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Water vapour atmospheres are known to raise the temperature of the endothermic reaction of clay minerals in differential thermal analyses. In isothermal kinetic studies of dehydroxylation, it is shown that a stationary state is attained depending on the operating temperature. The rate of dehydroxylation is a function of specimen size and shape, which suggests a controlling influence by entrapped water vapour. Even at constant reaction temperatures, the rate of dehydroxylation varies greatly between the interior and the exterior of a specimen. The bearing of these results on thermal analyses and on kinetic studies generally is discussed.

In the preceding papers of this series, Part I (Hoare 1934) gave an account of the experimental method and results for sodium and potassium halides, Part II (Brindley and Hoare 1935) gave results for lithium, rubidium and caesium halides and a general discussion of this group of salts. The present paper deals with the halides of the alkaline earth metals. Although a few values have previously been obtained for these salts, this is the first time that a comprehensive systematic series of measurements has been made. When comparing the results for the halides of Mg, Ca, Sr and Ba with those for the alkaline halides, the effect of the positive ions being doubly charged must be considered. The positive and negative ions will be drawn closer together than if both ions were singly charged, but the change of interatomic distance on this account is not large owing to the intrinsic repulsion between adjacent ions varying as a high inverse power of their separation; the change will in fact be about 6-10% for the ions under consideration. The effects of the ionic charges on the electron distributions of the adjacent atoms have also to be considered; the positive ions will be subjected to approximately the same effects as the positive ions in the alkaline halides, but the negative ions will be subjected to larger effects owing to the positive ions being both doubly charged and at relatively smaller distances than in the alkaline halides. Whatever the effects of positive and negative ions on each other in crystal lattices may be, it seems unquestionable that these effects will be greater for the negative ions in the alkaline earth halides than in the alkaline halides, but will be of the same order for the positive ions in the two series of crystals.

From a wide survey of the available experimental data for the diamagnetic susceptibilities of salts in aqueous solution, the effective susceptibilities of the ions in solution are obtained and their additivity is established to within 1 per cent of the observed values. There are shown to be systematic differences between the susceptibilities of ions in solution and in crystals. Small univalent ions and all bivalent ions have effectively smaller susceptibilities in solution than in crystals owing to the effect of the ions on the surrounding water molecules; this effect is correlated with the field at the boundary of the ions. Large univalent ions have effectively larger susceptibilities in solution than in crystals, owing mainly to the change in coordination of these ions when they are dissolved. New measurements are given of the susceptibilities of a number of hydrated salts. It is shown that the susceptibilities of salts obtained from results for hydrated crystals, by assuming the water of hydration to have the same susceptibility as ordinary water, approximate more closely to the effective susceptibilities of salts in solution than to the true susceptibilities of the anhydrous salts.

Little exact information exists on the relation of mineralogical composition of clay materials to their properties after firing. Property-composition diagrams are constructed for linear shrinkage and apparent porosity of quartz-kaolinitemica mixtures fired at 1100° and 1200°C.; contours of equal shrinkage and equal porosity are derived. Sixty natural clays and shales are analyzed semiquantitatively for mineral content, and their shrinkage and porosity values are obtained after firing to 1100° and 1200°C. The ceramic properties of the natural materials are compared with those of the prepared mixtures. Conclusions are drawn regarding the influence of mineralogical composition on the ceramic properties. The mica content is shown to be particularly important because of its role in the for mation of a glassy component.

The two methods by which the intensities of X-ray reflections from fine powders can be measured absolutely by comparison with reflections from a standard substance are critically examined. The applicability of the mixture method is limited by the necessity for having powders of extremely small particle size, of the order of 10-5 cm. or less. In the substitution method, the powders are kept separate and the use of such fine powders is not essential; the absolute accuracy of this method is limited by the fact that the absorption coefficients of the two powders have to be known. Experimental results obtained by the two methods for powders of different particle sizes are compared with theoretical results by Schafer for the effect of particle size on intensities measured by the mixture method. The agreement between the experimental and the theoretical results is sufficiently close to show the usefulness of Schafer's analysis in determining the magnitude of the error which may occur when the mixture method is used with powders which are not sufficiently fine. A general account is given of a camera designed for use with the substitution method. It allows an alternating interchange of the two powders with a corresponding movement of the photographic film in order to obtain a separation of the photographs corresponding to the two powders.

