Major and trace element and Sr, Nd and O isotopic data are presented for ferrobasalts, icelandites, rhyolites, mixed pumices and silicic xenoliths of the 1875 eruption of Askja. Trace element modelling and Sr and Nd data largely confirm previous major element calculations that fractional crystallization was dominant in the generation of the basalt-ferrobasalt-icelandite-rhyolite suite. Relative enrichment in Rb (and Th and U?), depletion in Cs, and low values of δ18O/16O, in the rhyolites are not explained by this mechanism alone. The silicic magmas were selectively contaminated by diffusion from partially molten granitic wall rocks, now found as xenoliths in the eruptive products, the process being particularly marked by lower δ18O and Cs/Rb ratios in the rhyolites than in the associated basalts. This is the first record of a combined fractional crystallization-selective contamination process in an Icelandic silicic complex.

Sapphirine-bearing assemblages occur in paragneisses in a 200 km long block in the Grenville province in Labrador-Quebec. The occurrence of some of these rocks was previously known, but their considerable extent is now recognised from regional mapping. The mineral assemblages, reactions, and compositions and the tectonic structure in the paragneisses of this block are surprisingly uniform. Within feldspar-quartz layers we recognise assemblages with sapphirine, quartz, iron titanium oxides, spinel, corundum, diaspore, orthopyroxene, sillimanite, cordierite, garnet, and biotite in metre to millimetre-thick layers. These minerals reacted with their matrix, especially quartz, during cooling and uplift. At least 11 retrograde reactions gave rise to spectacular corona textures and define a P-T-time trajectory from c. 8 kbar at 900 °C to 6 kbar at 700 °C which changed from early isobaric to late isothermal. Based on successive generations of sapphirine and orthopyroxene with constant XMg and decreasing Fe3+/Fe2+ ratio in recalculated formulae, we deduce an accompanying change from high to low oxygen fugacity along this trajectory. The isothermal section of the trajectory is consistent with predicted rapid uplift and with field evidence for thrust tectonics and mylonitisation.

Metanorite dykes intrude the Banded Gneiss Complex at various places in Rajasthan, N.W. India. They show neither chilled margins nor gradational contacts with the country rock amphibolite or granulite facies gneisses. They have ophitic to subophitic texture with strongly zoned subcalcic clinopyroxene and orthopyroxene, olivine and plagioclase, with subsidiary biotite. During slow cooling a series of reaction coronas developed with garnet forming round biotite, ilmenite and orthopyroxene; hornblende round pyroxenes and orthopyroxene, hornblende ± spinel round olivine, which may be totally replaced. It is inferred that the dykes crystallised from a tholeiitic magma at about 1100-1150 °C and were intruded during the waning stages of granulite facies metamorphism. The corona minerals grew at about 650–700 °C. A series of reactions to account for the development of the coronas is proposed using measured mineral compositions. Although these reactions do not balance for individual corona formation, metamorphism was probably isochemical with Ca, Na, K, Ti, Si and H2O only mobile on the scale of a thin section. Si and H2O were possibly mobile on a larger scale.

The plutonic complex in SE Jersey consists of a late Precambrian gabbro-diorite mass which has been intruded by several granites. The status of the diorites which, like the gabbros, often possess a layered structure, is not clear. New geochemical data, including rare earth and trace element analyses, suggest that acid-basic magma mixing was not responsible for the variety of intermediate rocks. Amphibole-controlled fractional crystallization of hydrous basic magma is also unlikely in view of the REE and HFS (high field strength) element distribution. The model which best fits all the available field, petrographic and geochemical evidence is one in which the chemical variation was initially produced by fractional crystallization of anhydrous basic magma and subsequently overprinted by granitic metasomatism. This superimposed calc-alkaline characteristics on the complex and produced hybrid diorites which, because they were above their solidus temperature, recrystallized with textures indicative of a magmatic origin.

A skarn mineral assemblage occurs at the junction between vent pyroclastics and a xenolithic Cretaceous chalk block which subsided into the collapsed caldera of the Central Ring Complex, Isle of Arran, Scotland. Adjacent to the metachalk marble an andradite garnet exoskarn zone has developed at the expense of the carbonate. An andradite grossular/diopsidic clinopyroxene endoskarn zone has formed in the surrounding agglomerate, and a magnetite exoskarn is present in places between the andradite and garnet/pyroxene zones. The andraditic exoskarn garnets have fluor-hydrogarnet components, indicating that fluorine was present in the metasomatic fluid. From petrographic evidence, three distinct episodes of exoskarn garnet crystallization can be recognized, in which the fluor-hydrogarnet component steadily increased as a function of time, which probably reflects falling temperature. The REE compositions of the exoskarn minerals are regarded as having been largely inherited from the carbonate, and the exoskarn garnets increasingly fractionated HREE with time. The endoskarn and agglomerate have also been epidotized. The REE signatures of epidotes appear to be inherited partially from precursor clinopyroxenes or feldspars, which have been replaced by epidote. Late-stage vein minerals include prehnite, laumontite and K-rich laumontite, and their REE compositions appear to have been derived from the marble, probably via REE fluoro-complexes in the fluid.

