When magmatic fractionation involves the settling and removal of crystals from the body of magma, the efficiency of the fraetionation process may be defined as the degree of separation of the solid from the liquid phase. An expression is given that relates efficiency to the amount of mesostasis, or crystallized trapped liquid, in an igneous cumulate. The uranium contents of samples from a 349-m-long drill-core of part of the lower and hidden zones of the Skaergaard intrusion are used as a quantitative indicator of the amounts of mesostasis in the cumulates. There are marked changes in the amount of mesostasis over the length of the core and the average efficiency of fractionation was 85 %.

A new analysis of the violet amphibole from the original locality where winchite came from agrees with another recent analysis in differing substantially from the original winchite composition and is (K0·17Na2·10Ca0·23)(Al0·19Ti0·00Fe31·27Fe20·01Mn0·07Mg3·34)Si8·01O22·05(OH)1·95. It is essentially a magnesio-arfvedsonite. Although it is possible that the original analysis was unreliable, evidence is given to support the alternative view that there is a wide range of blue alkali and soda calcic amphibole compositions at Kajlidongri and the name winchite should be restricted to compositions between ferri-winchite CaNaMg5Fe3+Si8O22(OH,F,Cl)2 and alumino-winchite CaNaMg5Al Si8O22(OH,F,Cl)2. X-ray, density, and optical data on the analysed mineral are given.

Compositions of nine hydrous aluminium phosphates are considered with respect to the AI, P, and H contents per 8 oxygens, and it is suggested that when the ratio (Al.P)/H is less than 0·11 (or (Al.P2)/H2 is less than 0.01) such compositions do not crystallize. Examples are bolivarite and evansite.

While undergoing 'dry' homogenization at 980°C the 2̄01 X-ray reflections of sodic lowalbite- microcline microperthites show that phase compositions recapitulate (but in a reverse timesense) the unmixing history of ordered feldspars about a solvus with a critical composition near 26 mole per cent KAlSi3O8.

A layered aipine-type peridotite, of pre-Scourian age(?), has been metamorphosed in the granulite facies then partially retrogressed to greenschist facies assemblages. Following granulite facies metamorphism of the dunite-harzburgite peridotite, granitization and metasomatism modified the peridotite by adding calcium and generated olivine-tremolite rocks. Anthophyllite-rich rocks developed from localized orthopyroxene-rich zones. The main epoch of serpentinization followed after tremolite formation and removed a portion of the calcium previously added, but with negligible effect upon the original trace-element content. Between tremolite generation and the main period of serpentinization greenschist facies assemblages formed. The layered rocks possess a repetitive, chemical variation normally attributable to basic igneous processes, although they may have formed either by vein metasomatism or by metasomatism of a disrupted layered igneous series. Twelve rock and five mineral analyses are presented, and trace element data for twenty-one rocks.

The garnierite, which occurs in the oxide zone overlying a nickel sulphide deposit, consists of talc-like and serpentine-like components, with the former predominating. Chemical and electron-probe analyses give NiO contents ranging between 39.6 and 43.9 weight %.

Two kinds of vitreous material with composition K2GeSi3O9 and K2Si4O9 were treated under conditions of high temperature and pressure. The X-ray diffraction patterns of the products were similar to that of K2TiSi3O9, which has the wadeite, K2ZrSi3O9, structure.

A natural crystal of K-feldspar was also treated under the same conditions. The product was found to consist of three phases, coesite, kyanite, and the above-mentioned K2Si4O9. Since Zr ions in wadeite are in a 6-coordinated site, one-quarter of Si ions in K2Si4O9 are considered to be in 6-coordination. This is the first silicate in which Si ions occupy sites of 4- and 6-coordinations in one crystal at the same time.

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.

