Blue pleochroic crystals obtained from basic slag were first described by Carnot and Richard. They published a chemical analysis, some of the optical properties, and a brief description of the forms they observed. The crystals were stated to be orthorhombic, with a composition of 5CaO.P205.SiO2. Several authors later described similar crystals. None, however, gave a thorough account of the optical properties. These various papers were summarized and discussed by Kroll, who accepted Carnot's formula, but concluded, from Bücking and Linck's examination, that the material was monoclinic. Kroll applied the rather unfortunate term ‘silico-carnotite’ to these crystals.

The present work shows that silicocarnotite occurs in only one modification–orthorhombic. The crystals drawn by Bricking and Linck and described as monoclinic are, in fact, identical with our orthorhombie crystals.

Phase-contrast methods in microscopy appear to have been developed mainly with the intention of applying them to biological subjects, and their application to the examination of mineral substances has received only slight attention. Thin sections of rocks viewed under a phase-contrast microscope have a fascinatingly strange appearance and it does not take long to realize that some features are shown up more clearly and others less clearly than when viewed under an ordinary or a polarizing microscope. A considerable amount of work may be necessary before it is possible to assess the value of the new instrument for petrological purposes or to interpret all the phenomena observed by its aid. Nevertheless, it seems fitting to set out the following short account as a contribution towards this knowledge.

There is general agreement as to the basis on which most major igneous rock-types are defined despite some difference of opinion as to quantitative limits. This is not true of diorites and gabbros (or of their hypabyssal and volcanic equivalents) in distinguishing between which at least four different criteria are in current use.

The occurrence of dickite in the gold-bearing rocks of the Witwatersrand system was first reported by Mr. H. C. M. Whiteside. In addition, among the many mineral samples given to me by students of the Witwatersrand Technical College, Johannesburg, were several dickite specimens from mines all along the Witwatersrand area. Dickite may therefore be regarded as fairly widespread in the rocks of at least the Main-Bird series, although not in large quantity.

The only occurrence of which a detailed record is available is that from the Daggafontein mine. Little is known of the other samples except from the immediate mineral associations. It is, however, clear that all the occurrences are of hydrothermal origin.

The first occurrence of this mineral in Britain was announced by L. J. Spencer who found it in the great baryte-witherite vein at New Brancepeth, near Durham. It was found later by Sir Arthur Russell in the witherite vein at Settlingstones, Northumberland.

Spencer proved the presence of antimony in considerable quantity and of sulphur, and returned the nickel as 27·87 %, a quantity which agrees well with the accepted formula, NiSbS; the specific gravity was given as 6·70. In view of the rarity of the mineral and its scientific interest, a complete analysis was deemed advisable, and I have carried this out on a specimen from New Brancepeth, which came into my hands not long after its discovery.

The abundance of the well-known potassium aluminium silicate minerals, orthoclase, microcline, and muscovite, stands in striking contrast with the rarity of the little-known potassium aluminium phosphate minerals. The only one of these which occurs as distinct crystals and which has yielded a satisfactory formula is minyulite, KAl2(PO4)2(OH,F).4H2O. This mineral was first described by E. S. Simpson and LeMesurier from Western Australia in 1933 and an examination of well-crystallized specimens from South Australia by L. J. Spencer et alii was published in 1943. Spencer's paper included X-ray single-crystal and powder data for minyulite. X-ray, optical, and specific gravity data were also determined at that time for a few other hydrous potassium aluminium phosphate minerals from the British Museum collections. These included palmerite from guano deposits in a large cavern, Monte Alburno, near Controne in Salerno, Italy, presented by E. Casoria who described the mineral in 1904, and taranakite from the Sugarloaves, near New Plymouth, Taranaki, New Zealand.

The crystals of artificial cassiterite, described below, were produced by volatilization of South African low-grade tin ores. In the sinter-pot method employed, air was blown through a charge of alternate layers of coke and of a 12 % metallic tin concentrate mixed with fine coke. This charge was placed above a basal layer of previously ignited coal. Occasionally sodium chloride was mixed with the concentrate to remove tin as chloride as well as oxide fume. The air-flow was carefully controlled to prevent slagging and was maintained until the evolution of copious fumes of tin oxide, and at times of chloride, ceased.

Optical pyrometer readings on the charges during the experiments gave temperatures of 1050°C. approximately, but it is certain that this value is a grave under-estimate of the actual temperatures in the interior of the sinter-pot.

