Migmatites are described from the Sillimanite-potash-feldspar Zone of the aureole around the Newer Basic suite of synorogenic intrusions. The lowest-grade migmatites are trondhjemitoid (characterized by the assemblage quartz-plagioclase-biotite) or muscovite-granitoid (quartz-plagioclase potash-feldspar-muscovite-sillimanite-biotite). With increasing grade, a transition occurs to cordierite-granitoid assemblages (quartz-plagioclase-potash-feldspar-cordierite-garnet-sillimanite-biotite), which persist to the highest grades observed, where there are also noritoid migmatites (quartz-plagioclase-orthopyroxene-cordierite-biotite). The trondhjemitoids are texturally simple because the minerals did not undergo dehydration reactions. Textural immaturity and consistently cotectic modal compositions indicate that their leucosomes originated as melts. Scatter of plagioclase compositions suggests that the partial melting occurred in small closed systems. The other migmatites have more fusible compositions, so it is deduced that they also underwent partial melting. Retrograde reaction textures are used to infer the sequence of reactions, involving muscovite and biotite, by which melting proceeded during prograde evolution. Whereas the fugacity of water probably varied among spatially associated trondhjemitoid leucosomes, in the muscovite-granitoids it was constrained to an approximately constant value, at given pressure and temperature, by the buffering effect of the mineral assemblage.

A minor picritic body at Suisnish Point, Isle of Skye, is described. The mass has a dyke-like form, and is rhythmically layered. Cryptic layering, however, is absent. From a study of the field relations, contacts, metamorphic aureole, and other physical features, it is concluded that the mass crystallized in situ (i.e. it is a true dyke) and is not an upfaulted block of pre-existing igneous rock. The main physical and chemical features of the layered units and their mineralogy are described. The problems of generating horizontal rhythmic layers in a relatively small, essentially vertical, planar intrusion are discussed. It is concluded that the surrounding country rock was maintained at a moderate temperature during, and after, intrusion of the picritic magma and that this allowed a combination of gravity settling and magma flow currents to produce a cumulate rock with characters normally only observed in much larger intrusions.

Chromiferous spinel and chlorite are associated and sometimes intergrown in serpentinized peridotites at Chimwadzulu Hill. The peridotites probably crystallized from a melt under the conditions of the spinel-lherzolite facies (O'Hara, 1967) in which the stable aluminous phase is spinel. The spinel that formed was probably a chromite with some Al2O3 and MgO. At lower temperatures aluminous spinel is unstable in the presence of olivine and orthopyroxene, chlorite being the stable aluminous mineral. It is thought that the chlorite crystallized under the conditions of medium-grade metamorphism that prevailed when the rocks were emplaced, and at the same time the spinel recrystallized, depleted in Al2O3 and MgO and correspondingly enriched in FeO, Fe2O3, and Cr2O3, giving rise to the observed chromite-chlorite association.

The iron-titanium oxide minerals of three troctolites from the Freetown layered gabbro have been analysed using an electron microprobe. Comparisons of the analyses of the iron-titanium minerals with the data of Buddington and Lindsley (1964) provide temperature and oxygen fugacity information relating to particular magnetite-ilmenite grains. Magnetite-ilmenite grains occurring in different silicate environments within any one single troctolite rock have equilibrated at differing times in the cooling history of that rock and this enables part of that history to be established. The partial cooling histories established for the three specimens can be clearly distinguished, and are strongly influenced by the appropriate buffer reactions.

A garnetite that occurs locally in phyllitic schists exposed by the Santorini caldera consists mainly of euhedral garnet crystals showing alternating isotropic and birefringent lamellae. X-ray diffraction revealed two garnets with unit cell dimensions a of 12·052±0·002 and 11·984±0·005 Å, corresponding respectively to almost pure andradite and a granditic phase. Mean compositions determined by microprobe analysis are An97Sp1Alm2 for the isotropic and about An63Gr33Sp1Alm3 for the birefringent layers.

The garnets are deduced to have been produced mainly by thermal metamorphism of the schists. Although cyclic variations in the composition of circulating fluids could have brought about the zoning, this may just as well be the result of temperature fluctuations, individual garnet compositions being indicative of the respective formation temperature.

Lodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi5O12. Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs13.8 with little variation. Olivine composition averages Fa12.6, but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found.

A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene.

Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies.

The Harvard University meteorite is a mesosiderite and not closely related to Lodran.

A flinty, spherulitic dyke of peralkaiine trachyte in a Precambrian dyke-swarm consists mainly of albite and aegirine. Within this, euhedral crystals of narsarsukite, Na2(Ti,Fe)Si4O10(F), occur as short prisms elongate parallel to c. This distinguishes them from all previously described narsarsukites, which have tabular habits. Minor phases present include micas, apatite, quartz, pectolite, nordite, and a thorium silicate. Probe analyses are presented for narsarsukite, aegirine, aibite, biotite, apatite, pectolite, nordite, and the thorium silicate.

On mineralogical and geochemical grounds it is unlikely that the narsarsukite-bearing dyke is a differentiate of supposedly mantle-derived basalt-trachyte-comendite magmas in the region. The dyke is colinear with a comendite dyke exposed at lower altitude and it is suggested that the narsarsukite- bearing dyke developed as a rhcomorphic fenite generated by reaction between quartz diorite basement and fugitive alkaline solutions from the subjacent comendite.

Seven chemical analyses of pink or green tourmalines belonging to the elbalte-schorl series, along with their physical, optical, and X-ray data are presented. Linear variation diagrams showing the relationship between composition and refractive indices, specific gravities, and cell parameters are constructed. Relationship between colour and chemical composition is also discussed.

Seven samples of pelletat glauconites with varying iron and potassium contents were investigated using the electron microprobe. In-sample variation of pelletal compositions is quite important, even for samples previously separated according to grain-size, density, and magnetic susceptibility. This observation should lead to a reconsideration of previous work based upon multigrain samples.

During the evolution of pelletal material towards micaceous glauconite compositions it is apparent that iron can be added to as wellas subtracted from the bulk pelletal composition. This transfer appears to be effected when the pellet contains a multiphase mineral assemblage as well as during the evolution of a single-phase mixed layered mineral.

A schematic phase diagram is presented which attempts to describe the phase changes as a function of K, Si, and Fe variables.

Two new occurrences, in quartz dolerite, are reported for julgoldite. Physical and chemical data is presented together with indexed powder data for fifty-five reflections, fifteen of them additional to those previously reported. The julgoldite infra-red spectrum possesses the same configuration as that of an epidote or clinozoisite but with large peak shifts due to the high iron content.

The occurrence of cookeite, a di/trioctahedral chlorite of composition (Al3.95Fe0.02Mg0.01Li0.81)(Al0.74Si3.26)O10(OH)8, in the Back Creek pyrophyllite deposit near Pambula is described. The mineral is restricted to a relatively narrow zone near the centre of the deposit and although diaspore is also present in this zone the two have not been found in intimate association, it is concluded that the introduction of lithium has been the principal factor controlling development of the cookeite.

The lattice types of omphacite-diopside series clinopyroxenes were determined for 9 Japanese, 2 Norwegian, and 1 South African samples. The boundary between the compositional ranges of P and C lattice clinopyroxenes lies at about 14·5 wt. % CaO in Japanese and Norwegian samples, and the compositional gap between them is very small, if it exists. The South African omphacite has C-lattice in the compositional range of P lattice omphacite in metamorphic eclogites. These data combined with the reported electron microscope studies suggest that the omphacite—diopside series clinopyroxene has a C lattice structure above 3 to 400°C and very slow cooling after regional metamorphism is responsible for the transition to the ordered structure with P lattice symmetry.

From field and petrographic evidence, some scoriaceous rock from the upper ignimbrite horizon of the Mount Rivers Volcanic Member of the Carboniferous Dyrring Formation in the Mirannie-Mount-Rivers District, New South Wales, Australia, is believed to have a recent origin.

By heating unaltered dacitic ignimbrite in a furnace it was possible to produce in the temperature range 600-1230°C changes in the rock similar to those observed in specimens of scoria collected in the field. Thus a temperature range for the alterations was established. Two possible sources of heat axe suggested as being responsible for the production of the scoriaceous rock, namely a bush fire and a lightning strike, with a preference for the former.