Compositions concerning sodic amphiboles (glaucophane) in the system Na2O-Al2O3-MgO-SiO2-H2O have been studied at PH2O ≤ 3 kb, 380 °C < T < 700 °C. The study attempts to establish the mineral parageneses under these physical conditions as they define the clay mineral (saponite-hectorite smectite) and greenschist (chlorite-albite) facies. The occurence of sodic amphiboles is defined by compositional parameters. Glaucophane does not appear in the present investigation.

The method of ‘reaction reversal’, i.e. using precrystallized mineral assemblages to form new ones, has been used to establish the three reactions: albite + chlorite → paragonite + saponite, paragonite + chlorite → saponite + cordierite, and saponite + paragonite → albite + cordierite. In general, the clay mineral → greenschist transformation is favoured by an increase in pressure in the system studied. The amphibole present is a sodic richterite with the type substitution

New occurrences of xenolithic alkaline rocks from three East African volcanoes are described. Micro-ijolites and alkali pyroxenites occur in the carbonate-rich tufts and lavas of Mount Elgon; kaersutite-rich pyroxenites and a suite of fenitized basement rocks in the caldera flows of Mount Meru, and apparently unique melanocratic hortonolite nepheline syenites in the central foyaite plug of Mount Kenya. The 87Sr/86Sr ratios indicate mantle origin, and all are enriched in trace elements such as Sr, Nb, and Zr. The xenoliths from Mount Kenya are cumulate equivalents of the glassy kenyte lavas of the mountain; the pyroxenites of Mount Elgon are also cumulates whose predominant titanaugite is of identical composition to that of the host lavas. The contrast between the 'dry' Elgon pyroxenites and the 'wet' amphibole-rich types from Mount Meru is believed to reflect different CO2/H2O ratios in the volatile phase accompanying the alkaline vulcanicity. Strongly alkaline, carbonate-rich suites such as that of Mount Elgon derive from fairly dry alkali-basaltic magma under high CO2 activity by dominant pyroxene fractionation, whereas the mildly alkaline lineage seen in some areas of Mount Meru forms under more hydrous conditions in which amphibole fractionation is important. Mount Meru in addition shows a transition to rock-types typical of the strongly alkaline lineage, and this is reflected in the occurrence of relatively amphibole-poor pyroxenites with the amphibole-rich types.

The granites of Sardinia have been analysed for copper, lead, and zinc, and each element has been found to show a regional variation. There is a close relationship between the copper and lead contents of the granites and the distribution of copper and lead mineralization in the island, but very little relation between the zinc contents of the granites and the distribution of zinc mineralization. This is attributed to a greater degree of lithophile character of zinc in the magmatic source region.

Fluorine, chlorine, zinc, niobium, zirconium, yttrium, and rubidium have been deter-mined on fifteen obsidians from Eburru volcano (Kenya Rift Valley), spanning the range from pantel-leritic trachyte to pantellerite. All pairs of elements show positive correlation coefficients, ranging between 0·769 and 0·998, but with most values better than 0·900. In spite of some very high correlations, only two of the twenty-one best-fit lines pass near the origin of the Cartesian coordinates. Linear distributions are found within two separate groups of elements: F, Zr, Rb; and Cl, Nb, Yt. Zn behaves in general as a member of the second group but seems to be subject to an additional variation. When an element from the fluorine group is plotted against one from the chlorine group the resulting pattern is non-linear. Therefore, although the elements in both groups would generally be considered ‘residual’ (partition coefficients between crystals and liquid approaching zero) there are clearly detectable differences in their variation, and hence their behaviour.

