A structural scheme is proposed for the synthetic Mg-Al hydroxycarbonates described in Part I (Gastuche, Brown & Mortland, 1967) based on chemical and X-ray powder data. The structure consists of positively-charged brucite-like layers in which Al replaces Mg up to a maximum of about one in three sites. The positive charge is balanced by an interlayer sheet containing carbonate groups and water molecules. The idealized structural formula for the Al-rich material is:

[Mg4Al2(OH)12]2+ [CO3.3H2O]2−.

A similar structural scheme seems to apply to many other materials including the minerals of the pyroaurite group, the compound 4CaO.Al2O3.13H2O and the related mineral hydrocalumite, a nickeliferous magnesium hydroxide mineral described recently and to many synthetic products that have been referred to as double sheet structures.

The relation between counter ion distribution, ion mobility and degree of contraction has been studied on hydrobiotite (2–5 µ size), equilibrated with mixed RbCl-SrCl2 solutions. From X-ray data, isotopic exchange and chemical extraction, it appears that the counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. The distribution of counter ions in the interlayer space follows a gradient from the margin of the mineral to the centre. This distribution is the result of the interruption of a diffusion process, caused by the contraction of layers during exchange. Isotopic exchange is rapid on the external surfaces and in the expanded portions of the interlayers, but very slow in the contracted part, where the rate of exchange amounts to only 0.002–0.02 m-eq/100 g/day, and decreases with time. On exchange with MgCl2 solutions, secondary expansion of the interlayer space occurs and some of the counter ions trapped in the contracted regions are released.

A study of soil formation on glauconitic parent materials in South-East England has included a mineralogical investigation of glauconite grains, heretofore identified solely by morphology. The grains have been subjected to X-ray diffraction, differential thermal and partial chemical analyses, and have been found to consist essentially of randomly interstratified micaceous and montmorillonitic clay mineral; thus all are glauconite sensu lato. Only small amounts of non-clay mineral impurities have been detected. The proportion of expandable layer material has been found to be related to the % K2O and cation exchange capacity of the specimens.

A mixed-layer clay mineral from a Permian sandstone at Maitland, New South Wales has been identified as 2:1 mica-montmorillonite structure with ‘imperfectly regular’ interstratification. The results from Fourier transform analysis and Fourier synthesis of 00l X-ray diffraction data have not fully elucidated the interlayering patterns but have indicated that there is complete alteration in the stacking and that the 1:1 (allevardite type) layer sequence relationship is strongly developed.

X-ray diffraction, differential thermal, thermogravimetric, chemical, cation exchange and electron microscopic data for the mineral are given. The chemical analysis for the Na+-saturated material gives the structural formula

K0.90 Ca0.06 Na0.49 [Al3.52, Fe0.183+ Mg0.27 Ti0.03 (Al1.24 Si6.76) O20 (OH)4] H2O

It is considered that most of the fixed K+ and Ca++ ions are probably held in the mica interlayers, while the exchangeable components are largely accommodated in the expanded montmorillonite zones.

In the quantitative X-ray diffraction analysis of a series of samples, the problems arising from the variable compositions and degrees of crystallinity of clay minerals were overcome to a great extent by the use of standards extracted from the samples. Procedures are given for separation of the montmorillonite standard by differential settling of Na-montmorillonite solvated in an ethanol solution and for isolation of the kaolinite, mica, and chlorite standards by density separation of their Na-forms in thallous formate.

Calibration curves were prepared from the X-ray diffractograms obtained for series of known mixtures of Ca-forms of the standards and the internal standard boehmite using both powder and smear-oriented mounting techniques.

The Dalradian meta-limestones of the Banffshire coast have been investigated as part of a study of the mineralogy of Scottish sedimentary and metamorphic rocks, the main purpose of which is to assess the extent to which soil minerals are inherited from such parent rocks. Saponite was found to be a common constituent of the non-carbonate fraction. Thin sections of the limestones were examined microscopically and the textural relationships between the saponite and the other minerals, particularly those of undoubted metamorphic origin, suggest that the saponite was formed during the metamorphism. Recent reports of the hydrothermal synthesis of saponite at elevated temperatures and pressures support this hypothesis.