Summary. The layer structure of the kaolin mineral dickite has been shown to differ significantly from the idealized arrangement of regular tetrahedra and octahedra. Since the structure of kaolinite can be studied only by X-ray powder methods, it cannot be refined beyond the idealized arrangement. By applying the diekite type of layer structure to the ease of kaolinite, an explanation for the triclinic symmetry of kaolinite has been obtained, and also an improved agreement with the observed X-ray intensities. A DETAILED structure analysis of kaolinite was first attempted by Gruner (1932), based on X-ray powder diagrams of the mineral. He concluded that the unit cell was monoclinic, with probable space group C~-Cc, and contained two structural layers related by a c-glide plane. However, further examination of powder diagrams of various kaolinites by Brindley and Robinson (1946) revealed that there were reflections that could not be explained in terms of this monoclinic cell, and they put forward a triclinic cell containing only one structural layer. So far, it has been proved that this triclinie cell fits very well with the observed features of the X-ray powder patterns of the mineral as regards both spacings and intensities of reflections. The structure of the composite layer of Si-O and AI-OH was assumed to have the geometrically idealized arrangement first proposed by Pauling (1930). At that time, there were no means to establish a refined structure for the layer itself in the absence of suitable single crystals. In terms of this idealized layer, it is impossible to account in the actual structure of kaolinite for the s-angle and for a small departure of the fl-angle from an idealized displacement of --a/3. Recently, Newnham and Brindley (1956, 1957) have made a detailed analysis of the mineral dickite, a polymorphic form of kaolinite, and have shown that the actual layer structure is distorted to some extent from the idealized arrangement. This has opened the way to an interpretation of the triclinic nature of kaolinite, since there are good reasons for thinking that the structures of the layers in dickite and kaolinite

SummaryElectron micrographs of scarbroite show thin platy crystals of about 1 μ size, having rhombic outlines with angles 66° ± 1° and 113° ± 1°. Single-crystal electron-diffraction patterns show rectangular net patterns, with d100 = 9·90 Å., d010 = 14·67 Å., γ* = 90° ± 0·05°. Strong hk0 reflections show a pseudohexagonal arrangement, but true symmetry is probably orthorhombic or monoclinic. Faces outlining rhombic forms are of type {11l}, .

Fuller's earth from near Dry Branch, Georgia, is shown by X-ray analysis to contain comparable amounts of montmorillonite and cristobalite, and only minor amounts of other minerals. The cristobalite resembles the high form, but may be a very disordered low form. Electron micrographs show the montmorillonite to be lath-like, and the cristobalite globular in form.

A three-month-old male infant with undiagnosed hemophilia presented with fever and irritability. During the diagnostic evaluation a venipuncture in the right antecubital fossa led to hemorrhage and a compartment syndrome in the brachium. Differential diagnosis included osteomyelitis and septic arthritis but a 99technetium bone scan revealed only delayed washout of radioactive material from the right brachium without increased uptake. Despite elevated pressures in the muscle compartments of the right arm, surgical decompression was delayed until a complete blood coagulation profile was obtained. After replacement of deficient blood products, the compartment syndrome resolved without surgical decompression. Defective coagulation leading to hemorrhage into the muscle compartments of the arm resulted in a significant elevation of compartmental pressure. The unusual clinical presentation of the patient, as well as a lack of a positive family history, delayed the diagnosis of hemophilia and the subsequent replacement therapy.

