15 K) = − 2926·7 ± 1·1 kJ mol−1 These results are consistent with observations indicating that imogolite, halloysite and gibbsite can co-exist in soils, but that imogolite is metastable relative to either halloysite or gibbsite in the long term. At temperatures above 25°C there is a widening range of silica concentrations in which imogolite is more stable than halloysite, although both are metastable relative to kaolinite.

IR spectroscopy has shown that adsorbed water is almost completely removed from ferrihydrite by evacuation at room temperature. Absorption bands at 3615 and 3430 cm−1 appearing thereafter are interpreted as arising from OH groups located respectively at the surface and deeper in the structure. These groups are readily converted to OD on treatment with D2O vapour and this has allowed the OH deformation vibration to be identified at 800 cm−1. It is proposed that OH groups in ferrihydrite are about half as numerous as those in akaganéite (β-FeOOH) and that they may occur in environments similar to those in this mineral. The formula for ferrihydrite proposed by earlier workers, 5 Fe2O3.9H2O, should thus be amended to Fe2O3. 2 FeOOH.2·6H2O in order to indicate the presence of structural OH groups. A re-appraisal of the ferrihydrite structure appears desirable.

Aluminium substituted goethites (Fe, Al)OOH were synthesized from Fe(III), Al systems (ferrihydrites) in m KOH at room temperature. A linear correlation existed between degree of Al substitution and log[Al] in solution. Incorporation of up to 12 mole % Al was obtained. The rate of conversion of ferrihydrite to goethite decreased as [Al] increased. The goethite consisted of thin elongated plates approximately 0·1 µm long with serrated edges and reduced growth in the crystallographic X-direction. The presence of salts and the nature of the starting material either an Al-ferrihydrite or a ferrihydrite in aluminate solution did not influence the degree of substitution, but only the rate of formation of the goethite.

IR spectroscopy has been used to study the interaction of alkali metal hydroxides with dioctahedral smectites having iron contents of 0·7% (montmorillonite) to 22% (nontronite), in connection with the alkali-stabilization of soils. The results have shown that the alkalis (LiOH, NaOH, KOH and CsOH) deprotonate hydroxyl groups co-ordinated to octahedral ferric iron, causing distortion of tetrahedral and octahedral layers. This concept of distortion is supported by a striking change in Mössbauer spectra and a large increase in b cell dimensions. The perturbation of the smectite structure, shown by shifts in the Si-O and O-H stretching vibrations, is greater for Na, K and Cs than for Li, suggesting that the larger cations interact with surface oxygens around the hexagonal hole. IR spectra suggest that the effect of NaOH on nontronite is substantially reversed by treating the smectite with CO2 or acetic acid vapour or by washing it with dilute salt solution, although Mössbauer spectroscopy and XRD indicate that the structure of the regenerated smectite is more disordered than that of the original.

En el miembro basal de la Formación Utrillas, que aflora en las cercanias de Nuévalos, se ha localizado un nivel constituido por ilita, montmorillonita, natrojarosita, caolinita, cuarzo y materia carbonosa.

Se aportan datos de análisis químico, termodiferencial, termogravimetrico, microscopico y de difraccion de rayos X de la natrojarosita.

Se le atribuye a dicho sulfato un origen sedimentario por precipitación en un medio de aguas estancadas, desarrollado sobre materiales margo-yesiferos del Keuper. El hierro aportado al medio, la materia orgánica, y los iones procedentes de la disolución de las evaporitas Triasicas, provocaron las condiciones fisico-quimicas necesarias para la precipitación de la natrojarosita.

Los minerales de la arcilla son heredados, a excepcion de la montmorillonita que se la podria considerar paragenetica con la natrojarosita.