This paper describes the occurrence of myrmekite in a graphic quartz-microcline-rock from the mica-bearing pegmatites of Kodarma, Bihar, India. The myrmekite has grown out from the quartz-mierocline junctions into the potash-felspar with the appearance of having replaced the latter. The two are in optical and crystallographic continuity and, owing to the large ‘single crystal’ character of the microcline, definitely oriented sections can readily be prepared. A detailed study of these sections shows that the myrmekite has been formed by the segregation and coalescence of microperthitic albite formerly held in solid solution in the potash-felspar. Successive stages in this process of segregation can be observed.

These sections also provide evidence of the paragenetic relationship between the quartz rods, the vein perthite, and the body microcline. This is supplemented by additional evidence from graphic granites and vein perthites obtained from other localities.

The Struy lead mines (Six-inch ordnance Survey, Inverness-shire, sheet 17) are situated on the west side of Strath Glass, about two miles south-west of Struy village, and comprise three distinct sets of small long-abandoned workings, two of them being in the nature of trials only. All three workings are shown on the six-inch map, and are on veins traversing the Moine schists. The Geological Survey Memoir of the district says that four veins may be traced on the hill face between the Dùn south of Struy school and Crelevan; if this is correct there are two outcrops I have not seen. The mines are on what was until a few years ago part of Lord Lovat's estate. The date of their discovery is not known, but they were apparently worked about the year 1818 by Thomas Alexander Fraser (afterwards 12th Lord Lovat) at the same time as his graphite mine in Strath Farrar; most of the work seems, however, to have been done by him between 1838 and 1845, when miners were brought from England and housed in cottages specially built for them in Struy village. In 1864 the Loch na Mèine workings were cleared and prospected, but since then no further work has been done.

Antigorite is a lamellar variety of serpentine, and is supposed to be a dimorphous form of chrysotile, which is finely fibrous. Its chemical composition is approximately H4Mg3Si2O9, which is taken as the basis of calculations here.

This study was undertaken primarily because it was hoped that knowledge of the structure of antigorite would throw some light on that of chrysotile. Certain similarities between the two structures have been established, namely in the c(7·3kX or 14·6kX), and b(9·2kX) directions. There are two main differences, however. Firstly, imperfections which cause line broadening in the X-ray pattern of chrysotile, are absent in antigorite (apart from certain ‘streaks’). Secondly, the a(43·4kX) axis of antigorite is approximately eight times longer than the corresponding axis in chrysotile. A complete determination of the structure has not been achieved, but the X-ray pattern has been described, and some suggestions made as to the explanation of the peculiarities observed. A further study of the outstanding questions is in progress.

Loch Bruicheach (One-inch Ordnance Survey of Scotland, popular edition, sheet 37; Six-inch Inverness-shire, sheet 18) is situated 7½ miles (about 9 miles by road) south-west of Beauly and is best reached by taking the road from that town to the keeper's house at Clunevackie and thence by a track to within about half a mile of the actual locality, a low cliff bordering the north-east shore of the loch. Attention was apparently first directed to the locality about the year 1884 by a Mr. John Ross, one of Lord Lovat's gamekeepers, and Mr. Thomas D. Wallace of Inverness, having received through him specimens from there of baryte and a mineral which he, Mr. Wallace, recognized as resembling fluorite, visited the spot with Mr. Ross in May 1885. In August of the same year Mr. Wallace gave a description of the occurrence in the pages of this Magazine, and in July 1886, also in this Magazine, there appeared a note and analysis of the fluorite by Professor W. Ivison Macadam of Edinburgh, who in collaboration with Mr. Wallace proposed for the mineral, without any sort of justification, the name bruiachite. Macadam's analysis of the fluorite was very inaccurate and showed only 10% of fluorine.

During the course of a study of the interaction of certain pure refractory oxides at high temperatures it became necessary to obtain accurate values for the lattice parameters of α-alumina and chromic oxide. Values obtained by previous workers, listed in table I, did not attain a greater accuracy than one part in 2000, whereas the use of large-diameter powder cameras calibrated with quartz now yield lattice parameters accurate to at least one part in 30,000.

Samples of the calcined oxides were examined in a 19 cm. diameter powder camera, alumina with cobalt Kα radiation and chromic oxide with chromium Kα radiation. Results are given in tables II and III.

Bustamite forms an important constituent of the manganese ores of the Treburland mine near Altarnun, Cornwall. The rediscovery of this mineral locality is due to Sir Arthur Russell, who has already presented an account of the rhodonite and tephroite of the mine (Proc. Min. Soc. 1944, p. xix). Since then the writer has noted bustamite and bementite from the same locality (ibid. 1944, p. xxvi), and it is the purpose of the present paper to discuss the composition and paragenesis of this bustamite.

