Chemical analyses of the Lorne volcanic rocks show that the basalts and andesites are a closely related suite of lavas, notably rich in alkalis, especially potassium. The rhyolites of the Lorne area do not appear to form a continuous series with the more basic rocks. The various lava types are compared with the plutonic rocks of the area, and a relationship is suggested between the basic lavas and appinitic intrusions. The acid lavas are not comparable to granites in the area.

The late orogenic igneous activity in this part of the Caledonides therefore appears to involve three generations of magma, produced separately but within a short time of one another. The assumption that all the late Caledonian igneous rocks of the Scottish Highlands are differentiates of a common parent magma is not justified.

The liquidus temperature (1198 °C) and equilibrium phase relations of a sample of Columbia River basalt from the Picture Gorge section have been determined at I atmosphere by heating in a controlled atmosphere. When this basalt is cooled from above its liquidus temperature the liquidus phase (plagioclase) may fail to crystallize depending on the degree of undercooling and the duration of the experiment. A field in temperature-time space in which plagioclase fails to crystallize on cooling is separated from another in which plagioclase always crystallizes by a third in which the nucleation of plagioclase is unpredictable in terms of temperature and time. The extent to which this basaltic liquid can be supercooled without the crystallization of plagioclase is independent of the time it is held above the liquidus or the temperature in excess of the liquidus to which it is heated.

The exceptionally long times required to ensure the nucleation of plagioclase at or near the liquidus temperature suggest that many so-called ‘equilibrium’ phase relations determined from experiments of a few hours' duration could be in serious error if the ‘equilibration’ involves a nucleation process.

It is demonstrated that, over a range of cooling rates, the temperature at which plagioclase begins to crystallize on cooling varies markedly and the temperature and times required for both possible and certain nucleation of plagioclase are calculated for a range of constant cooling rates. The range of cooling rates over which the nucleation temperature of plagioclase varies is likely to occur in nature only in certain lava flows and small minor intrusions. In such cases this could lead to changes in the order in which the minerals appear on cooling and other petrologically significant effects.

Grain boundary migration and the exsolution of albite from alkali feldspar, followed by the precipitation of myrmekite from the same source on to adjacent plagioclase grains, will explain the reversed zoning of these phases in contrast to the normal zoning seen in igneous rocks.

Zeolites (eight species) and the associated secondary minerals (minerals akin to zeolites, chlorites and related minerals, silica, and calcite) in the Deccan Traps of Western India have been examined in some detail on the basis of chemical, optical, and X-ray studies. The three zeolite zones (laumontite, scolecite, and heulandite, in ascending order) suggested by Walker have been recognized. Efforts have been made to understand their genesis. On available field and laboratory data it is suggested that the different zones of zeolitization are the result of increasing depth and the action of the circulating fluids on the rocks. The activity of such solutions was probably enhanced in the wake of the structural disturbances and intrusive activity that affected the rocks of this area.

Staurolite crystals from the aureole of the Ardara pluton have been analysed using the electron microprobe. Compositional variations of TiO2, MnO, and MgO take the form of irregular zoning, differences across sector twin boundaries, and variable marginal compositions, and can be attributed mostly to disequilibrium and localized diffusion.

Several uncommon phosphate minerals including tarbuttite, parahopeite, scholzite, and collinsite occur in near-surface gossans at the Reaphook Hill zinc prospect. Electron-probe microanalyses of the tarbuttite and scholzite compare well with published analyses of specimens of these minerals from the type localities in Zambia and Bavaria respectively. However, microanalyses of the parahopeite are significantly different from published analyses of parahopeite, and suggest that it may be one member of an isomorphous series in which six-coordinated Zn can be replaced by other cations, including Fe, Mg, and Mn. Furthermore, crystals of the Reaphook Hill parahopeite are compositionally zoned with their centres enriched in Fe and Mn relative to the margins, which are enriched in Mg.

Zincian collinsite occurs at Reaphook Hill as concentric crusts composed of very fine grained radially oriented plates. Electron-probe microanalyses of this material are significantly different from published analyses of collinsite from the type locality in British Columbia: Zn instead of Fe occupies the divalent cation sites; this material may be part of an isomorphous series that includes fairfieldite. Microanalyses also indicate that the Reaphook Hill collinsite crusts are compositionally zoned such that Zn:Mg ratios vary from the base to the outer edges of the crusts. This zoning is interpreted as the result of variation in the availability of Zn and Mg during growth. Zoning of similar type, but due to variation in the Fe: Mg ratio, was observed in concentric crusts of collinsite from the type locality in British Columbia.

The carbonate phase of impure sideritic concretions from a sequence of argillaceous sediments from the Westphalian of Yorkshire has been analysed using an acid-dissolution technique and corrections allowing for the decomposition of non-carbonate minerals applied to obtain accurate compositional formulae.

The siderites show considerable compositional variation, some containing Ca in excess of anticipated substitutional limits. Whilst these may be metastable it is suggested that coupled Mg-Ca substitution could provide an explanation.

Lattice parameters have been determined for two coexisting siderite phases in concretions of Westphalian age. Chemical evidence indicates that, for one concretion at least, the phase of smaller cell dimensions is a magnesian siderite with around 25 mole % MgCO3. Both abundances and compositions of the two phases vary between concretions whilst a single phase only is present in disseminated siderite. It is thought unlikely that the two phases represent an equilibrium assemblage and a mechanism is suggested by which magnesian siderite could have formed later.

The electron spin resonance spectrum of a sample of natural, orange-coloured fluorite from Broken Hill, Australia, has been observed and shown to arise from the presence of manganese ions. From an analysis of the structure of this spectrum it has been possible to estimate the various contributions to the ionic energy level splitting and to show that these are in good agreement with comparable results for synthetic material. It is evident that the manganese ions have entered the lattice substitutionally at Ca2+ sites and the nature of this site is discussed.

Further lines of the anomalous optical absorption spectrum in low metamict zircon have been observed in the infra-red. Low-temperature polarization studies have enabled a clear distinction to be made between lines due to the anomalous spectrum and those due to the normal spectrum of U4+ in zircon. Magnetic circular dichroism measurements showed, for the anomalous spectrum, only contributions due to the mixing of the energy levels by the magnetic field; thus the A spectrum results from singlet → singlet transitions. The anomalous spectrum appears to derive from U4+ incorporated in polycrystalline zirconia.