X-ray and thermal studies in the system MnO-O-H2O show that the transformations groutite(α-MnOOH)-ramsdellite(MnO2)-pyrolusite(MnO2) are topotactic, with the a, b, and c directions of all three corresponding. The reactions proceed by migration of manganese atoms and, where appropriate, protons. Pure single crystals of ramsdellite could not be prepared by heating groutite, even though the two structures are very similar.

In the Donegal Granite garnet occurs as an accessory mineral in muscovite-granties, pegmatites, and aplites. The composition of the garnet in fourteen samples of granitic rock has been determined from the physical properties and three new chemical analyses are given. All the samples examined are intermediate in composition between almandine and spessartine. The reason for the high manganese content in garnet from granites is discussed.

In this paper we give data for An and Sr in plagioclase to demonstrate the presence of numerous amphibolite xenoliths within parts of a basic igneous mass where, after initial reconnaissance mapping, none was suspected. The data also show that the concentration of Sr in plagioclase from the margin of some amphibolite xenoliths is appreciably higher than in plagioclase from either the cores of the xenoliths or the enclosing metagabbro even though the modes and An contents of the plagioclase are similar. An explanation is offered, which implies that some of the xenoliths were amphibolites rather than anhydrous equivalents prior to their enclosure in the basic magma.

A method is presented for use with a 3- or 4-axis stage whereby a biaxial mineral, after being oriented into the cardinal position by means of the inner vertical, NS., and EW. axes, can be placed into an orientation suitable for measurement of the 2 V as required by the Berek method. A table of the necessary manipulations is given. The stereographic projection is only used to determine rotational corrections.

Monazite and sphene from the central zone of the Damara orogen of South-West Africa, have yielded ages of 500 ± 20 Myr and 600 ± 25 Myr respectively; these results are discussed in relation to the late Precambrian to early Palaeozoic tectono-thermal history of the region. Although chemically similar to that found in carbonatites, the monazite is probably genetically related to the widespread granitic pegmatites that characterize the central zone of the orogen.

The theory of a new method for the determination of reflectivity at truly normal incidence is described. A parallel light beam falls on the specimen after passing through a glass cube with semi-silvered diagonal mounted at the centre of an optical goniometer. The various reflected beams are measured by a photomultiplier fixed to the telescope of the goniometer. Experiment has proved the validity of the method and the spectral reflectivity of pyrite in air and in oil has been investigated.

The four principles governing the cation distribution in various octahedral metal sites in the amphibole structure are: Ionic size—the M4 position prefers large cations like Ca2+, Na+; charge balance—when the M4 position is occupied by a monovalent ion, like Na+, Li+, the neighbouring M2 position is occupied by a trivalent ion like Al3+, Fe3+; role of the strongly polarizing cations—cations like Fe2+ prefer the M4 site, since they tend to form covalent bonds with the O4 oxygens; and the role of the (OH) ions—these form part of the brucite layer and repel large or strongly polarizing cations, which tend to distort the regularity of the sheet structure.

The Al-Si distribution in the two different tetrahedral sites are governed by three principles: relief of the Si-O-Si bond strain—Al3+ in the Si1 site causes fewer Si4+ cations to share more than two anions with other Si4+ cations; charge balance—with the introduction of alkali ions in the ‘A’ position, Al3+ is introduced in the Si1 position, since the coordinating oxygens around the A position mostly belong to Si1; and steric considerations—occupation of the M4 site by a large cation tends to rotate the Si2 tetrahedron anticlockwise; Al3+ substitution in the Si1 site rotates the Si1 tetrahedron clockwise and thus the regularity and the constancy of the repeat distance of the silicate double chain is maintained.

Based on these principles, a scheme of cation distribution in the M1, M2, M3, M4, A, Si1, and Si2 positions in the amphibole structure has been derived for most of the common amphibole varieties, except richterite, whose position is anomalous from a crystallochemical point of view.

The transformation of ankerite during heat treatment has been studied by single-crystal and powder X-ray methods. The mechanisms of formation of calcite at 750° C and of periclase at 950° C are discussed from a structural viewpoint.

Reflectivities of a series of oriented polished sections through a single crystal of pyrrhotine (Fe1−αS) from Trepča, Yugoslavia, were measured by means of a spectral photometer attached to a microscope. This pyrrhotine consists of two phases, namely intermediate pyrrhotine (∼ Fe9S10) and monoclinic pyrrhotine (∼ Fe7S8), which were probably formed by exsolution. The phase relations of pyrrhotine at low temperatures are discussed. Optical data are given for the intermediate pyrrhotine from 440 to 660 mµ. From the reflectivities measured in air and oil against a carborundum standard, values of n, k, and κ were calculated for ω and ε and represented in dispersion curves. Also, sections through the various ‘indicating surfaces’ at 546 mµ are shown. The accuracy of the values, which should be approximately the same for other pyrrhotine phases, is discussed.

