The new mineral theophrastite, Ni(OH)2, from Unst, is Mg-bearing and occurs associated with a very poorly crystalline Ni-containing mixed hydroxide of the pyroaurite type and/or zaratite on chromitite. X-ray powder diffraction data show a shift in d spacing toward brucite compared with pure Ni(OH)2. Infrared, thermal, chemical, optical, and physical data are presented together with indexed powder data for fifteen lines ranging from d 4.66Å to 0.90Å. The solid-solution series brucite-theophrastite is briefly discussed.

Two contrasting types of Hercynian U-rich granites can be distinguished by their accessory minerals: (1) subalkaline potassic granites (amphibole-biotite±pyroxene) characterized by apatite-zircon-thorite-sphene-allanite and rare ThO2-rich uraninite, in order of appearance in the magma, and (2) leucocratic granites (biotite and muscovite) which contain monazite-zircon-apatite-xenotime-ThO2-poor uraninite and Ti oxides. Intermediate accessory mineral associations may also exist.

The mineralogy, crystallization history, and geochemistry of two high K-U-Th granites from the Vosges, France, is described. Their assemblage of U- and Th-bearing accessory minerals is listed, in order of appearance in the magma, in Group 1 (above). Trace elements (U, Th, Zr, Hf) and preliminary data on REE are discussed. In a layered sequence of the Crêtes granite, thorite, with up to 15% UO2, is shown to be important during differentiation. There is a decrease of the Th/U ratio corresponding to a slight decrease of the Th content whereas there is an increase of the U content at the end of differentiation. The Crêtes and Ballons granites have REE patterns characterized by LREE enrichment with a La/Yb ratio of about 28 (Crêtes granite) and a slight negative Eu anomaly. These relationships are typical of subalkaline potassic granites of the Hercynian. The trace element associations are similar to those of the shoshonite association with high U, Th, LREE and Ba, Rb, and Sr contents. A hypothetical model for uranium mineralization associated with such granites is discussed in the light of these data.

Major, minor, and trace elements were measured with electron and ion microprobes for eighteen anorthoclase megacrysts from five locations. Comparisons with concentration ranges tabulated by Smith (1974) show excellent agreement except for consistently lower values of Mg and B. The discrepancy for Mg probably reflects Mg-rich impurities contaminating samples analysed by bulk methods. The cause of the discrepancy for B is unknown and deserves further study. Samples from Mt. Erebus, Antarctica, contain glass inclusions from which the following crystal/liquid partition coefficients ensue: Mg, 0.008; P, 0.04; Ti, 0.08; Fe, 0.04; Ba, 5.2; Sr, 8; Rb, 0.27.

Kûngnât Fjeld is an intrusive complex resulting from three intrusive events involving trachyte, mafic trachyte and hawaiite magmas tapped successively from a single magmatic source. In situ differentiation produced gabbroic, syeno-gabbroic, syenitic, and quartz syenitic cumulates showing strong cryptic variation. Layered sequences of syenitic rocks total c. 3500 m. The compositional range is extended by late-stage minor intrusions of microsyenite and peralkaline granite. The principal cumulus minerals are feldspar (Ab40An60 to Or45Ab51An4); olivine (Fo56 to Fo1) and clinopyroxene (Di70Hd27Ac3 to Di6Hd90Ac4 followed by an abrupt change of trend towards Di3Hd78Ac19 in the layered series and nearly pure aegirine in some peralkaline residues). Intercumulus phases are amphibole (hastingsite to ferro-edenite to ferro-actinolite in the layered series with riebeckite to arfvedsonite in some associated granites) and biotite (annite31 to annite98.5).

Over 100 samples of ‘pegmatite’ and associated host rocks from the Cree Lake Zone of northern Saskatchewan have been studied. The degree of U and Th enrichment of the pegmatites appears to depend on the parent material of the anatectic melts produced during the Hudsonian thermotectonic event. Evidence for Archaean granulite-facies depletion of U and Th is inconclusive; however, important enrichment of U and Th in the basal Aphebian is associated with metamorphosed black shales and possibly evaporates.

The Currywongaun and Doughruagh masses of northern Connemara represent a section through a syntectonic, partially layered intrusion formed of ultrabasic cumulates all having plagioclase of around An94. Differentiation and contamination of the magma from which these cumulates were precipitated ultimately produced an acid residuum which was responsible for the hydrous metasomatism of the earlier formed ultrabasic rocks resulting in the extensive formation of quartz-amphibole-anorthite gneisses in the petrogenetically ‘upper’ parts of the intrusion. Coexisting Ca-rich and Ca-poor amphiboles in the gneisses are interpreted as ‘frozen’ reaction intermediates that were in local equilibrium. The bulk-rock chemical data presented confirm that the compositions of the quartz-rich gneisses can be attributed almost entirely to the addition of Si to the ultrabasic rocks, coupled with the removal of Mg and Ca. The data also suggest that Zr, which reaches around 1000 ppm in some of the gneisses, may have been ‘mobile’ under these conditions of hydrothermal metasomatism.