An investigation of the clay mineralogy of the molasse formations of Azerbaijan has been carried out. The results have been correlated with climate, the geochemistry of the basin of deposition, the parent rocks and the tectonic stability both in the area of erosion and the zone of accumulation. A knowledge of the relationship between these factors and the clay mineralogy of the sediments may help in the solution of paleogeographic problems.

Results from previous work on clay-mineral complexes with dimethyl sulphoxide are taken as a starting point for the development of a method to identify kaolin in the presence of chlorite. The use of this method with artificial mixtures of these minerals, and with soil clays allows the detection of kaolin when it is present in an amount higher than 5%, through the formation of a mono-layer complex with a basal spacing of 11.8 Å. Details for the use of the method are given.

Three soils of the Strathdon area, Aberdeenshire, derived from a biotite-rich quartz-gabbro and representing well-drained, imperfectly drained and poorly drained types, were studied. The 50–200 µ fractions contained mainly plagioclase feldspar, amphibole (tremolite and hornblende), biotite and quartz and the following order of relative stability was established: quartz > amphibole > plagioclase feldspar > biotite. In all soils biotite was found to weather to a 14 Å aluminous vermiculite-chlorite with zones of kaolinite. The clay mineral composition of three soils was identical, consisting of trioctahedral vermiculite-chlorite, trioctahedral illite, kaolinite and gibbsite. These minerals originated in various ways through the decomposition of biotite which thus plays a key role in the origin of the clay fraction. It is suggested that the clay mineral constituents of the soils were formed in a regolith predating the last glacial period, and that post glacial soil-forming processes have modified them only slightly.

An X-ray diffraction study of over 600 specimens has shown that the Tertiary sediments of the Hampshire Basin are divided into two clay mineral provinces. The western province is characterized by kaolin from the West Country granites whilst the highly montmorillonitic sediments of the eastern province were partially derived from the dissolution of Chalk by tropical weathering.

Amorphous alumino-silicates of different compositions were aged at 175° C under acid and basic conditions. At high pH, crystallization of two zeolites, phillipsite and chabazite, was recorded. Under acid or neutral conditions, a demixing into boehmite and a more stable alumino-silicic gel has been observed.

These results are discussed in relation to the affinity of silica for alumina.

The alteration products of five sub-marine tuffs from the Mount Carmel were analysed chemically, by X-ray, D.T.A. and I.T.A. techniques and their mineralogical composition determined. Smectites are dominant in all the samples. In the less altered tuffs the smectite seems to consist of a mixture of saponite with a smectite from the beidellite-nontronite series. In the more intensely altered tuffs the saponite disappears and a swelling chlorite appears. The formation of the swelling chlorite is attributed to the degradation of the saponite. These mineralogical changes are accompanied by the loss of a large part of the alkali and alkaline earth cations.

When phlogopites containing only a little iron are artificially altered with sodium tetraphenylboron solutions, the vermiculite-like products contain fewer interlayer cations than the original micas. Structural formulae of the products are calculated from the chemical analyses, together with infrared examination of the angular vibration band of water at 1650 cm−1 to distinguish between water of hydration and hydroxyl groups. The formulae obtained are consistent with the weight-loss curves and show that the products contain, more than 4(OH,F) per formula unit. The interpretation of these formulae is that the silicate layers lose net negative charge by protonation of structural oxygen anions to form new hydroxyl groups.

Addition of aluminium to the ferrous sulphate mother solution used to synthesize lepidocrocite, γ FeOOH, yielded a substituted, though excessively hydrous (Fe,Al)OOH structure. Apart from reducing X-crystallinity, the main effect of isomorphous incorporation of aluminium in the lepidocrocite lattice was a shift, to higher values, in the temperature at which the lepidocrocite dehydration to maghemite was complete. Increased thermal stability of this substituted lepidocrocite, deduced from measurements of magnetic attraction made after serial heating, was consistent with the presence of a boehmitic component in solid solution. Magnetic susceptibility data for an oxisol indicated that a similar ferri-aluminous complex had developed pedogenically in the upper solum. This constituent was, however, amorphous to X-rays.