Intraoperative fractures of the proximal femur occurred in 40 (38 patients) of 630 (6.3%) biological ingrowth total hip arthroplasties performed between January 1984 and July 1986. Twenty-three of these fractures occurred during 131 revision arthroplasties (17.6%) and 17 during 499 primary arthroplasties (3.5%). All but two of the fractures were recognized during surgery. Thirty-seven were treated with either Parham bands or cerclage wires. Bone graft was added to the fracture site in 31 fractures. All of the fractures healed. Three of the patients failed to achieve stable fixation and have required revision surgery. An additional patient has thigh pain with 2 mm of subsidence noted with serial roentgenographs. Femoral fractures can be prevented by preoperative templating of roentgenograms containing markers to measure magnification, routine overreaming of the femoral canal when implanting long-stemmed prostheses, and prophylactically applying wires or bands to femurs requiring the removal of screws.

The effects of alterations in serum calcium levels on the volume of distribution of a calcium tracer in bone and on bone blood flow were investigated in dogs by manipulation of parathyroid status. The volume of distribution increases with increasing serum calcium. This implies that there is a concentration-dependent binding mechanism in the extravascular space of bone, which is not saturated at the levels of serum calcium achieved in these experiments. The skeletal exchangeable calcium was increased in hypercalcemic dogs and decreased in hypocalcemic dogs. Cancellous bone contains proportionately more exchangeable calcium than cortical bone. Bone blood flow was decreased by 50% 15 min after injection of a bolus of parathyroid hormone (PTH) but returned to control values by 240 min. Fifty-two hours after the induction of hypercalcemia with repeated doses of PTH, there was no difference in blood flow over control animals.

Aliphatic chain molecules may be adsorbed on montmorillonite surfaces with the zigzag of their carbon chain either parallel or perpendicular to the surface. These orientations are designated αII and αI, respectively. Fourier syntheses have too few terms to distinguish clearly between these orientations. The problem is studied therefore by consideration of the basal spacings. These spacings are appreciably less than the sum of the van der Waals cross-section of the molecules plus the thickness of the montmorillonite layer. Chemical bonding or geometrical packing, or both, appear as possible explanations. Previous studies have shown that CH activity influences the shape of the adsorption isotherms, but infrared studies of the adsorbed molecules have shown no appreciable change in the CH-stretching frequencies. This suggests the importance of the CH activity for organic-organic interaction on the clay surface rather than for clay-organic interaction itself. The possibility of geometrical packing is studied by means of molecular models and it is shown that an appreciable contraction can occur at a clay-organic interface owing to a keying of the molecules into the surface structure. This is possible equally well for the αII and the αI orientations. From the information available, it appears that the αII arrangement is adopted by aliphatic chain molecules containing strongly polar groups in order to place the dipole parallel to the silicate surface, while the αI arrangement is favored by other aliphatic chain molecules possibly because this provides a favorable organic-organic interaction.

I believe that the answer to this question raised by Gy. Pokol [1] can only be 'NO' in all cases where reactions are concerned involving the loss of water vapor from finegrained solids such as clay minerals. The reason for the emphatic 'NO' arises from the dependence of the reaction not only on temperature and time, but also on the variable water vapor pressure generated by the reaction itself. Perhaps in an ideally dispersed sample the water vapor pressure might be controlled sufficiently by the temperature of the surrounding equipment, but in any actual experiments where even small samples are loosely contained in sample holders, some variation of p(H20) must inevitably occur within the sample studied. In extreme cases, the effects of the generated water vapor pressure can be very large and exemplify the nature of the effect. Thus, with a pressed disc of kaolinite about 3 mm thick heated ~sothermally at 517 ~ , the exterior surface can be about 95% dehydroxylated when the interior is no more than 40% reacted. Discs prepared under similar pressure conditions and ranging in thickness from about 0.4-2.7 mm thickness and heated isothermally at 497 ~ showed dramatically different rates of overall dehydroxylation [2]. These experiments, now old history, showed strikingly the effect of entrapped water vapor on the reaction rates. These inherent problems are greatly reduced by using small samples of loosely packed material but can it ever be asserted that in kinetic experiments which involve the removal of water from surfaces and from the ambient atmosphere wholly eliminate the effect of the water vapor released by the reaction? Experiments under isothermal conditions with small and loosely dispersed samples under conditions of controlled water vapor pressure offer a better solution to these inherently difficult questions [3, 4], but they take much longer to carry out.