The existence and relationships of the polymorphs of CaSO4 have been studied by X-ray methods. Only two distinct phases are found: no crystallographic evidence is available to support the suggestion that the γ-CaSO4 structure can have two forms, and it is shown that most previous data on the existence of the high temperature α-CaSO4 polymorphs are likely to be due to decomposition of the sulphate.

Some observations on the γ to β-CaSO4 transformation are reported. The structural implications of these results and their relevance to the nature of the transformation are discussed.

Tables are given which enable a quick estimate to be made of the true thickness of kamacite plates from measurements of the Widmanstetter figure on a single random plane. The method is applied to the following meteorites (width in mm following name): Arispe, 2·90; Brenham, 0·91; Duketon, 1·13; Henbury, 0·74; Ogallala, 1·53; Thule, 0·89; View Hill, 0·62.

Graphite occurs in the Darkainle nepheline syenite complex, Somali Republic, as small drop-like grains in an altered ankeritic carbonatite; it has been recorded previously from carbonate-bearing nepheline syenites and kimberlites. Experimental evidence indicates that siderite decomposes to a stable association of magnetite and graphite within the PT ranges 500 to 10 000 bars and 455 to 745° C. The graphite in carbonatite and other alkaline igneous rocks may have been formed similarly by the decomposition of siderite or iron-rich carbonates.

An investigation of the system Cu-Fe-O has established the existence of a phase near 3Cu2O.Fe3O4 and the non-existence of Cu2O.Fe2O3 a in the system over the range of oxygen pressures investigated.

The X-ray diffraction patterns obtained from four samples of natural delafossite have been found to be similar to that obtained from a synthetic sample of the 3Cu2O.Fe3O4 phase. It has also been shown that, when heated under equilibrium conditions in the thermobalance in air, a sample of natural delafossite and the synthetic 3Cu2O.Fe3O4 behaved similarly.

The isotopic ages of three biotites from the Kondapalli area, Andhra Pradesh, India, have been determined by the 40Ar/40K method. The results (440–524 Myr) indicate that the basic charnockite and khondalite of pre-Cambrian age have been subjected to at least one subsequent metamorphism the age of which must be 440 Myr or younger. The significance of the results is discussed in the light of the available geochronological evidence from Peninsular India.

In order to study the orientational relationship between Ca(OH)2 and CaO, the transformation was carried out in stages inside an electron microscope by condensing the electron beam, and the process was monitored by electron diffraction. It has been found that at the initial stage all the hexagonal spots of Ca(OH)2 split into two ; on further heating one set of spots disappear. The remaining set of spots corresponds to the CaO structure, which has an orientational relationship to the hydroxide structure. To estimate the degree of orientation, a sample of brucite was treated in a similar way to the Ca(OH)2; a comparison shows that a high degree of orientation is preserved in the conversion of Ca(OH)2 to CaO. This observation is contrary to earlier reports.

Flood basalts of the Isle of Mull were sampled in areas outside the metamorphic aureole of the central intrusions. Alteration products of olivine and pyroxene in these rocks are talc, chlorite, serpentine, iron oxides, brucite, calcite, and ‘iddingsite’. Plagioclase feldspar in the same rocks is unaltered. ‘Iddingsite’ is less common than mixtures of chlorite and serpentine.

In two flows the only alteration product near the bottom is ‘iddingsite’ but at higher levels in the same flow green ehloritic material replaces olivine. This variation is accompanied by an increase in the FeO/Fe2O3 ratio in the rock towards the flow top. Green alteration products may be converted to material optically identicaI with ‘iddingsite’ by heating in air at 600° C. Zoned patches of chloritic material have formed by crystallization of the late stage magmatic fluid, modified in composition by reactions with olivine and pyroxene.

Details are given for the construction of a continuous, density separator from standard laboratory glassware and equipment. The separator can treat up to 5 g/min of granular material from 150 to 2000 µ in grain size, and the liquid used is continually recirculated by a simple airlift.

The subsolidus phase equilibrium of the lithiophilite-tephroite system was studied in the temperature range 500 to 900° C. The solubility of tephroite in lithiophilite is limited and increases with temperature. The reaction is very sluggish and has peculiar kinetic properties. No reverse solubility was observed. Similar asymmetry was found in the LiMgPO4-forsterite system of which a preliminary study was made. Apparent absences of silicate phosphate olivine solid solutions in nature are related to our experimental findings and to geological factors.

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.