The clinozoisite to be described is found in veins traversing a metamorphosed basic intrusion exposed at the top of Camaderry Mountain, Glendalough, Co. Wicklow. This intrusion is a small boss of Lower Palaeozoic age which has been metamorphosed both by earth movement and by the intrusion of the Leinster granite. The rock exposed is now an amphibolite (fig. 1) within which are found the veins carrying clinozoisite with albite and a little prehnite, sphene, and tremolite. These veins, which are up to an inch in width, were probably introduced during the late stages of the basic intrusion as veins of lime-rich plagioclase.

Preliminary examination under the microscope shows that the clinozoisite is well crystallized, fresh, and free from inclusions. The colour in the hand-specimen is grey-white; in thin section it is colourless. Individual crystals are found up to 5 mm. in length. Twinning and intricate zoning are common.

The Egyptian clays under consideration are obtained from two different localities. The Aswan clays are mainly concentrated in the neighbourhood of Aswan town in Upper Egypt and the Sinai kaolins from the Sinai Peninsula. The Aswan clays, namely: red Aswan clay, white Aswan clay, and siliceous Aswan clay, are transported clays, while Aswan kaolin is a residual clay. The Sinai kaolins are of obscure origin but most probably are transported clays. As is shown later, these are alunite-bearing clays not previously recorded in Egypt.

A thin belt of serpentine up to 400 yards wide and about 6 miles long runs SSW. from Portsoy, Banff, and parallel to the general structure lines at that locality. Near Damhead, 1½ miles SSW. of Portsoy, an old quarry has recently been reopened in this belt and worked to a depth of 98 feet for the exploitation of talc. The general association in this quarry, stated in order of quantitative importance, is serpentine, talc, dolomite, the last being present in very minor amount but of an unusual character meriting description. In the following account the particular task of one of us (E. M. P.) has been the preparation of chemical analyses, the other author being responsible for descriptions and discussion.

Mineralogists have long been accustomed to describe hardness with the aid of a scale devised by Friedrich Mohs, who lived from 1773 to 1839. The test is qualitative, each mineral in the scale being capable of scratching those that precede it, but the ten minerals have held their ground as a useful representative series with which it is now interesting to compare another method of estimating hardness.

A new tin mineral, which has been given the name ‘nigerite’, was discovered in June 1944 during au investigation of the tin-bearing pegmatites of Kabba Province, central Nigeria. The chemical and physical properties of nigerite have been determined by F. A. Bannister, M. H. Hey, and H. P. Stadler, and their results will be published in the next number of this Magazine. Dr. Hey's analysis shows SnO2 25·33, Al2O3 50·91, Fe2O3 11·90, ZnO 4·51 %, &c., and the unit-cell contents are given by the formula (Zn,Mg,Fe″)(Sn,Zn)2(Al,Fe‴)12O22(OH)2.

The name ‘halloysite’, first used in 1826 by Berthier, is derived from that of Omalius d'Halloy, who found the mineral in Angleur, Liége, Belgium. In 1935, Mehmel discovered that there were two distinct substances included under the name halloysite, one of which contains loosely bound water and shows a 10 Å. basal reflection on an X-ray diagram, the other none and shows a reflection at about 7 Å. Mehmel preserved the name halloysite for the hydrous mineral, and for the product of its dehydration he proposed the name ‘metahalloysite’. Both substances may occur naturally (though this may not have been known to Mehmel at the time). Previous to this work it had been suggested by Hofmann, Endell, and Wilm that the 10 Å. material on dehydration at a low temperature gave kaolinite. Mehmel showed that this was wrong; and that the error was due to the similarity of the X-ray diagrams of kaolinite and metahalloysite. Hofmann and co-workers, and Mehmel, all tacitly assumed that the name halloysite was correctly applied to the 10 Å. material rather than the 7 Å. material.

The affinity between certain organic compounds and clays has been known for many years. Lloyd reported the adsorptive capacity of fuller's earth for alkaloids in 1916 without theorizing extensively about the mechanism. Smith reacted organic bases and their salts with bentonite and presented evidence that the reaction was one of base-exchange or metathesis, while Hauser has discovered that certain organic compounds of bentonite have the property of swelling and dispersing in organic liquids. This paper reports observations on the rather striking conversion of bentonite from the hydrophilic to the organophilic condition and proposes a mechanism to explain the observed phenomena.

In his work on the mineralogy of soils colloids G. Nagelschmidt, suggests the possibility of a reversible change of the type

We have attempted to establish experimentally the existence of this transformation following fixation or departure of the potassium from between the fundamental structural sheets of the mica type. The initial material was montmorillonite from Camp Bertaux, Morocco, separated from its impurities by sedimentation. As it was evident a priori that the alteration of montmorillonite into illite necessitates (1) saturation of the base-exchange capacity of the minerals by potassium, and (2) the departure of the layers of water from between the structural sheets of the montmorillonite, we finally, after several attempts, were led to make use of the following technique.