MANY solid-state reactions involving oxide systems take place by re-organization of cations within a framework of oxygen ions and the rate of the reaction is controlled by ion diffusion rather than by nucleation and grain growth processes. This general concept has been well illustrated by recent work of Bernal et al. 1 on oriented transformations of iron oxides and hydroxides, and of Steadman and Youell2 on the thermal transformation of an iron silicate of layer-lattice type.

The AlKα emission wave length is determined by the state of coordination of Al atoms and a clear distinction can be made between Al atoms in fourfold and in sixfold coordination to oxygen and also between Al atoms in fourfold coordination to oxygen and to nitrogen. Measurements of Δ(2θ) (the difference in diffraction angle for AlKα radiations from a sample and from metallic aluminum) are used to distinguish Al in fourfold and in sixfold coordination in normal and in heat-treated kaolin minerals. The Al atoms in meta-kaolin are shown to have fourfold coordination to oxygen in agreement with the structure proposed by Brindley and Nakahira. After heat-treatment at 1024°C., the coordination of Al reverts toward sixfold, and this is more marked for disordered kaolinite and halloysite than it is for well-crystallized kaolinite and dickite. Infrared absorption measurements by Stubican and Roy provide further confirmation of the coordination changes of the Al ions in these reactions.

THROUGH the kindness of the Editor, I have been allowed to see Prof. Herrmann's letter prior to its publication. I entirely agree that the X-ray results do not exclude the possibility that the outermost electrons of the atoms in metallic zinc and cadmium may depart from having spherical symmetry ; one would not expect the completed inner electron shells to have appreciable asymmetry. The possibility of the outermost electrons being asymmetrical, however, is not to be regarded as an alternative explanation of the experimental results, but as something additional to the explanation already given in terms of asymmetrical lattice vibrations. The contributions of the outermost electrons to the scattering factors of zinc and cadmium at all angles for which measurements can be made are so small that one could scarcely hope to measure an asymmetry in their distribution by means of X-rays. On the other hand, any asymmetry in the lattice vibrations will affect the entire atom, and at large scattering angles, measurable effects may be expected if the asymmetry is sufficiently great, and this appears to be so for zinc and cadmium.

A summary is given of a structure analysis of chloritoid. The structure contains continuously linked octahedral sheets cross-linked by discrete SiO4 units. Comparisons are made with mica structures and with corundum.

The dehydroxylation reaction of kaolinite in a vacuum at 425 degrees C is halted by introducing a water vapor pressure of 47 mm-Hg, and is resumed when the vacuum is reestablished. The sample gains weight corresponding to an approximately monomotecular layer of water on the kaolinite surface. At the temperatures and vapor pressures involved, the sorption is considered to be a chemisorption process.

DURING the past few years, it has been shown that the diamagnetic susceptibilities of ions in crystals are approximately additive, and their values are in fairly close agreement with values calculated theoretically from the wave-mechanical structures of free ions1. Attempts to interpret the experimental results in more detail have been handicapped by a lack of essential data, for although results are now available for many substances, some have been obtained for solutions and others for crystals, and the two are not directly comparable. With the view of obtaining more precise information for a particular group of crystals, we have studied the susceptibilities of the alkaline halides, using a method of measurement previously described2.

Two articles and an editorial note in the August and September 1971 issues of Physics Bulletin indicate that physicists in Britain are not wholly satisfied with traditional procedures relating to PhD theses and oral examinations. My experience of 24 years in a British university and 18 years in an American one leads me to think that the American system has many advantages which might be considered if changes are to be made in British arrangements.

