In order to aid the graphical presentation of analyses of igneous rocks, a pseudo-quaternary system R2O3XO-YO-ZO2 has recently been used (O'Hara, 1968, fig. 4). To test its ability to present the analytical data of tholeiitic basalts, the analyses of selected lavas have been projected into the system and then sub-projected into two planes. Lines, which represent the loci of liquid compositions in certain isobaric, pseudo-univariant equilibria, have been drawn on the basis of the melting relations of the basalts. After reading the pseudo-ternary phase diagrams resulting from the sub-projections, some genetic aspects of the lavas of Kilauea and Mauna Loa are discussed from a phase equilibrium standpoint.

New analyses of a garnet-clinopyroxene rock (‘eclogite’) and its constituent minerals from the Sittampundi complex of India are presented. The rock has a composition comparable with an alkali-poor tholeiite and this composition, as well as the garnet (Py 51·5 %; Alm 32·7 %) and pyroxene (Jd 7·4 %; Ts 10·8 %) are very similar to those from South Harris.

The apparent distribution coefficient (K′Fe/Mg) for the garnet-clinopyroxene pair suggests a temperature of equilibration near 850 °C. Projection of the experimental data of Green and Ringwood (1967) down to this temperature suggests a pressure of about 7–8 kb. This is well below the pressure of the granulite-eclogite transition.

The staurolite-bearing schists of the area were metamorphosed in the staurolite-kyanite zone of the amphibolite facies. The various assemblages of the pelitic schists are represented in the AKF and Thompson's AFM diagrams. Within the PT conditions of stability of staurolite, this mineral develops in pelitic schists that have a high Al2O3 (excess)/(K2O+FeO+MgO) ratio so that the plots of analyses lie above the muscovite-garnet join in the AKF diagram. The three-phase field of staurolite-kyanite-biotite in the AFM diagram migrates towards the lower FeO/MgO side with increasing oxidation ratio and thus staurolite-bearing rocks are not necessarily restricted to pelitic schists with high FeO/MgO ratio as is commonly believed. On the other hand, the three-phase field shifts towards the higher FeO/MgO side with increasing grade of metamorphism assuming oxidation ratio and pressure to be constant, thus reducing the composition field of staurolite schists in the AFM diagram.

The recently described electrolysis of molten basalt (Oppenheim, 1968) gave indefinite results with regard to the behaviour of iron. Repeating the experiments in an inert atmosphere has shown that ferric iron moves towards the anode, and ferrous iron to the cathode. All metallic ions of valency three and more are now seen to be concentrated towards the anode, with relative enrichments in decreasing order: Fe3+, P, Ti, Si, and Al; and oxygen gas is released at the anode. Metals of valency one and two are concentrated towards the cathode with relative enrichments in decreasing order: Na, Fe2+, K, Ca, Mn, and Mg.

The series expected to be differentiated within a magma consequent upon its electrolysis is redefined. The absence of natural analogues suggests either the non-operation of suitable electric currents within magmas, or the destruction of their effects by convection, fractional crystallization, or both. An electrolytic process is feasible at the mantle-core boundary, leading mainly to deposition of iron on the core and release of oxygen upwards.

The effects of hydrothermal treatments on gel mixtures and synthetic minerals in the C3S2-M3S2 and C2S-M2S regions of the CaO-MgO-SiO2 system have been investigated. The products formed in the temperature range 180-350°C at pressures up to 220 N/mm2 were identified. The reactants generally behaved as simple mixtures of binary silicates and their products agreed well with existing data for reactions in the CaO-SiO2-H2O and MgO-SiO2-H2O systems. An unidentified phase that resulted from the hydration of åkermanite could be a new compound in the quaternary CaO-MgO-SiO2-H2O system. There was no evidence to indicate that a magnesium analogue of kilchoanite can be formed from materials that are isostructural with γ-Ca2SiO4.

Authigenic envelopes up to 0·18 mm thick on clastic potassium feldspar grains from arkoses and subgreywackes of the Devonian Nannyarra Greywacke have been examined with the four-axis universal stage. The plane of the optic axes is parallel to (010) in some segments of individual envelopes and normal to (010) in others, and the range of 2Vα over all envelopes is from close to 0° to 65°. The optics are ascribed to modifications of potassium feldspar corresponding to high-and low-sanidine and orthoclase.

