A group of ultrabasic dykes occurring in south-west Skye is typified by often abundant and diverse ultrabasic xenoliths composed of essentially the same minerals as the dykes. The xenoliths, which vary considerably in shape and size, are in sharp contact with the host rocks and exhibit no evidence of remelting by, or reaction with, the dykes. Although extensive axial concentration of olivine occurred in the dykes during intrusion, the xenoliths appear to be randomly distributed throughout all but the extreme margins of the dykes and often exhibit a preferred orientation. The orientation and distribution of the xenoliths are attributed to the suppression of rotation and axial migration of the xenoliths during the emplacement of the dykes because of the relatively high viscosity of the suspending medium.

The ultrabasic xenoliths, unlike many of those occurring in basalts, kimberlites, etc., are considered unlikely to represent primary upper mantle material and it is suggested that they were probably derived by the disintegration of layered ultrabasic rocks genetically related to the dykes and hence are of cognate origin.

Many examples of epitaxic relations between coexisting pyroxenes are found in the picritic dykes that cut the Lewisian gneisses of the Lochinver area, Sutherland. In some dykes augite forms oriented overgrowths on the (100) face of orthopyroxene; in others orthopyroxene forms similar overgrowths on augite. A number of unusual variants of the first case are described together with a complex ‘induced’ twinning found in the augite overgrowths. Analyses of the coexisting pyroxenes suggest that the overgrowth relation has been facilitated by increased solid solution between the pyroxenes, so reducing the crystallographic misfit along the c-axis. However, crystallization of the pyroxenes in an ordered sequence rather than simultaneously also seems to have been an important factor.

Mosaic olivine textures, with straight triple grain boundaries meeting at angles of 120°, are common in dunites. This is a well-known feature of annealed metals, and is the result of grain-boundary migration during recrystallization in which the system tends toward a state of minimum interfacial energies. Geometrically similar textures are present at the boundaries between such mosaic grains and strain-banded relict olivine grains. The unstrained mosaic grains make angular projections into the strained grains exactly at the junctions between the deformation bands. These are interpreted to be triple junctions with the two differently oriented bands acting as separate grains, and thus also due to recrystallization.

Petrographic descriptions, bulk chemistry, and partial analyses of several mineral phases in each stone are presented. Medanitos, previously classed as a eucrite, is now shown to be a howardite: Putinga is an equilibrated olivine-hypersthene chondrite.

Chemical, optical, and X-ray data are presented for forsteritic olivine, bronzite-enstatite, diopsidic augite, anorthite, and spinel from ultramafic nodules at seven localities in Auckland province. Olivine has a composition range from Fo79–93 for large primary crystals to Fo80–100 for smaller interstitial grains. Orthopyroxenes (En83–96) carry exsolution lamellae of clinopyroxene parallel to (100); clinopyroxenes are mainly diopsidic augites with exsolved bronzite. The plagioclase (An97) has high-temperature characteristics and a lattice structure probably transitional between primitive and body-centred anorthite. Spinels include chromite, picotite, ceylonite, hercynite, and magnetite. Element distribution indicates that the ultramafic assemblages attained equilibrium at temperatures near 1250 °C and at moderate pressures, corresponding to the melting interval of anhydrous basaltic magmas within the uppermost mantle.

Well-resolved Mössbauer spectra of several dioctahedral and trioctahedral micas have been obtained and subjected to detailed computer analysis. Most of the spectra can be resolved into two ferrous quadrupole doublets and one ferric quadrupole doublet. In dioctahedral micas, ferrous iron is seen to occupy the larger, more symmetric octahedral site in preference to the smaller, less symmetric site, confirming predictions made on the basis of structure alone. In trioctahedral micas there is still a tendency for ferrous iron to occupy preferentially the more symmetric octahedral site even though the two octahedral sites are no longer distinguished by size. A lithium-rich biotite gives a spectrum typical of that expected from a mica with a zinnwaldite structure, but the spectrum of a more authenticated zinnwaldite could not be resolved sufficiently for detailed structural interpretation. The spectra of pyrophyllite and talc give results that can be related to the corresponding micas by considering their respective structural differences. In none of the micas studied was there evidence of ferric iron in tetrahedral coordination. The ferrous: ferric ratios obtained from the Mössbauer spectra do not always agree with the chemical values. In several cases the spectra show more ferrous iron than found chemically. It is suggested that the spectral values are more accurate, oxidation on chemical analysis being the most likely source of error.

The biotites in the pegmatites are Fe2+ type. X-ray analyses indicated that they are 1M or 3T polymorphs. In the colour of the biotite iron plays an important role. From the Fe/(Fe+Mg) ratios, the analysed biotites are assumed to have crystallized between 760 and 500 °C.

Specimens of tavistockite fall into two groups: true tavistockite from the George and Charlotte mine, Tavistock, Devon, and wavellite from the Stenna Gwyn mine, St. Austell, Cornwall. Both were sold as tavistockite by the discoverer, Richard Talling.

Tavistockite proper is a fluorapatite, as shown by optical and X-ray examination, and the alumina and water in the original analysis are certainly derived from kaolinite with which the apatite is intimately associated.

The published optical properties attributed to tavistockite were determined by E. S. Larsen on Stenna Gwyn material, and are those of wavellite.

Re-examination of a portion of Buttgenbach's type bialite, which he related to tavistockite on optical grounds, shows it to be wavellite.

Grandidierite has been found in hornfelses around a diorite stock on Cuvier Island. Physical properties and electron microprobe analyses are given for two Cuvier Island and two Madagascan grandidierites. The Cuvier Island grandidierite has crystallized under conditions of the K-feldspar-cordierite hornfels facies at temperatures of approximately 600°C. and pressures (Pload) of less than 300 bars.

A new variety of garnet, a chromian hydrogrossular, has been found in a loose block of diopside-albite rock in a quarry of serpentinite in Jordanów, Lower Silesia. Its chemical formula is given. Although the water content of this garnet barely exceeds 1 %, the X-ray, thermal, infra-red spectrophotometric, and optical examinations give evidence that this water, in the form of (OH)4 groupings, substitutes for the SiO4 tetrahedra. The chromian hydrogrossular from Jordanów belongs to the numerous calcium-rich silicates that were formed there during the process of alteration of serpentinites by lime-rich hydrothermal solutions.

The constituent manganese minerals of an ore body located at the Lecht mines near Tomintoul include cryptomelane and lithiophorite. The latter, now known to occur extensively in soils, was characterized by X-ray, thermal, and electron microscope techniques. Lithiophorite yields a distinctive differential thermal curve and this was studied in some detail. A platy morphology is revealed under the electron microscope and from selected-area electron diffraction patterns it was concluded that the b parameter of the mineral is 8·73 Å, three times the value determined by Wadsley (1952). As suggested by Wadsley, this could arise from the distribution of lithium ions. Chemical analysis shows that the Lecht lithiophorite is slightly unusual in that it has a high iron and zinc content but is relatively low in cobalt. The associated cryptomelane contains a significant amount of thallium.

The thermal decomposition of alunite and of mixtures of alunite with dickite or quartz or both has been studied, and X-ray determinations of the products at various stages of the decomposition have been made. Between 600 and 700°C the product from alunite is a mixture of anhydrous alum and amorphous alumina; between 700 and 770°C the anhydrous alum breaks down into aluminium sulphate and alkali sulphate; then in the prominent 850°C endothermic reaction three-quarters of the sulphate is lost, leaving a mixture of alkali sulphate and γ-alumina. The area of the 850 °C endothermic peak may be used as a measure of the percentage of alunite in alunitized rocks (alunite-quartz-dickite mixtures).