Recent research on the polymorphism of calcium orthosilicate has broadened and to some extent modified the picture of the phase relations of this substance as presented in the pioneer studies of Day and Shepherd (1906) and Rankin and Wright (1915). These investigators recognized three distinct forms, α, β, and γ, related by transformation as follows: α⇄β at 1420° C. and β⇄γ, at 675° C. The high-temperature α form was described as monoclinic or triclinic and intimately twinned, the β form orthorhombic, and the low-temperature γ form as probably monoclinic. The optical properties of these α and β forms as reported are so closely similar that were it not for the twinning, regarded as diagnostic, the two forms would be distinguishable only with difficulty. This optical similarity was later found to be paralleled by a close agreement of the X-ray powder diffraction patterns of the two phases early noted by Hansen (1928) and confirmed among others by Insley (1936).

In the course of examining the cores of a boring for water at Hesleden Dene, near Castle Eden, Co. Durham (no. 2 borehole), Mr. A. Fowler noticed a cream-coloured flaky mineral of talcose aspect coating joints in the Magnesian Limestone (Permian), at depths below surface of 246 to 250 feet. The rock at these depths is an impure dolomite mudstone (Geological Survey Slice, no. E. 21304) resembling the Marl Slate, and carrying ostracods, foraminifera, and sponge spicules. Investigation of the coatings in specimens sent in by Mr. Fowler and by Mr. W. Anderson has shown that the flaky mineral is dickite, and that it is associated with fluorite, baryte, galena, zinc-blende, pyrite, and collophane. Dr. J. Phemister had previously concluded on optical evidence that dickite might be present among the minerals of metalliferous affinities discovered in the Magnesium Limestone by Mr. Fowler.

In the progressive thermal metamorphism of siliceous dolomites, tremolite and calcite are usually considered the earliest of the new mineral phases formed by reaction of the primary materials, dolomite and quartz. An early formation of forsterite has also been recorded, as, for example, in the dolomitie zones of the Durness limestone surrounding the Tertiary Beinn an Dubhaich granite of Skye. In this aureole, however, the relation of forsterite to tremolite has hitherto not been closely investigated. As Bowen has pointed out in his systematic study of the progressive metamorphism of siliceous limestones and dolomites, a first formation of forsterite from dolomite and quartz could not rank as an equilibrium reaction since forsterite and quartz can have no stable coexistence. Tremolite has accordingly been regarded as the first metamorphic product in an equilibrium series of reactions of increasing decarbonation affecting siliceous dolomites in an advancing higher temperature environment at any given pressure.

This paper concludes the account of our experiments on halloysite and is to be read in conjunction with the two previous parts, to which frequent reference will be made.

The effect of heat-treatment on metahalloysite was examined with the primary object of ascertaining if a recrystallization or ‘kaolinization’ of metahalloysite could be achieved.

Cuspidine was first described by Scacchi (1876) in the ejected blocks of metamorphosed limestone at Vesuvius, but a detailed and systematic morphological description of well-terminated crystals was given first by vom Rath (1883) from material at Cozzolino in Resina, Vesuvius. The monoclinic character of the spear-shaped crystals was then established with a:b:c = 0·7243:1:1·9342, β = 89° 22′, and twins on (100) giving = 1° 16′. Scacchi (in Arzruni, 1877) had earlier indicated the possibility that cuspidine he had described as orthorhombic was monoclinic and twinned when he recognized a re-entrant angle of 178° 42′ on the cleavage truncating the acute ends of the crystals.

These two minerals are analogous silver compounds with similar pseudo-hexagonal monoclinic form and similar compositions: polybasite, 8(Ag,Cu)2.(Sb,As)2S3, and pearceite, 8(Ag,Cu)2S.As2S3. Cu replaces Ag in considerable and varying amounts in both minerals, and in polybasite there may also be substantial replacement of Sb by As. However, the rather numerous existing analyses do not indicate continuous chemical variation from arsenian polybasite to pearceite, and therefore it is proper to recognize the two minerals as distinct species rather than end-members of a series.

A recent report has appeared describing a process developed in Germany for the production of a crystal described as synthetic mica. The details are given for a process used at Ostheim and reference is also made to a Siemens–Halske technique. Through the courtesy of Mr. Kendall of Metropolitan-Vickers we have secured a few small samples of the Siemens-Halske material. This is reported to be a fluor-phlogopite with the chemical composition KMg3AlSi3O10F2 and free from water of constitution. Claims made by the manufacturers stated that the synthetic material exhibits the cleavage characteristics of natural mica.

The few samples we have obtained have areas somewhat less than a square centimetre and we have selected thin cleavage flakes and submitted them to intefferometric examination with the multiple-beam technique.

The method described is one that can be used to measure the refractive indices of minerals when no refractometer is available. The readings are consistent to ±0·002 and are in reasonable agreement with those obtained using a refractometer.

The principle of the method is to mix two liquids with different refractive indices. One of these is a typical immersion liquid with a very low volatility. It must have a refractive index greater than that of the mineral under investigation. The other, more volatile liquid has a refractive index less than that of the mineral. The two liquids selected must be miscible in all proportions, but must not react together. The mineral is completely immersed in a mixture of the two liquids.

The plagioclase which forms the subject of this investigation was presented by Dr. C. Stansfield Hitchen, director of the Kenya Geological Survey, who has kindly sent the following note of its occurrence:

In X-ray studies of fireclays we have frequently observed the presence of anatase to the extent of 1 or 2% with little or no rutile, and no brookite. Chemical analyses of these clays usually record 1–2% of TiO2 and occasionally more (Ennos and Scott, 1924). The mineralogical form of the TiO2 is usually not stated, but a number of references to the observance of rutile needles give the impression that rutile is the form in which titania occurs in these deposits. Our own observations based on X-ray powder photographs lead us to conclude that anatase is commonly present and that rutile is either absent or present in smaller quantities. We have not detected brookite.

