In a recent paper Taylor has shown that three distinct hydration levels exist within the ‘calcium silicate hydrate (l)’ group of artificial preparations (Taylor, 1953a, p. 168). The individual hydrates may be distinguished from the position of the 002 reflection in X-ray powder photographs. The d spacings corresponding to the three hydrates of this group examined during the present investigation are 14.6, 11.3, and 9.6 Å. These hydrates have H2O : SiO2 molar ratios of approximately 2.0, 1.0, and 0.5 respectively.

A recent examination of the minerals crestmoreite and riversideite (Taylor, 1953b) has shown that both these minerals are intimate inter-growths of members of the above hydrate series with the mineral wilkeite. Both the artificial preparations and the crestmoreite-riversideite intergrowths have failed to provide material suitable for detailed optical study.

Selfridge (1936) examined by chemical, optical, and X-ray powder methods a large number of specimens of serpentine minerals (ideal formula H4Mg3Si2O9), many of which were known as antigorite. He found that nearly all gave similar powder photographs, indicating a close structural relationship, but that there were noticeable differences on the basis of which the specimens could be separated into two groups, one giving a photograph similar to that of chrysotile asbestos, and the other containing the varieties called antigorite. Gruner (1937) came to similar conclusions, and in addition he indexed most of the reflections using the cell dimensions a 5.33, b 18.5, c 2 x 7.33 Å., β 93°, remarking in particular on the correspondence between 001 reflections in the two groups.

Mr. A. Steiner has kindly invited the present writer to study by X-ray methods the new mineral wairakite described by him in the preceding pages. The material provided includes an incomplete octahedron 15 ram. across with small modifying ieositetrahedron-like faces, and some smaller partial icositetrahedra. Faces are dull and give diffuse reflections oil the optical goniometer, but, by attaching small pieces of glass cover-slip to improve the reflections, interfacial angles approximating to those of the cubic forms {111} and {211} were measured. As pointed out by Mr. Steiner, however, the mineral is birefring'ent, biaxial, and finely twinned. A preliminary small-diameter X-ray powder photograph taken by Mr. N. Wells of the Soil Bureau, Wellington, showed an apparently cubic, analeime-like pattern (personal communication).

The association of orthoclase and microcline in the same rock has been commented on by many writers, and from these occurrences attempts have been made to deduce the relative stability of these two low-temperature forms of potash-felspar. Ahnost all investigators have concluded that nlicrocline is the lower-temperature modification, but difficulties in explaining the relationship of adutaria to orthoclase and microcline have been encountered.

The identification of the monoclinie potash-felspar in association with microcline is almost always based on optical properties, viz. straight extinction in the zone [010] or the absence of visible multiple twinning.

The four amphiboles described in this paper were collected during the summer of 1950 from the Sitasaongi mine. This mine, which produces high-grade manganese ore, is situated in the Chikla area, Bhandara district, India. The ore band is 8 to 18 feet thick and is strongly folded and faulted at many places (fig. 1). The most common rock types in the area are: calc-granulites, calc-gneisses, mica-schists, quartzites, gondite (quartz-spessartine-rock), injection gneisses, and pegmatites. The pegmatites are the youngest and cut through the manganese-ore band as well as the country-rocks. All are of Archaean age.

There appear to be few published data correlating X-ray powder patterns of chemically analysed plagioclase felspars with composition. Tuttle and Bowen (1950) have examined the powder patterns of low-temperature and high-temperature modifications and find that pure albite from Amelia, Virginia, when heated for ten days at 1050° C., gives a powder pattern identical with that of synthetic albite. They have not given, however, complete data over the whole range from An0Ab100 to An100Ab0. Claisse (1950) selected six reflections from the powder patterns of eight plagioclases of composition extending over the complete range, and plotted line separations against composition determined optically, but did not distinguish between high- or low-temperature forms.

In the course of the investigation of the nepheline-kalsilite system by Tuttle and Smith an X-ray method for determining the KAlSiO4 content of synthetic nephelines of composition (Na,K)AlSiO4 has been developed. This method is rapid and sensitive to about 1 % in composition ; details will be given in a later publication.