Calderitic garnet and manganoan deerite have been found in Al-poor, Fe-Mn-rich quartzites associated with metabasic schists on the island of Andros, Cycladic blueschist belt, Greece. Calderitic garnet (30–45 mole % calderite, 55–70 mole % andradite, 0–2 mole % spessartine) occurs in paragenesis with quartz, rhodonite, rhodochrosite, Ni-Mn-bearing magnetite (2–20 mole % trevorite, 6–12 mole % jacobsite, 73–91 mole % Fe3O4), baryte, and orthite. Deerite is associated with quartz, garnet (rich in spessartine, almandine, and andradite), phengite, aegirine, magnetite (containing up to 12 mole % jacobsite), and crossite/riebeckite. Deerite contains 33.0–34.4 wt. % SiO2, 0.53–5.5 wt. % TiO2, 0.09–0.31 wt. % Al2O3, 11.9–33.9 wt. % MnO, 17.5–44.2 wt. % FeOtot, and 1.5–2.8 wt. % MgO. MnO contents correspond to 29–77 cation % Mn2+ per total R2+ cations. Deerites from Andros are thus closer to the hypothetical end-member mangano-deerite, Mn122+Fe3+6Si12O40(OH)10, than any other analysis of deerite so far published.

Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims.

Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastnäsite. These are briefly described and microprobe analyses presented.

The occurrence and mineral chemistry of apatite, magnesian ilmenite, and an Na-Fe-Ti oxide in lavas from the Leucite Hills are reported. Magnesian ilmenite and apatite occur as xenocrysts and as crystals in amphibole-mica-pyroxenite xenoliths. Na-Fe-Ti oxide and also rutile occur as inclusions in ilmenite. The latter mineral contains up to 7.2% MgO and shows evidence of oxidation by, and reaction with, the host magma. The apatite differs from that which occurs as phenocrysts and microphenocrysts inasmuch as REE were not detected. The occurrence of these minerals, which are important repositories for REE and High Field Strength elements, together with phlogopite in the upper mantle source regions of ultrapotassic lavas, is important and may explain some unusual aspects of the geochemistry of such lavas (low K/Rb, P2O5/Ce, Ti/Zr, high Ti/V, Zr/Nb). The source regions must be grossly heterogenous and a two-component model is suggested for the source. This model is similar to that suggested for the source regions of other alkaline magmas and is capable of explaining the unusual Nd-Sr isotopic characteristics of the Leucite Hills lavas.

Millimetric, ellipsoidal monazite nodules found within Lower Palaeozoic sedimentary rocks in Wales, south-west England and Brittany are characterised by a pronounced zonation of light and heavy REE, an inclusion fabric of low-grade metamorphic minerals indistinguishable from the host rock and a low Th content. They are interpreted as the product of in situ recrystallization of detrital monazites derived from pegmatitic or granitic source rocks and are potentially useful as indicators of Lower Palaeozoic sedimentary rock provenance.

Caryinite from Låmgban, Sweden, and arseniopleite from both the Långban and Sjö Mines in Sweden, are re-examined chemically and crystallographically. Both species are monoclinic, with space group C2/c, and similar lattice parameters. The general formula for caryinite is Na4Ca4Ca4(Mn,Mg)8(AsO4)12, and that for arseniopleite is Na4Ca4Mn4(Mn,Mg)8(AsO4)12. The species status of arseniopleite remains uncertain; it may have Mn dominant in the M(1) site.

The surfaces of fresh and tarnished (through exposure to the atmosphere) bornite have been studied by measurement of reflectance and quantitative colour parameters in the visible region, X-ray photoelectron and Auger electron spectroscopy, and conversion electron Mössbauer spectroscopy. The results of these studies, in combination with those of previous workers, indicate that a surface coating of an iron hydroxy-oxide forms on bornite, leaving beneath it a layer with a copper sulphide composition. A model to explain this alteration process is proposed based on a suggested surface reconstruction in bornite, leading to metals in essentially trigonal planar coordination. Resulting destabilization of iron relative to copper in this crystal chemical environment is suggested to account for its preferential oxidation.

The thermal behaviour and mechanism of formation of different texture types of intergrown As-Sb alloys have been studied by DTA and annealing experiments performed on natural samples. The constant composition of the As-rich component and of the stibarsen in the intergrowths, and the large compositional range of the homogeneous solid solution obtained after heating in the DTA cycle, have been established using the linear relationship between cell volume and composition. The high-temperature features detected in the DTA studies of the natural samples confirm previously published phase relations for the synthetic As-Sb system. The low-temperature features can be correlated with the homogenization reaction which leads to the formation of a complete solid solution. Study of TTT plots based on the annealing experiments clearly shows that a diffusion mechanism is involved in the homogenization reaction. This has been further substantiated by fitting the experimental data to kinetic equations for diffusion-controlled processes. The kinetic parameters evaluated from the ending time for the 520, 480, and 420 °C annealing experiments, using both the Arrhenius and the transition state theory formalisms, suggests a rather rigid activated complex for the rate-determining step of the process.

The Vale das Gatas tungsten mine is situated at the contact between Hercynian granite and Cambrian-Precambrian crystalline schists a few kilometers north of the village of Sabrosa, near Vila Real, in northern Portugal. Wolframite has been the principal product of the mine although several other metals of economic interest are known to occur there. The silver content of the ores was attributed by earlier workers to the presence of tetrahedrite and schapbachite, whilst this work shows matildite, pavonite, acanthite, and argyrodite to be present, together with Ag-bearing galena. The textures and compositions of these phases are described with preliminary results of fluid inclusion studies.

The Tieschitz chondrite is a disequilibrium assemblage of silicate, metal and sulphide occurring together or separately as chondrules and clasts that generally have opaque rims (matrix) of fine-grained material. Translucent, silicate-rich matrix fills channels that occur sporadically between chondrules and clasts. An angular fragment of twinned ‘clinoenstatite’ was found to be rimmed with chromium- and manganese-rich augite and surrounded by opaque matrix. The whole is set within the largest area of translucent (white) interchondrule matrix encountered. It is suggested that reaction between crystallizing undersaturated translucent matrix, opaque matrix, and clinoenstatite probably led to the partial replacement of clinoenstatite by chromian-manganoan augite at high temperature.