The manganese ores of the Benallt mine are pervaded by an orange-coloured mineral which occurs very poorly crystallized as patches and small specks in the matrix. Less commonly it occurs as minute flakes in the ore and sometimes as a constituent of very thin veins which traverse the ore. In some specimens the mineral is better developed. It then shows a good cleavage and resembles in appearance in thin section a weakly pleochroic mica or chlorite. Patches of this mineral in the matrix frequently enclose very small rounded garnets which are believed to be spessartine.

In 1932 Dr. A. F. Hallimond examined some sections of ore from the old Benallt mine for Dr. C. A. Matley and suggested that one of the constituent minerals might be a manganiferous chlorite.

Of the many small manganese deposits which have in the past been worked in both Cornwall and Devonshire that of Treburland is from the mineralogical point of view by far the most remarkable, its especial interest being due to the variety of minerals which it has afforded and to the fact that it and one other are the only manganese deposits in the west of England which are known to lie on the contact of igneous and sedimentary rocks and which have consequently been vitally affected by contact metamorphism.

The following observations are based on frequent visits to the spot since the year 1906, when I first stumbled across the locality, which, strange to say, has only received very cursory mention by the Geological Survey and has altogether escaped mention in mineralogical literature.

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.

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′.

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.

Calcium is one of the most widespread and abundant of metals, coming fifth on the list of elements and third among the oxides of the outer lithosphere; it is an essential constituent of a great many rock-forming minerals and is, therefore, almost always present in rocks. Barium and strontium come very much lower in the list of abundance of elements, but nevertheless barium is extremely widespread and occurs in most rocks, igneous, metamorphic, and sedimentary, while strontium is also very widespread. It has been stated that the average content in igneous rocks of both of these elements is of the order of 0·05%.

Recently Dr. M. H. Hey published in this journal a method to construct graphically a projection showing the relation of two etched surfaces M and N to the crystallographic axes of a mass of the Gibeon meteoritic shower. Upon those faces could be distinguished the traces of four octahedron planes 0l, 02, 03, 04 which are inclined at the following angles, measured clockwise from the intersection [MN] of the etched surfaces: on surface M 22°, 62°, 98°, and 135° and for the same octahedron planes on surface N 106°, 38°, 150°, and 86°, respectively.

The two faces M and N are at an angle of 84° one to the other. Hey's solution of the problem is based on a reversal of the Goldschmidt construction for drawing crystals from the gnomonic projection.