For some time research workers have directed their attention to the study of ferriferous minerals. Researches on iron ores have led us to take up this problem. We shall give here the results of our observations on some minerals considered to be pure, belonging to the collection of the Museum National d'Histoire Naturelle, and on samples extracted by sedimentation from various ores.

This shows that the different samples belong to three types, with 10Å, 14Å and 7Å spacing. We will consider only the last two.

1. 14Å type (chlorite).—This includes the Chamoson chamosite, the bavalite from Bas-Vallon, metachlorite from Radauthal, and the chlorites extracted from ores from Soumont (Calvados), Auboue, Angevilliers, Andun-le-Tiche, Godbrańge (Lorraine).

Amongst hitherto described clay minerals, there are a number which have not yet been placed in clearly defined categories. In the course of an investigation of antigorites (Caillere, 1936), one of us has already shown that the thermal curves of bowlingites from Blackburn, Linlithgowshire, and Catkin Hills, Lanarkshire, of saponite from Catkin Hills and of diabantite from Bergen Hill, New Jersey, U.S,A., are very similar to the thermal curves of montmorillonites. We have therefore taken up again the study of these minerals with a view to determining more precisely their nature. We may recall that prior to the work already mentioned (Caillere, 1936), bowlingite and saponite were classed with antigorites, while the mineral from Bergen Hill, regarded as a diabantite, was classed with the chlorites.

The kieselguhrs or diatomaceous earths are composed essentially of more or less well crystallised silica, as well as a small quantity of alumina, iron oxide, and traces of other oxides. These earths are largely used as filtration aids; they can serve as supports for certain catalysts, and have also excellent calorifugic properties.

A study of different sorts of kieselguhrs used as supports for nickel and cobalt catalysts in the Fischer-Tropfsch synthesis has recently been carried out by Anderson et al. (1947). These authors find that a previous treatment of the raw kieselguhr by hot hydrochloric and nitric acids gives more active catalysts than those obtained using the same support untreated, and especially than those prepared from calcined kieselguhrs.

During an extensive survey of brickmaking clays by the differential thermal analysis method (carried out from 1947 onwards) it was observed that the clays of the Keuper Marl gave a series of very similar thermal curves which could not be explained by the presence of known clay minerals. Dr D. M. C. MacEwan of Rothamsted Experimental Station kindly carried out an X-ray examination of one of the samples for us, and he has reported the results of this examination, and the examination of a Keuper Marl sample from another source, to the last meeting of this Group. He postulated the existence of a “hydrated chloritic material”—sometimes exhibiting an expanding lattice stabilised by glycerol.

Since then the Keuper Marls have been further studied at the Building Research Station both by thermal analysis and X-ray methods and this paper is an interim report of this work. MacEwan's findings have been confirmed and some additional information is presented.

Recent examination of clay-materials from several different sources has suggested to us the existence of a type of chlorite capable of the same kind of expansion along the c-axis by imbibition of glycerol, as is shown by montmorillonoids, vermiculites and halloysites. The expansion occurs within the range of 14 kX to 18 kX.

It is necessary to discuss the definition of chlorite in this connexion, since such expansion is not regarded as characteristic of any of the minerals to which the name has been given. The characteristics by which chlorites are generally identified from their X-ray diffraction patterns are twofold: (1) they give basal reflexions at approximately 14 kX and submultiples, the first four orders being usually strong and of comparable intensity, though differing in different species of chlorite; (2) these reflexions persist after heating at 500°C.

We investigated two earths sent to the Institute for Glass and Ceramics at the Technical University in Prague by the Inspectorate for Research and Mining at Presov in Slovakia. The earth from the region of Kuzmice is a “metabentonite.”

In the literature there has as yet been no mention of the abnormal properties of suspensions of such clays. We found that suspensions with a concentration of over 15 per cent (measured by volume) behave as a gel with a very low limit of flow. This is the reason why no signs of thixotropy show themselves. Between certain limits the viscosity does not rise with increasing concentration above the 15 per cent mentioned above, which we explain as a consequence of structural flow of the gel. At lower concentrations the particles are rather distant from one another so that they can move freely (rotate) when flow takes place and the viscosity increases with the concentration of the suspension. Above 15 per cent the particles draw so near to one another that their swollen and easily deformable surface layers can touch one another. Such particles deform during their motion; they orientate themselves in the direction of the flow and they glide over one another so that the viscosity does not increase. The limit of flow being very low, the gel has not got such a strength as is necessary to solidify perfectly; this is the reason for the absence of signs of thixotropy.