Possible evolutionary paths of alkali-rich carbonatite magma may be related to the production of sodic and potassic fenites. It is concluded that the latter are generated not from separate Na- and K-rich carbonatites, but by the earlier preferential loss of Na from a sodi-potassic carbonatite magma. Trends of both Fe and Mg enrichment in carbonatites are illustrated from a number of centres, and ascribed to crystal fractionation, although metasomatism may be partly responsible. Current nomenclature of carbonatites is unsatisfactory and a division into sövites, magnesiocarbonatites and ferrocarbonatites is proposed, based on rock chemistry.

A mineral closely resembling eudialyte in single crystal X-ray pattern and chemistry has been found in one granite block from the 5 Mile Post area on Ascension Island. To our knowledge, eudialyte has not previously been recognized in this suite of granites although they contain two other rare zirconium silicates, dalyite and vlasovite. Textural evidence suggests that the eudialyte, which is remarkable for its high yttrium and REE content, crystallized from very late-stage magmatic fluids. Rare-earth patterns are only slightly fractionated (CeN/YN = 1.8–2.5) compared with those of allanites and sphenes from Skye granites which presumably have similar parageneses. In contrast, the whole-rock REE pattern shows greater enrichment of light REE (CeN/YN ∼ 10). This difference in REE pattern is considered to be due to the effects of REE and other element complexing related to the peralkalinity of the granite liquid.

The Cadomian igneous complex of N. Guernsey includes gabbro, diorite, granodiorite, and adamellite. The gabbro is layered, shows variations in colour index and texture, contains abundant amphibole, and is cut by the diorites. The diorites show little variation in mineral compositions despite considerable variation in modal proportions. They contain numerous thin and roughly parallel veins of leucodiorite and granodiorite, and layers of meladiorite occur in several places within the veined diorites. These meladiorites, previously interpreted as separate intrusions, form individual layers from a few centimetres to several metres thick which can be traced for some tens of metres along strike. The upper surfaces of the meladiorites are planar but the lower tend to be indented and irregular. The layers thin out laterally and usually pass into the host diorite by way of a zone containing dark spots and both the spots and the meladiorite layers are composed of actinolitic hornblende and Mg-biotite.

Chemical and petrographic data are presented for most of the rocks and minerals especially for the meladiorites and their host, the veined diorites. The rock chemistry and particularly the compositions of the amphiboles and micas of the meladiorites and the mafic spots, taken together with the presence of unmixing lamellae in the amphibole and the details of the field relationships, indicate that the rocks tended to equilibrium under subsolidus conditions and are most probably of metasomatic origin.

Acid pipes and pegmatites occur within and are restricted to the meladiorites. The forms of the pipes were previously taken to indicate that they were magmatic at the same time as the meladiorites. The metasomatic origin for the meladiorites proposed here calls into question this magmatic interpretation and it is suggested that the pipe formation is connected with the metasomatic process. It is envisaged that the alteration of the veined diorites took place under conditions in which high gas pressures allowed acidic melt to be produced at temperatures only a little above the solidus of the diorites. The thermal impulse for this recrystallization may have resulted from the polyphase intrusive history of the complex as a whole.

Petrographic, microscopic, and X-ray investigations of a uranium occurrence near Tiraun, Graubünden, Switzerland, are described. The uranium mineralization occurs in greenschist facies metamorphic rocks of the Tavetsch massif and consists essentially of uraninite of various habits with minor chalcopyrite, linnaeite, millerite, galena, marcasite, pyrite, hematite, magnetite, and skutterudite. The ore structure, characterized by concentrations along foliation planes, boudinage, and rotated (helicitic) textures of uraninite porphyroblasts, is indicative of a synmetamorphic (Alpine) remobilization, recrystallization and enrichment of the ore, which was probably derived from an original ‘protore’ in psephites and psammites (? sandstone-type mineralization) of at least Variscan age. The synmetamorphic emplacement of the ore took place under medium- to high-temperature conditions (approx. 350–400°C), as demonstrated by the lattice constants of the uraninite phases and by the ore paragenesis. Late to postmetamorphic tectonism resulted in a partial remobilization (over cm distances) and redistribution of the ore along veinlets that cut the foliation planes. The characteristics of the ore from Tiraun are similar to those from the synmetamorphic ore from Preit, northern Italy, as described by Cevales (1961).

The occurrence of pumpellyite in Late Cretaceous volcanic rocks of the Coloured Series, NE of Neyriz, is reported for the first time. The form of the pumpellyite and its chemistry are described. The mineral assemblages of the prehnite-pumpellyite facies of metamorphism are present, indicating that burial metamorphism has occurred, a new factor to be considered in the tectonic evolution of the Zagros suture zone.

Cr-rich Mg-sapphirines, which are more aluminous in composition than R2+:R3+:Si of 7:9:3 and are therefore considered to be peraluminous, are shown to have an inverse linear relationship between octahedrally co-ordinated Cr and Al. This balanced substitution, which has not previously been described, may be used to explain the major chemical variations exhibited in the described minerals. Since the peraluminous Cr-rich sapphirines are idioblastic it is suggested that they are stable members of the assemblages within which they occur. Peraluminous sapphirine is usually considered to be metastable but in this instance it is possible that chromium may assist in stabilizing its development.