Many syntheses have been carried out in order to establish the possibility of replacing manganese in spessartine, Mn3Al2(SiO4)3, by various elements (Be, Mg, Ca, Sr, Ba, Pb, Cu, Ni, Co, Fe2+, Sc, Yt, Zn, Cd) as well as aluminium by Sc, Yt, Zr, Ti, Cr, Fe3+ by a solid state reaction. The syntheses were done by sintering pellets of the desired composition at about 1100 °C for 5–10 hours under normal pressure. It has been found that an inert atmosphere creates more favourable conditions for the syntheses of spessartine analogues, compared to the results obtained in the open-air reaction, where often spinel was obtained. All the reaction products were submitted to X-ray and chemical analysis and to IR spectroscopical examination.

Wardite, NaAl3(OH)4(PO4)2. 2H2O, has a = 7·03 Å, c = 19·04 Å; space group P41212 or P43212. Its crystal structure was solved by a three-dimensional Patterson function computed using intensity data photographically collected by the Weissenberg method, and refined by successive Fourier maps and least-squares cycles to a R index 0·062 for 316 independent observed reflections.

The wardite structure is formed by layers of Al and Na coordination polyhedra sharing vertices and edges. These sheets, parallel to the a axes, are connected to each other in the c direction by PO4 tetrahedra and H-bonds. This structural feature accounts for the perfect {001} cleavage of wardite and explains the change that occurs in lattice parameters when Al is substituted by Fe in avelinoite. The relationships with the minerals of the trigonal families of crandallite, woodhouseite, and jarosite are also discussed.

Well-resolved Mössbauer spectra of several dioctahedral and trioctahedral micas have been obtained and subjected to detailed computer analysis. Most of the spectra can be resolved into two ferrous quadrupole doublets and one ferric quadrupole doublet. In dioctahedral micas, ferrous iron is seen to occupy the larger, more symmetric octahedral site in preference to the smaller, less symmetric site, confirming predictions made on the basis of structure alone. In trioctahedral micas there is still a tendency for ferrous iron to occupy preferentially the more symmetric octahedral site even though the two octahedral sites are no longer distinguished by size. A lithium-rich biotite gives a spectrum typical of that expected from a mica with a zinnwaldite structure, but the spectrum of a more authenticated zinnwaldite could not be resolved sufficiently for detailed structural interpretation. The spectra of pyrophyllite and talc give results that can be related to the corresponding micas by considering their respective structural differences. In none of the micas studied was there evidence of ferric iron in tetrahedral coordination. The ferrous: ferric ratios obtained from the Mössbauer spectra do not always agree with the chemical values. In several cases the spectra show more ferrous iron than found chemically. It is suggested that the spectral values are more accurate, oxidation on chemical analysis being the most likely source of error.

Grandidierite has been found in hornfelses around a diorite stock on Cuvier Island. Physical properties and electron microprobe analyses are given for two Cuvier Island and two Madagascan grandidierites. The Cuvier Island grandidierite has crystallized under conditions of the K-feldspar-cordierite hornfels facies at temperatures of approximately 600°C. and pressures (Pload) of less than 300 bars.

The constituent manganese minerals of an ore body located at the Lecht mines near Tomintoul include cryptomelane and lithiophorite. The latter, now known to occur extensively in soils, was characterized by X-ray, thermal, and electron microscope techniques. Lithiophorite yields a distinctive differential thermal curve and this was studied in some detail. A platy morphology is revealed under the electron microscope and from selected-area electron diffraction patterns it was concluded that the b parameter of the mineral is 8·73 Å, three times the value determined by Wadsley (1952). As suggested by Wadsley, this could arise from the distribution of lithium ions. Chemical analysis shows that the Lecht lithiophorite is slightly unusual in that it has a high iron and zinc content but is relatively low in cobalt. The associated cryptomelane contains a significant amount of thallium.

A new occurrence of varlamoffite is described, and the results of chemical analysis, differential thermal analysis, and infra-red spectrographic analysis are presented. It is suggested that varlamoffite is of secondary origin resulting from the attack of meteoric waters on stannite.