Of the results which have so far been achieved by study of the packing of equal spheres one of the most remarkable, both by reason of its simplicity and of its fundamental importance, was that announced by Barlow in 1883. He called attention to the fact that equal spheres can be most densely packed in two ways, one possessing cubic symmetry and the other hexagonal (fig. 1).

Already in 1862 Tait had investigated the piling of marbles of equal size and had noticed that ‘there are two obvious ways of constructing the layers, and two of applying layer to layer’: nevertheless his two densest arrangements are in fact identical. He saw that the cubic structure could be begun either upon a square base or a triangular base but failed to perceive the possibility of the hexagonal arrangement.

In an earlier paper the authors described the occurrence of artificial kalsilite in the hearth of no. 3 blast-furnace of the Kettering Iron Company. That paper contained a detailed description of the furnace campaign together with the peculiar conditions observed in the excavated hearth which was found to exhibit zoning in a vertical direction. The third zone containing kalsilitc was described in detail, but the only reference to the second zone stated that it was 2 inches deep (fig. 2, p. 77, this vol.) and consisted of a black glassy slag containing numerous crystals of cordierite (p. 78). The detailed examination of this zone now forms the basis of the present paper.

Mineral localities in north-western England were visited during the second excursion organized by the Society in recent years, from April 14 to 18, 1947, the headquarters being the George Hotel, Penrith. Members taking part included: The President, W. Campbell Smith (London), O. J. Adamson (Oslo, Norway), W. Anderson (Newcastle), F. A. Bannister (London), D. W. Bishopp (Dublin), C. E. N. Bromehead (London), G. F. Claringbull (London), T. Deans (London), W. A. Deer (Cambridge), K. C. Dunham (London), N. R. Goodman (Nova Scotia and Oxford), G. S. Gowing (Stockton-on-Tees), S. E. Hollingworth (London), W. Hugill (Stoke-on-Trent), H. Neumann (Oslo and Leeds), C.W. Pegg (London), F. H. Rathmann (U.S.A.), P. L. Rumsby (London), R. D. S. Shrimpton (Slough), J. H. Taylor (London), E. A. Vincent (Durham), L. R. Wager (Durham), Miss D. E. Wisden (Southampton). Dr. Claringbull undertook the administrative arrangements.

A laboratory investigation in connexion with some blast-furnace slags, led to the preparation of the five synthetic melilites for which X-ray data are provided. The five compounds represent gehlenite, åkermanite and three members of the intermediate series of solid solutions, corresponding to 25, 50, and 75 % of åkermanite.

The binary system gehlenite–åkermanite was studied by Ferguson and Buddington, who established relationships between refractive indices, density, and composition and determined solidus and liquidus curves. The crystal structure of the melilite group of compounds was investigated by Warren, who showed that the structure was based on a tetragonal lattice.

The important influence that the radioactive decay of the radio-elements uranium and thorium has had on the geochemical distribution of lead is now well appreciated, but little is known about the possible geochemical implications of the radioactivity of rubidium.

Like its alkali metal homologue potassium, rubidium is β radioactive and slowly disintegrates into a stable isotope of strontium: small quantities of strontium have therefore been accumulating in various minerals since the earth's crust first consolidated. This paper will describe firstly, the abundance of radiogenic strontium in the earth's crust as a whole, and secondly, the distribution of radiogenic strontium in various minerals and rocks, in particular those which are of igneous origin.

Extinction is the increase in absorption that is to be expected when Bragg reflection of X-rays takes place in single crystals, and the corresponding decrease in intensity of reflection observed.

The increase of absorption was directly observed in 1914 in two ways. Using the ionization spectrometer, W. H. Bragg found that the amount of radiation transmitted by a crystal section was notably decreased when the transmitted beam had suffered reflection from a set of crystal planes.

Ganophyllite was first detected in the Benallt mine as small fibrous patches varying in colour from pale pink to brown, in the thin vein cutting ore in the Ty Canol incline from which the manganese-rich chlorite, pennantite, was described. These were identified by X-ray photographs. It had previously been found at the neighbouring Nant mine by Sir Arthur Russell in 1911.

It is a familiar fact that in dikes, lava-flows, and sills the grain-size of the individual minerals normally varies according to the distance from the contacts. At or near the margins of the igneous body the grainsize is usually very much less than in the centre, and this phenomenon has been attributed to differences in the cooling-velocities at these spots. On the basis of measurements carried out by Queneau (8) the opinion seems to have been formed that the crystal-size always shows a progressive increase from the contacts towards the centre. This is not so, however, for as Lane (6) has shown, the crystal-size may increase to a maximum at a certain distance from the margin, and thereafter decrease again towards the centre of the intrusion. Apart from these investigations, carried out by Queneau and Lane, the only additioual quantitative data oil crystal-size variation known to the writer concerns a series of measurements made on an olivine-diabase by B. H. Dollen, under the direction of H. L. Alling.

From the beginning of classical times it was well known that the mineral conveniently called loadstone, whether magnetite in the strict sense or some other compound, attracted or, better, adhered to iron ; also that this property was imparted to a piece of iron in contact with a loadstone, so that a chain of disconnected iron rings could be suspended from it. This action aroused such wonder that many superstitions gathered round the stone which was reckoned throughout tlle Middle Ages as a gem. A genuine economic value was given to loadstone when the further discovery was made that a suitably shaped piece of the stone, or of iron touched with it, if free to move, would always aline itself approximately north and south.