Uranium has been detected in fluid inclusion decrepitates from quartz of several granites of the British Isles and from vein quartz associated with the Hcrcynian granites of SW England using ICP. Material, ejected from the inclusions during decrepitation on heating the sample, is transferred into the plasma for qualitative analysis via a stream of argon. Several other elements have been detected in the decrepitate, of which carbon is of particular interest. It shows a strong positive correlation with U and indicates the importance of C (presumably as carbonate complexes) in the transport of U in hydrothermal systems. Approximate order of magnitude estimates of the average U contents of fluid inclusions from the SW England samples, based on the assumption that U in the decrepitates is principally derived from the fluid inclusions, range from less than one to over a thousand ppm. Fluid inclusions may therefore be important in contributing to the levels of U reported in quartz (0.1 to 10 ppm).

The scale and nature of the exsolution textures developed in some titanomagnetites have been used to calculate the rate at which the oxides were cooling at the time of exsolution. The cooling rates were calculated by evaluating a kinetic model, which describes the growth of an ulvöspinel-rich lamella in titanomagnetite during exsolution. The model was evaluated both by numerical techniques and by an approximate analytical method.

The cooling rates during titanomagnetite exsolution of the Taberg, Skaergaard and part of the Mt. Yamaska intrusions were calculated to be approximately 130, 12 and 6000°C per 1000 years respectively. These values are in good agreement with cooling rates calculated from heat flow models of the intrusions.

Scattered dunite and lherzolite nodules occur in one of the youngest basanitoid lavas on Ubekendt Ejland. They have protogranular to porphyroclastic textures. The dunites are composed of olivine (Fo93.2−91.9b), enstatite (En93.4−92.8) low in Al2O3 and CaO, and Cr-spinel (61-13% Cr2O3 and 3–55% Al2O3). A solitary lherzolite module has olivine (Fo94.7–94.1), enstatite (En94.7–94.2), Cr-rich spinel, Ti-phlogopite (11% TiO2), and hyalophane. Petrographic evidence suggests that the two latter minerals have not been introduced by magmatic injection from the host in spite of the refractory nature of the coexisting phases, and metasomatic processes prior to the last deformation are therefore indicated. Partial melting of such mantle material would presumably produce small amounts of alkaline liquids even at very high temperatures. Another lherzolite nodule from a lamprophyre dyke has minerals with lower Mg/(Mg + Fe) ratios which, together with its preserved igneous textures, suggest a high-pressure precipitate. The lowest well-esablished equilibrium temperatures of 700–830°C for both dunites and lherzolites indicate a pressure regime of 12-17 kbar, according to the oceanic geotherm, whereas unrealistically high pressure (20–5 kbar) are suggested using the continental shield geotherm.

A method is presented for determining the composition of the material being instantaneously abstracted from the liquid during equilibrium crystallization of a solid solution in a binary system. It shows that, except when the solidus and liquidus curves are straight and parallel, the composition of the material being instantaneously abstracted from the liquid does not lie on the straight line joining the composition of the first crystals and last liquid for that bulk composition. Despite this, the latter straight line is a reasonable approximation to the actual curve.

The crystal morphology of plagioclase feldspar in seven mafic rocks has been studied by isothermal supercooling and constant rate cooling experiments at 1 atmosphere using the ‘wire-loop’ suspension method. There is a systematic variation in crystal morphology with supercooling (−ΔT), but the transitions between the different morphologies are gradual, and consecutive forms sometimes occur together. As −ΔT increases, the sequence of plagioclase morphology is generally laths → skeletal forms → fan spherulites. During isothermal supercooling the position of the plagioclase morphological fields is shown to be dependent on −ΔT, superheating (+ΔT), and also on time to a greater or lesser degree. Variations in melt composition and viscosity affect the position and slope of the boundaries of the fields. In the constant rate cooling experiments, the morphological type is dependent on the cooling rate and can be used to a limited extent as an indicator of the range of supercooling over which the first crystals nucleated and grew.

The Duobblon rhyolitic ignimbrites, of middle Precambrian age are 60 m thick. They consist of at least three flows with varying degrees of welding and have undergone devitrification, producing lithophysal and spherulitic textures. They are overlain by thick fluviatile red-bed-type conglomerates and sandstones, which in turn are capped by terrestrial acid volcanics.

Uranium enrichments of up to 3000 ppm U occur within two or three peneconcordant tabular horizons which are mostly lithophysae-bearing. Fission-track investigations of the ignimbrites and overlying conglomerates and sandstones, which supplement earlier mineralogical studies, show that U occurs as fine pitchblende disseminations; as complex uranotitanates associated with Fe-Ti-Mn oxides; and as coatings associated with matrix sericite. Small amounts of O are present in such primary accessory minerals as sphene, apatite, and zircon.

It is suggested that oxidizing U-bearing solutions, generated partly during devitrification of the ignimbrites and partly from the overlying volcano-sedimentary pile, produced the U enrichments with later sulphide deposition, along the more permeable lithophysal horizons in the ignimbrites.