Psilomelane has been redefined from single crystal study to be of orthorhombic symmetry with the space group P222, the dimensions a 9·45, b 13·90, c 5·72 Å, and two molecules of per unit cell, where A represents Ba2+, Mn2+, Al3+, Fe3+, Si4+ etc., and X6 stands for (O,OH)6 with OH about 5. This formula is very closely followed by 14 analyses of psilomelane, and the calculated density agrees fairly well with the observed density. The powder data of psilomelane cannot be indexed with Wadsley's monoclinic cell and the conditions for the space group A2/m, proposed by Wadsley for his crystal, are not satisfied by the indexing. Wadsley's crystal was not psilomelane but probably an altered phase of Vaux's neotype specimen.

The electron micrograph of shining platy crystals of psilomelane shows some almost square-faced crystals whereas that of the botryoidal material consists only of particles with irregular edges. The dehydration product of psilomelane shows gradual shrinkage of the orthorhombic cell up to 500° C, marked structural changes at 600° C, and transformation at 800° C to a monoclinic phase, similar to hollandite.

The existence and relationships of hydrated forms both to one another and to anhydrous CaSO4 have been studied by X-ray methods. Detailed study of the hemi-hydrate reveals its true cell to be monoclinic and does not confirm the previously suggested polymorphism.

Dehydration studies of the dihydrate and hemi-hydrate are reported; cyclic dehydration and hydration of single crystals of the latter show a variable temperature interval between the formation of γ-CaSO4 and the reconstitution of the hemihydrate at approximately constant humidity. An explanation of this behaviour is offered, and the structural implications of the results of the dehydration studies are discussed.

Rhombohedral (3R) molybdenite has hitherto been reported from only one locality. Its mode of occurrence at a new locality is reported in this paper. At Panasqueira it occurs as a rare accessory in greisenized granite with which wolframite-cassiterite-quartz veins are associated. In the veins themselves only normal hexagonal (2H) molybdenite has been detected.

Rocks containing a high percentage of potash feldspar are described for the first time from the Toror Hills in the Karamoja district of eastern Uganda. They include feldspathic fenites, intrusive feldspathic fenite-breccias, potashtrachytes, and orthoclasites. Petrographic data and chemical analyses show the close similarity in composition of these rocks despite their widely differing textures and modes of occurrence. The potash metasomatism associated with the emplacement of carbonatite, and the subsequent mobilization of these feldspathic fenites are compared with similar phenomena observed at other carbonatite centres. The problem of the mechanism of mobilization is discussed.

A test of the linearity of a photomultiplier has been made using crossed polars as an optical attenuator. An expression is derived that allows for the measurable transmission of a polarizing filter in directions other than the permitted vibration direction. The linearity is shown to be adequate for accurate reflectivity measurement.

Two nickel arsenates from South Terras mine, St. Stephen-in-Brannell, Cornwall, have been shown to be identical with the minerals xanthiosite and aerugite, described by C. Bergemann in 1858, and with two synthetic products described by J. B. Taylor and R. D. Heyding (1958). Xanthiosite is pure golden-yellow, monoclinic, with a 10·174, b 9·548, c 5·766 Å, β 92° 58½′, space group P21/a; with D154=5·42±0·10, the unit cell contains 4[Ni3(AsO4)2] (Dcalc 5·388±0·003). Aerugite is deep blue-green, monoclinic, with a 10·29, b 5·95, c 9·79 Å, β 110° 19′, space-group C2, Cm, or C2/m; the ideal unit cell formula is 2[Ni9As3O16], with one-sixth of the arsenic trivalent; the natural mineral has a lower Ni:As ratio, around 2·6. Both xanthiosite and aerugite are minerals new to Britain.

The Equivibration Curve technique of Joel (1963) is used to determine the orientation of a zoned mineral on the spindle stage axis and an average value for 2V. A bracketing method enables the extreme values of each refractive index to be determined for sodium light on a single grain, using the Focal-plane Screening technique of Cherkosov (1960).

An examination has been made of the composition and structural state of the feldspars in each of the units of the Rosses granite complex. The compositions of the plagioclases are systematically related to the compositions of the rocks, but the alkali feldspars are much more uniform as a result of postmagmatic recrystallization. In the granites both alkali feldspar and plagioclase are in low-temperature structural states, but alkali feldspar phenocrysts from one of the porphyry dykes associated with the complex show a higher-temperature state.

Four orthopyroxene: calcic-pyroxene pairs from pyroxene-hornblende- and pyroxene-granulites from the Precambrians around Saltora, eastern India, have been chemically analysed. The distribution coefficients with respect to Mg-Fe (KD) are found to be 0·56, 0·57, 0·60, and 0·61, a set of values well within the limits characteristic of metamorphic parentage.

Stannian andradite occurs with pyroxene and carbonates, with blende and pyrite as accessories in a skarn deposit in the Plavno mine, Krušné Hory Mts., NW. Bohemia. The green crystals are rhombic dodecahedra up to 3 cm in size. Sp. gr. 3·78; n 1·879; a 12·079±0·002 Å. Quantitative chemical and spectrographic analyses are given; the composition is (Ca2·96Mg0·04Mn0·02)3·02 (Si2·95Sn0·04)2·99O12. The content of tin (SnO2 1·07 %) is isomorphously bound in the lattice, and according to crystallochemical calculations the entrance of tin into the garnet lattice is most probably due to substitution of The presence of tin in the andradite shows that this element was already present in the metasomatic solutions during the process of skarnization.

Two unusual cavities at a Northumberland colliery are described. Although apparently of tectonic origin they do not seem to be related to any major fault. Sideronatrite, not previously recorded in this country, is thought to result from the alteration of the marcasite and pyrite lining the cavities by underground saline waters. These waters are the source of the baryte and gypsum that are also found in the cavities and of the salt stalactites in the disused roadways of the colliery.

Pyrite concretions from an arid area of Southern Israel are described. The alteration products include alunite, gypsum, jarosite, baryte, celestine, goethite, hematite, sulphur, and sideronatrite. Sideronatrite is described in detail. The pyrite alteration is considered to be recent and the sideronatrite is preserved owing to climatic aridity. The puzzling appearance of sulphur is tentatively explained by microbiological reduction of the sulphates.

A comparison has been made of the relative speed, accuracy, and precision of several methods of measuring large optic axial angles with the universal stage. It is concluded that a method based on the direct location of a single optic axis and the application of the Biot-Fresnel law will frequently be the most satisfactory when only the standard, low refractive index centre plate for the stage is available. If a centre plate of high refractive index is employed, however, good results can normally be obtained by using the more rapid method based on the direct location of both optic axes.

Garnets from twenty-eight pegmatites, schists, and gneisses from the crystalline massif of Eilat have been examined. Refractive index, unit-cell edge, MnO and FeO content were determined in 19 samples. Three chemical analyses are given. The garnets fall into two distinct groups, the pegmatite garnets (spessartine-rich, almandine to spessartine) and metamorphic garnets (nearly pure almandine). Garnets occurring in pegmatite-like granitoid veins cutting through the schists are shown to belong to the second (metamorphic) group. The pegmatite garnets again fall into two groups of different spessartine content.