Natural nephelines usually do not have the ideal composition (K,Na)AlSiO4, for they often contain excess silica and certain substituted atoms like Ca and Fe. We have, however, used this X-ray method on natural nephelines to see whether it is also applicable to them. If the method proved satisfactory, information on the chemical composition of natural nephelines could be rapidly obtained.

Qualitative studies of the optical properties of crystals under the polarizing microscope and refractive index measurements by immersion methods are made on slides placed upon the microscope stage. These procedures have the disadvantage that the crystals have a fixed orientation. Quantitative properties, such as optic axial angles, are measured on an accessory to the microscope, such as the universal stage, which although allowing rotation of the crystal necessitates auxiliary stereographic projection and is often considered laborious in operation and is not widely employed.

In a recent paper, Cole, Sörum, and Taylor described X-ray measurements on a number of plagioelases with compositions ranging from pure albite to pure anorthite. As a result, they were able to make more precise, and extend, the structural interpretations proposed on the basis of earlier X-ray investigations of the three typestructures, albite, anorthite, and intermediate plagioclases (Taylor, Darbyshire, and Strunz; Chao and Taylor). At the same time, new complexities were revealed which called for more detailed examination, among them the fact that the anorthite-type structure, with c-axis ∼14 Å., may have either a body-centred or a primitive unit cell. The body-centred cell was observed with a material containing 71.5% anorthite, the primitive cell with a nearly pure anorthite, but there was not sufficient evidence to show whether the difference was due entirely or largely to the difference in chemical composition, nor was a detailed investigation made of the effects of heat-treatments.

During an investigation of stibiconite and cervantite (Vitaliano and Mason, 1952) we examined many specimens of antimony ochre. The commonest mineral in these specimens proved to be stibiconite, but others were found, and to establish their identity it became necessary to study the secondary products as a group. The mineralogy of these oxygenated secondary antimony minerals presents unusual problems on account of their earthy and often inhomogeneous nature. Our experience indicates that without X-ray powder photographs it is difficult to establish the true identity of much of this material. The purpose of this paper is to present the results of our investigations and to provide more adequate descriptions of some little-known species.

This investigation was concerned with two aspects of the colour-zoning in tourmaline: firstly, the origin of the green and pink colours ; secondly, the reason for the apparent incompatibility of these two forms. Much of the previous work on the colour of tourmaline has involved the use of emission spectra. This method provides information as to which elements are present; it does not, of course, indicate which of the twenty or so elements identified is responsible for the colour, although it may be possible to infer this. Absorption spectra, on the other hand, are directly concerned with colour. They have been employed quite extensively in Germany, and more recently in Russia, but the earlier data and instruments were often inadequate. The last decade has seen marked improvements in the technique of absorption spectroscopy, particularly in the development of photo-electric methods for the quantitative recording of spectra, and the method is likely to prove one of increasing value.

For several years now it. has been generally appreciated that plagioclase felspar formed at high temperature differs optically from that formed at lower temperatures. Much of the evidence for this has been provided by the Austrian workers, notably Köhler (1-3), Scholler (4), and Tertsch (5-7), who have prepared revised curves to supplement the universal-stage stereograms of Reichert (which are for use with lowtemperature material). There are a number of interesting petrological points which have not yet been fully investigated. Much work has been done on synthetic material, and it is not clear to what extent the hightemperature forms occur naturally. There may be two distinct and separate types of plagioclase structure, or the two forms may grade into one another. It may be possible to explain the optical anomalies as an order-disorder phenomenon; a more disordered state has been 'frozenin' in those felspars which ha~e formed a.t a high temperature and cooled rapidly.