THE axial ratio, c/a, of the majority of metals having a hexagonal close-packed type of structure differs appreciably from the value 2 ” = 1·633 which corresponds to a hexagonal close-packed system of spheres. The two most notable deviations are for zinc and cadmium, for which c/a = 1·86 and 1·89 respectively, while for beryllium, titanium, zirconium, hafnium, yttrium, ruthenium and osmium, c/a lies between 1·58 and 1·59. These facts have sometimes been expressed by describing the atomic domains as having the forms of prolate and oblate spheroids when c/a is respectively greater than and less than 1·633 ; but it is only recently that experimental evidence has been obtained which throws light on the nature of the asymmetry in these metals.

IN a recent investigation of the ionisation processes in methane, Hogness and Kvalnes (Phys. Rev., 32, December 1928), using a mass-spectrograph method, find that at 14.5 volts only CH4+ ions are formed, but at 15.5 volts two processes occur; either stable CH4+ ions are formed or unstable CH4+ ions which dissociate spontaneously into CH3+ ions and neutral hydrogen atoms, the probabilities of the processes occurring being approximately equal over a wide range of pressure.

CHAMOSITE is a hydrous ferrous silicate, of common occurrence especially in bedded ironstone deposits. While it has generally been regarded as belonging to the chlorite family of minerals, evidence has accumulated tending to show that it is not a normal chlorite. To obtain more certain information on this question, a detailed analysis has been made of the X-ray powder diagrams of a number of chamosite specimens ; the results obtained lead me to conclude that chamosite is not a chlorite but has a structure of the kaolin type. Two other iron-bearing minerals, namely, amesite and cronstedtite, originally classified as chlorites, have also in recent years been shown by Gruner1 and by Hendricks2 respectively to be kaolin-type minerals. While these are comparatively rare minerals, chamosite is a very common mineral. Its recognition as a kaolin-type structure is of general interest in that it brings the kaolin group of structures more nearly into line with other groups of clay minerals in which iron-bearing members are quite common.

X-RAY studies of these minerals in relation to (a) their reactions with organic liquids (MacEwan), (b) the effect of heat treatment (G. W. B. and K. R.) have led us to certain conclusions which can best be expressed in a joint communication. Meta-halloysite has the same formula as kaolinite, Al2Si2O5(OH)4, and from the work of Hendricks and others1 is generally considered to have a closely similar structure, consisting of layers of Si-tetrahedra and Al-O,(OH) octahedra which in kaolinite2 are arranged regularly above each other, but which in meta-halloysite are stacked with “considerable disorder”1 or even, as we believe, with almost complete disorder as regards the orientation of the layers. In what follows we set out briefly the evidence for this view of the meta-halloysite structure.

Single-crystal electron-diffraction data reveal features of metakaolin. The basal plane parameters increase 2.2 percent in formation of metakaolin produced by heating kaolinite in air at 700 degrees C for 12 hours. This increase results from removal of the distortion of sheet structure (relaxation).

A SHORT time ago, experimental results were published by the present writer and F. W. Spiers on the X-ray scattering factors of nickel, copper and zinc atoms for copper K. radiation. While the observed values for nickel and copper lay on smooth curves when the scattering factors were plotted against (sin )/ in the usual way, in the case of zinc the scattering factors for certain reflections were found to lie well off the mean curve. It was noted at the time of publication that these discrepant results were consistently obtained and it has since been found that similar results are obtained with cadmium; both metals have a closed packed hexagonal structure with axial ratios c/a = 1.856 for zinc and 1.886 for cadmium. Cadmium is a more favourable metal to investigate for, having larger lattice spacings, it gives more reflections than zinc and they are relatively stronger.

THE banded character of many of the X-ray ‘lines' from such clay minerals as montmorillonite and halloysite has been attributed to their structures being equivalent to a random stacking of two-dimensional silicate sheets1. The relevant theory has been developed by Warren2, who introduced simplifying assumptions in order to obtain explicit expressions for the intensity distribution. We have sought a more general expression than Warren gives, and have avoided certain of his approximations by expressing the results in a form requiring numerical integration.