The discovery of nigerite, a new tin-bearing mineral, by R. Jacobson and J. S. Webb claims more than the passing interest of an addition to the few known species containing appreciable tin. Nigerite is the first mineral to be discovered in which both tin and aluminium are essential constituents. Its chief interest to mineralogists lies in its close relationship to minerals like spinel and corundum based on a close-packed or nearly close-packed assemblage of oxygen ions.

Samples of nigerite submitted to us for investigation consist of brittle, lustrous, brown, weakly magnetic, hexagonal plates up to 5 x 2 mm. in size but mostly much smaller. These samples also contained grains of cassiterite, columbite, gahnite, sillimanite, quartz, andalusite, and chrysoberyl, which were easily separated by hand-picking.

In a group of earlier publications details have been given of the application of multiple-beam interference methods to the study of mica. The characteristic structure of this mineral is exhibited by the Fizeau fringes which are produced when one side of a mica slip is silvered and placed against a similarly coated optical flat. The fringes show that the surface is violently contorted and crossed by cleavage steps, the heights of which appear to be integral multiples of the crystal-lattice spacing perpendicular to the basal plane of cleavage. If, however, a mica slip (thickness circa 1/100 mm.) is silvered on both sides then fringes of a totally different appearance are observed. It is frequently found that, despite the distortions on the surface, the Fizeau fringes are large areas of uniform illumination bounded by cleavage ‘lines’.

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 mineral tilleyite was first discovered by Larsen and Dunham in thermally metamorphosed limestones from the contact zone at Crestmore, California. It is associated there with merwinite, wollastonite, spurrite, and gehlenite, and the rock is traversed by streaks of idocrase with grossular. The amount of tilleyite in these rocks was apparently small, but in certain thermally metamorphosed limestones from Carlingford it is abundant and some specimens are practically pure tilleyite-rocks, it has been possible, therefore, to make a more detailed optical and chemical examination of this mineral. The tilleyite occurs as subidioblastic crystals when abundant, or as rounded grains in calcite-rich rocks, or as irregular plates.

In recent years the clinopyroxenes of the system clinoenstatite–diopside–hedenbergite–clinoferrosilite have received a great deal of attention. As research in this field advanced, the need for a balanced and generally acceptable subdivision of the system has made itself increasingly felt. Hess (1941) proposed a classification for these clinopyroxenes, which was subsequently extended by Benson (1944).

Interest in the orthopyroxene series is evidenced by the great number of publications on that subject which appeared in the literature. Correlations of the chemical composition and optical properties were attempted by Winchell (1923, 1933), Walls (1935), Henry (1935), and Hess and Phillips (1940). The nomenclature of the series was discussed by Winchell (1923), Alling (1936), and Hess and Phillips (1940). The obscure phenomena exhibited by certain orthopyroxenes in polarized light, attributed by some to twinning, by others to exsolution, was noted by a great many petrologists, of whom are mentioned here Lewis (1908), Holmes and Harwood (1928), Guimarães (1933), Scholtz (1936), Hess and Phillips (1938, 1940), Kuschke (1939), Wager and Deer (1939), Nel (1940), Walker (1940, 1943), Walker and Poldervaart (1940, 1941), Edwards (1942), Henry (1942), and Poldervaart (1944, 1946).

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.