The methods used for identifying clay minerals at Göttingen are passed in review. Stress is laid on the use of several different methods, each one alone being liable to errors. In particular, the optical microscope can be very valuable. The phase contrast method has greatly increased the scope of this instrument, especially for determining small r.i. differences (photographs).

Combined electron micrographs and electron diffraction diagrams of single crystals of hydrated kaolins from Hongkong, North Carolina and Indiana show that they have tubular crystal form, but a structure different from that of halloy-site, or of kaolinite, being two-layer triclinic, with β=83°, α=91·8°. They differ in degree of crystallinity, and show some inter-layer water.

A sericite from Unnan Mine, Shimane Prefecture, Japan, has a three-layer structure, perhaps similar to three-layer phlogopite and lepidolite, but with various degrees of disorder in inter-layer stacking, in different samples. A classification of polymorphic and disordered forms, showing their relationship, is proposed.

Clay minerals have been prepared by synthesis in solutions of comparable strength to their solubility (10–20 mgm./l). Syntheses have been carried out in boiling water and at normal temperatures, at various pH's, and with various components; the products have been examined with X-rays and the electron microscope. In general it is found that the proportion of the components has only a small effect on the product, this being determined mainly by the pH. The low temperature gives less crystalline products. Some results are:

Si-Mg syntheses.—Montmorillonite: “crushed paper” type at pH 9 (Fig. 1); “star” type at pH 8 (Fig. 2 from boiling solutions, Fig. 3 at ordinary temperature).

Si-Mg-Al syntheses.—Al as aluminate at pH 8 gives montmorillonite; with more Al, “swelling chlorite” (and 7Å mineral?) (Fig. 4). With cationic Al at pH 8·5 a filmy, almost amorphous product (sepiolite?) (Fig. 5); at pH 7·7 boeh-mite (Fig. 6). Anionic Al with KCl in great excess gives micas (Fig. 7).

Syntheses with iron.—Iron-rich saponites, montmorillonite minerals, and Fe2O3+“swelling chlorite” have been obtained. In NaCl excess, mica-like (but isotropic, poorly crystalline) products (Fig. 8).

The authors consider that the brucite-gibbsite layer forms first in these syntheses, and the silicate layer attaches itself to it: analogous products have been made with aluminate and chromate in place of silicate.

Careful (00l) fourier syntheses have been made on muscovite, pyrophyllite, and montmorillonite (dehydrated), in an effort to discover the cause of the different (00l) spacings of these minerals. The oxygens of the outer (tetrahedral) layer superpose exactly in these syntheses, those of the inner (octahedral) layer almost exactly. Thus the differences are entirely in the inter-layer spaces, and are probably due to electrostatic repulsion of the negatively charged multiple layers. As the negative charge increases (and with it the repulsion, and hence the inter-layer spacing), in the series pyrophyllite-montmorillonite-mica, the net energy of binding of the sheets increases also. The two variations are not in contradiction.

It follows from this work that the shifts in cell size due to ionic substitutions are entirely taken up in the plane of the layer, its thickness remaining the same.

The adsorption of phosphate by montmorillonite and kaolinite (Na+ and Ca++) shows a maximum at pH 4–5, being 18 ± 2 mgm. eq./100 gm. of calcined montmorillonite (assumed as H2PO4−). This agrees with the calculated value for edge-adsorption, allowing for some steric hindrance (assumed crystallite diameter 300 Å). It is in accord with this idea, that pyrophyllite is found also to fix phosphate, but not talc and hectorite.

Acid treatment causes natural “swelling chlorites” (like the similar synthetic material prepared by the authors) to change into montmorillonite. Some labile chlorites from Lorraine iron ore show a similar change on treatment with sodium hydrosulfite + sodium tartrate, or on electrodialysis, although certain of these show no swelling initially, The corresponding transformation with true chlorites is very difficult to achieve. Results have been checked by thermobalance (figs.), D.T.A., X-rays.