Major-element variations in the obsidians are such that a vapour (fluid) phase would be needed to account for any magma evolution. The trace-element patterns are also impossible by closed-system crystal fractionation and suggest that this fluid may have been rich in halogens, with the metallic elements forming preferred ‘complexes’ with either F or Cl. The F-Zr-Rb ‘complex’ also varies quite independently of the important major oxides (e.g. A12O3) in the rocks. In the case of Rb this is but one aspect of a more significant anomaly, in which there is no sign of any influence of alkali feldspar (which partitions Rb) in the variation. This is remarkable because trachytes and rhyolites have normative ab+or > 50 %, and any evolutionary process controlled by crystal ⇋ liquid interactions must be dominated by the melting or crystallization of alkali feldspar. The results on the Eburru obsidians show that if they are an evolutionary series then either, the process was not crystal ⇋ liquid controlled, or that any such process has been overriden (or buffered) by other processes that have superimposed the observed trace-element patterns. In the latter event, the buffering phase may have been a halogen-bearing vapour.

The same considerations must apply to other pantellerite provinces where Rb appears to have behaved as a ‘residual’ element.

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 volatilities of SiO2, Na2O, and Fe2O31 increase with increasing thermal function within the range 1600 to 2100°C during the melting of a sandy-silty clay. The remaining melt is to some extent enriched in Al2O3, K2O, CaO, and MgO. Chemical analyses of the sandy-silty clay, of the glasses melted from the latter, of the silicate material evaporated and newly condensed and their correlation with increasing thermal function and bulk density of glasses, as well as with the hitherto known quantitative data on moldavites, testify to the fact that in moldavites evaporated and newly condensed silicate material substantially prevails over material that passed directly from a solid to a fluid state. Evaporation of the silicate material from the rocks of the impact crater caused a significant change in relations between major oxides compared to the original relations in the source rocks. Thus, the hypothesis that tektites are directly remelted rocks of the Earth's crust, based on over-all chemical similarities of tektites and some rocks in major oxides, appears incorrect.

A plot of seventy-five new analyses of members of the helvine group and fifty-seven analyses from the literature indicates complete miscibility between the Fe and Zn members (danalite and helvine) and between the Fe and Mn members (danalite and genthelvite). The new analyses include essentially pure helvine and genthelvite (for both of which X-ray data are presented) and the nearest approach to pure danalite (86 at. % Fe). Two new localities for genthelvite, the rarest member, are reported.

Fibrolitic sillimanite in some high-grade gneisses from Broken Hill, Australia, shows microstructural features suggestive of compatibility with the main coarser-grained assemblage, namely: framework-silicate interfaces either meet (110)-sillimanite/framework-silicate interfaces at approximately 90° or are attached to the ends of 'fibrolite' rods; framework-silicate interfaces are deflected by 'fibrolite' grains; and interfaces of inclusions of one framework silicate in another are deflected by 'fibrolite' grains, producing irregular, rather than rounded, inclusion shapes. These criteria suggest that the 'fibrolite' in some rocks is a compatible member of the main metamorphic assemblage. Its persistence as small grains may be due to a high proportion of relatively low-energy prismatic boundaries in the fibrolitic aggregates.

The mineral tacharanite has been recently re-examined by Cliff et al. (1975). They worked on material from the original locality, Portree, in Scotland (Sweet, I961) and from a few new European localities described between 1971 and 1974. None of the post-Sweet authors, however, refer to the second discovery of the mineral from Tasmania where it was identified by this writer in 1961 and recorded in a number of publications (Sutherland, 1965; Sutherland and Corbett, 1967; Sutherland and Hale 1970; Mines Department of Tasmania, 1970; Sutherland, 1973b). The first Tasmanian report was communicated to J. M. Sweet, who kindly arranged for the Department of Mineralogy, British Museum, to send the writer a small piece of the Portree material for study. As the Tasmanian find does not seem to be known in the later literature, some more detailed information on its occurrence is presented here for comparison with the European localities.

A study has been made of a large clinoptilolite deposit in the Lesser Caucasus (Azerbaidzhan SSR). The geological conditions have been examined and a number of samples investigated by optical, electron-microscopic, chemical, X-ray, thermal, and infra-red methods. Based on these investigations and on comparison with previous literature, mineralogical, chemical, and geochemical conclusions are drawn and conditions for clinoptilolite genesis are considered.