Rhodonite and tephroite form the chief silicate constituents of the manganese ores exposed On the dumps adjacent to the old Treburland mine shaft, but bustamite-bearing assemblages are not uncommon.

Dana classifies gümbelite in the kaolin division under pyrophyllite, and expresses the view that it may be an impure pyrophyllite. The present study, however, has shown that the unit cell of gümbelite is very similar to that of muscovite. Also the chemical analysis reveals a Closer similarity to muscovite, particularly to a variety called hydromuscovite.

The most complete succession of Carboniferous rocks found in Co. Limerick crops out in the Limerick basin, which is situated between Limerick and Tipperary. The zonal ranges of the chief lithological divisions of the Lower Carboniferous rocks have been determined, and two volcanic episodes, represented by the Lower and the Upper Volcanic Groups, are now known to have occurred early in S1 and early in D1 times. The porphyritic pyroxenes have been isolated from representatives of the lower group, which ‘constitute a graded series of olivine-basalts, trachybasalts, trachyandesites and trachytes, comparable with the Lower Carboniferous Scottish suite. Among the olivine-basalts, all the Scottish types have been recognised, with the exception of Craiglockhart type and mugearite.

Unpublished work carried out in this laboratory several years ago had shown that the first-order basal X-ray reflections of illite from South Wales occurred at a spacing of about 10Å both with potassium- and calcium-saturated samples. Recently Jackson and others observed that hydrated mica samples having a 10Å basal spacing when saturated with potassium, showed a 12 to 13Å. spacing after saturation with calcium. They interpreted their results by ascribing to illite a limited capacity for reversible lattice shrinkage and expansion, identical in type with that known for montmorillonite.

In order to study the problem further three illite and two glauconite samples were treated with concentrated calcium chloride solutions for five weeks under frequent renewal of the solution.

The Bikita tin and tantalum field is centred about 43 miles east-north-east of Fort Victoria, in the south-eastern part of Southern Rhodesia. It forms the end of a tongue of basic and sedimentary schists surrounded on three sides by granite. Tin and tantalum minerals have been known in the area for over thirty years, and at various times attempts to work them have been made, but with indifferent success. During recent years the demand for tantalum has led to the re-opening of the field and several tons of tantalite and simpsonite concentrates have been exported to America. The area was mapped geologically by Mr. R. Tyndale-Biseoe in 1942, but the publication of the map and report have been held up during the war. The principal workings are the Mdara mine and the Al Hayat claims situated about three-quarters of a mile apart.

A possible structure for kaolinite was first put forward by Pauling (1930) in a paper dealing generally with the structures of layer-type minerals, chlorites, micas, &c. He suggested that the structure consisted of a sheet of Si–O tetrahedra arranged in a hexagonal network with a superposed sheet of Al–(O,OH) octahedra, the two together forming a layer of composition Al2Si2O5(OH)4 (fig. 2). Ross and Kerr (1931), from a detailed microscopic study of the kaolin minerals, concluded that the symmetry of kaolinite is ‘probably monoclinic’. The first attempt at a detailed structure analysis was made by Gruner (1932a) from X-ray powder photographs. The reflections, some 40 altogether, were indexed on the basis of a monoclinic cell with a 5·14, b 8·90, c 14·51Å., β 100° 12′.

In the course of an investigation on refractory materials, in which the identification of minerals by their X-ray diffraction patterns had a prominent place, an interesting observation was made on a zircon of gem quality which was lent to us by the courtesy of Mr. B. W. Anderson.

It may be recalled that the existence of three allotropes has been postulated in order to explain the considerable variations of physical properties which are found when comparing different specimens of zircon. In particular, densities ranging from 3·95 to 4·86 have been observed, the density of zirconium silicate, calculated from X-ray measurements, being 4·71.

It is well established by X-ray crystal analysis that the mineral constituent of bone and of the enamel and dentine of teeth is essentially hydroxy-apatite Ca5OH(PO4)3, and that hydroxy-apatite has a structure differing only in small details from that of the well-crystallized mineral fluor-apatite Ca5F(PO4)3. The main difference, and in fact the only fully established one, between the two structures is that the hydroxy-apatite has a slightly larger unit cell than that of the fluor-apatite. The structure of the latter has been determined by Náray-Szabó, and by Mehmel who finally agrees with Náray-Szabó. We have made what is probably a more accurate determination of the structure, using more extensive X-ray data.