The characteristic feature of “swelling chlorites” is the easy passage back to the montmorillonites from which they were probably formed; not all such labile chlorites show swelling.

Preliminary investigation of the minerals in sediment core 15 taken at a depth of 1990 metres in the Gulf of Naples shows:

1. (i) Volcanic materials, especially glass, occur throughout the core, but only in certain horizons >50%. These horizons represent periods of volcanic activities in the region and further investigation is required to correlate them.

2. (ii) No mineral in the core shows any post-depositional alteration, and most of them are clear and fresh.

3. (iii) Among the clay minerals illite is predominant, but kaolinite and mont-morillonite are also present.

Plasticity is characteristic of systems with “bound water” films in contact. It can only be properly compared for clays of different mineralogical composition if suitable mixtures (different in each case) are used, such that f/ηγf is a maximum, where f=elastic limit, η=viscosity, γ=max, elastic deformation.

1. (1) Pholerite is considered to have the kaolinite structure with partial Al-for-Si substitution.

2. (2) Anauxite has “holes” in the octahedral positions; the idea of intercalated silica layers is untenable.

3. (3) Montmorillonite minerals probably form continuous series with micas, talc and pyrophyllite. The swelling properties of montmorillonite may be due to the presence of positive inter-layer ions in excess, together with some OH groups.

4. (4) A chlorite-talc series probably exists also, and vermiculite may have the same relation to this series as montmorillonite has to the mica-talc series.

Microscopic observation is valuable for elucidating diagenetic phenomena in clays subsequent to deposition. Examples:—

1. (1) Pressure can enlarge the regions of parallel orientation in kaolinite (Fig. 1), even giving zoned regions (Figs. 2, 3).

2. (2) Solution and re-precipitation may give vermicular kaolinite (Figs. 4,5), “fluidic” halloysite (Fig. 6), or vermicular dickite on nuclei of hydrargillite (Fig. 7).

3. (3) Montmorillonite may form from halloysite by action of basic siliceous waters (Fig. 8); or (together with halloysite) from kaolinite (Fig. 9).

4. (4) Illite may recrystallise, giving large hexagonal forms (Fig. 10).

Adsorption isotherms have been determined for mixed Ca-Na montmorillonites, with various values of f, the proportion of (½ Ca) to Na (Fig. 1). These are coincident from f=0 to f=0·3, and then diverge. This is interpreted as meaning that “demixing” occurs at f=0·3, a system being formed in which some interlayer spaces are occupied mainly by Ca, others mainly by Na.

X-ray analysis confirms this view. Plots of the water uptake against f, for various relative humidities (Figs. 2–5) show that, for R. H.'s lower than 75%, the demixing is incomplete.

The observations can be explained by the occurrence of a limited number of doubly-charged sites (by Al-for-Si exchange spots being accidentally contiguous). For the montmorillonite used, the statistical probability of occurrence of such sites is found to be 0·2 if the layers have symmetry C-2/m and 0·33 for C-2, so that the observations are in favour of the latter symmetry.

(1) Long spacings (29·6 and 26·7Å) are confirmed from certain clays from Japan. (2) These clays have very complicated clay mineral compositions including kaolin minerals, chlorite, sepiolite and montmorillonite. (3) Each long spacing can be explained by a regular interstratification between two kinds of lattices selected from the following ones: montmorillonite (cell ht. 15·4Å), a magnesian chlorite (14·5Å), sepiolite (12·7Å), and a new lattice (14·0Å); the last is a regular interstratitication of gibbsite and silicate (pyrophyllite unit) layers. The new lattice is proposed not only to explain the spacing but all other mineralogical properties of the clay. Thus the 29·6Å spacing agrees well with that of a 15·4/14·0Å super-lattice, and the 26·7 Å spacing with a 12·7/14·5Å super-lattice. (4) These long spacings are liable to be found in a clay in which several kinds of clay minerals co-exist, and such a complex clay tends to be found in special conditions, such as a boundary area between two different kinds of alteration zones, and/or an area affected by successive hydrothermal alteration.