The bluish-green epidote-like mineral, pumpellyite, was first described by Palache and Vassar (1925) from the copper-bearing amygdaloidal lavas of the Lake Superior region. It has since been reported from many parts of the world in rocks which have undergone low grades of metamorphism, especially types such as amygdaloidal spilites and diabases (Burbank, 1927; Waldmann, 1934; Tsuboi, 1936; Quitzow, 1935, 1936; Klein, 1939; Laeroix, 1942; Seether, 1942; Zavaritsky, 1944:; Korzhinsky, ]944; de Roever, 1947a, b; Kvasha, 1950) and in glaueophane-schists and even eclogites in which pumpellyite is often associated with lawsonite (Irving, Vonsen, and Gonyer, 1932; Quitzow, 1935, 1936 ; Lacroix, 1942 ; Switzer, 1945 ; de Roever, 1947a, b, 1950). W. P. de Roever (1947a, 1950) has suggested a pumpellyitic facies of metamorphism as a lower-grade equivalent of his lawsoniteglaucophane subfacies, characterized by high confining pressure, but subordinate shearing stress and thermal influence, and passing into unmetamorphosed rocks.

Towards the end of 1952 mining operations at West Sleekburn colliery, Bedlington, Northumberland, involved the driving of a drift through a 'whin' dike. In the process of doing so an unusual, large cavity was penetrated. When the cavity was first entered, by the manager and under-manager, it was found to be water-free but full of 'gas', which rendered them almost unconscious. Their exploration was therefore delayed until the gas was removed by pumping. Unfortunately no sample of the gas was collected, but it is thought to have been 'fire damp'. After examination the cavity repeatedly filled with gas, leading to a suspicion of a hidden extension to the cavity, under the roadway.

Wadeite has been found only in a suite of leucite-lamproites from the West Kimberley area of Western Australia. The leucite-bearing rocks occur at a number of isolated volcanic vents that have in every instance broken through sedimentary deposits of Permian age. The nature of the mineral, wadeite, has been described by Prider. It is a potassium-zirconium silicate occurring as highly birefringent prisms up to 1 mm. in length with hexagonal-shaped basal sections. It is invariably replaced in part along the poor pyramidal cleavages by calcite.

Physico-Chemical data clearly show that the presence of tridymite in a rock indicates the prevalence not only of high temperatures, but also of relatively low pressures during the formation of that rock. Whereas many authors (e.g. Shand, 1927; Larsen, 1936) have used tridymite as an indicator of temperature, the importance of tridymite as an indicator of pressure has been largely overlooked.

Although the presence of tridymite is by itself sufficient to enable the pressure, and hence the depth, at which a rock formed to be estimated within certain limits, the presence of tridymite in association with some other pressure-or temperature-indicating mineral allows much closer limits to be set.

The mineral truscottite was discovered at the Lebong Donok mine, Benkulen, Sumatra, by Hövig in 1914. He considered it to be a hydrated calcium silicate of composition CaO.2SiO2.1/2H2O. Later work by Grutterink suggested a different formula, 2(Ca,Mg)O.3SiO2.2H2O, the water content being reduced to 1-3 molecules after drying at 120°. Grutterink noted that the composition, general character, and optical properties indicated a similarity to gyrolite. Flint, McMurdie, and Wells concluded from X-ray powder photographs that the two species were identical, although they detected quartz as an impurity in the truscottite examined. All known specimens of truscottite come from the original locality, and no further investigations have been reported.

A Dark brown (blackish-brown) mineral forms a thin crust on manganese ore on the specimen [B.M. 1944,35] from no. 5 orebody of the Benallt mine, Rhiw, Carnarvonshire, North Wales. This is the specimen on which the first measurable crystals of banalsite were discovered in 1944 (Min. Mag., 1945, vol. 27, pp. 63-64). The mineral occurs as closely-packed, small rosettes, at most 0"5 ram. across, and is partly coated with a thin layer of baryte. An X-ray photograph by Dr. Bannister suggested that the mineral might belong to the chlorite group. Its optical properties were determined and nothing further was done with it at the time.

A limited amount of careful optical work has been carried out on X three analysed felspars of gabbros belonging to the main layered series of the Skaergaard intrusion, east Greenland (Wager and Deer, 1939). The work was begun with the intention of providing precise optical data to be used in further defining the relationship between composition and optics. The primary precipitate felspar crystals, hitherto thought to be devoid of zoning, were found, however, to possess a zoning which prevents the data being used in this way.