Tertiary alkali basaltic rocks from the Southern Highlands, New South Wales, contain two types of clinopyroxene crystals, which exhibit excellent morphological development of hour-glass zoning. These are small, quench clinopyroxene crystals and large, abundant, euhedral phenocrysts. Electron microprobe analyses show that the different zone sectors derive from compositional differences, with variation in TiO2 and Al2O3 contents inversely with SiO2 content being most significant. Increased depth of colour of titanaugites depends on increased coupled substitution of Al for Si in tetrahedral sites and of Ti3− in an M1 site, while pleochroism is enhanced by increasing Fe content. It is suggested that the formation of hour-glass zoning is determined by an environment of crystallization where initial crystal growth is rapid relative to ionic diffusion in the melt, resulting in different chemistry for different growth directions within the crystal. This implies that, under certain conditions, kinetic factors, as well as the chemical composition of the host magma, may be significant in determining the chemical composition of phases precipitated, particularly where more than one crystallographic direction of growth is possible.

The mineral chernovite, described as a new mineral from the Urals by Russian mineralogists in 1967 (Goldin et al, 1967), has been found at three different localities in mineral fissures in the Binnatal, Switzerland, and a little to the south, in Italy. The mineral occurs in greenish-yellow bipyramidal crystals up to about I mm. A detailed single-crystal study of several specimens showed that the lattice constants vary considerably on either side of those published for the original chernovite. The refractive indices, too, display some variation. Microprobe analyses of this material, and of xenotime specimens from the same region, revealed that there exists at least partial solution between chernovite and xenotime. The three chernovite samples have molar percentages of 63·2, 73·2, and 82·0 % YtAsO4 (the Urals chernovite has about 95·2 %). These studies prove the existence of an isomorphous series between xenotime and chernovite, at least in the As-rich portion.

A new variety of heterogenite was found in samples from Mindigi, Shaba, Zaïre. Photometric reflectance varies from 0·235 to 0·165 at λ 5890 Å in air. The bireflectance is 0·070, the degree of anisotropy is 0·7 and the main index in air is 2·7. A thermal weight-loss curve and a microprobe analysis are given and discussed, indicating the formula (Co,Ni) O.OH.

The new variety, referred to as heterogenite-H, is hexagonal with cell dimensions a 2·85, c 8·80, space group P63/mmc and cell-content 2 [(Co, Ni)O.OH)]. Indexed X-ray powder data are given. A crystal structure is suggested based on CoO.OH octahedra layers and indicating a polytypic relation between heterogenite-H, the 2H form, and normal heterogenite, the 3R form.

A kinetic and X-ray study of the dehydroxylation of datolite, CaBSiO4(OH), has shown that the decomposition occurs very rapidly above 700°C in air, with an activation energy for the reaction of the order of 200 kcal mole −1. The transformation is topotactic, the dehydroxylated phase being tetragonal with a 7·14 Å, c 4·82 Å, and particularly well formed even at the lowest temperatures of decomposition. Single-crystal studies have shown that two orientations of the new phase exist and that the original a of datolite becomes the unique axis of the tetragonal phase while the tetragonal a axes are oriented either parallel to or at 45° to the b and c axes of datolite. The new phase appears to be a boron-containing analogue of the melilite structure, composition Ca2SiB2O7, but is metastable. The basic sheet structure is preserved during the transformation but a reorganization of the tetrahedral layer from the 4- and 8-membered rings of datolite to the 5-membered rings of the new phase is involved, together with effective removal of protons and some silicon. The transformation can be explained in terms of an inhomogeneous reaction mechanism involving migration of calcium and boron into the new phase domains and counter-migration of silicon and protons, but with only minor readjustment of oxygens. The high activation energy of the reaction is explicable in terms either of the effort necessary to remove silicon from the domains of the new phase or of the difficulty of moving the large calcium ions through a relatively close-packed structure.

Single-crystal diffraction patterns produced by transmitted radiation, both X-rays and electrons, reveal varying degrees of disorder and long-range order in common phyllosilicates from several different rock types. The transmission electron micrographs and their selected-area diffraction patterns demonstrate the presence of numerous stacking faults parallel to (001) of muscovite and chlorite. Individual stacking faults can be recognized by the diffraction-contrast fringe patterns they cause, and partial dislocations can be seen where such faults terminate inside a crystal. Long-range order of muscovite explains what seemed to be spurious, high ‘background’ levels that are sometimes encountered in the analysis of rock fabrics by transmitted X-rays.

Osumilite of composition closely approaching that of the K,Mg end-member occurs as a significant constituent of tridymite-enstatite-feldspar-cordierite-hematite-magnetite buchites at the contact of the Tertiary dolerite plug of Tieveragh, Co. Antrim; it is proposed that the mineral be known as osumilite-(K,Mg). Despite laboratory demonstration of the metastability of osumilite-(K,Mg) over a wide pressure-temperature range, the unaltered character and apparently constant composition of the Tieveragh osumilites raises the possibility of an osumilite stability field within the system K2O-MgO-FeO-Al2O3-SiO2.

An unusual paragenesis of chromite, nickeline, cordierite, and red-brown mica occurs in veins, pods, and schlieren in serpentinized ultramafic rocks at La Gallega and Los Jarales in Máilaga province, Spain. The chromites in this paragenesis are low-ferric, ferrous-rich aluminous chromites with exceptionally high vanadium content ranging between 1·8 and 2·9 wt % V2O3 and high zinc content ranging between 0·5 and 1·0 wt % ZnO. A comparison of chromite composition with available experimental data, together with paragenetical and textural evidence, suggests that the chromite, cordierite, and red-brown mica crystallized at magmatic temperatures between approximately 1300° and 1000°C at low oxygen fugacities and maximum pressures below 10 kb, probably near 5 kb.

Barea and Emery are typical mesosiderites, whereas Dyarrl Island is unique in having the lowest metal content and the highest FeO/(FeO + MgO) ratio of any meteorite of this class. Average pyroxene compositions have been determined for the individual mesosiderites, and show a sequential increase in FeSiO3 content, ranging from a mean of 21 mole % for Veramin to 45 mole % for Dyarrl Island. Steinbach may possibly be classed as a mesosiderite, since its mineralogy is identical except for the absence of plagioclase. Dalgaranga and Mt. Padbury may be a paired fall. The chemical and mineralogical composition of the silicate material of the mesosiderites is similar to, but not identical with the howardites; the overall range of composition is greater in the mesosiderites.

The Pre-Cambrian granitic rocks of Bygland are structurally and texturally concordant with the metamorphosed country rocks. Both these rock types plot close together in Q-Or-Ab and Ab-An-Or normative diagrams; trace element distribution is also similar. The granite has been reconstituted from the country rocks under epidote-amphibolite facies conditions. Plagioclase feldspars show that the granite is not a ‘mature’ rock; they are, however, ‘mature’ in the country rocks and are products of typical regional metamorphism (Ohta et al, 1968). Plagioclases in the granite are relicts of country rocks showing traces of a premetamorphic crystalline state. They have been rejuvenated with the introduction of alkali material during the reconstitution.

In the Dahomeyan of the western Accra Plains mafie gneisses, amphibolites, and ortho-amphibolite dykes in granitic gneiss occur. During the latest recrystallization, metamorphic conditions ranged (north to south) from oligoclase-epidote amphibolite subfacies through andesine-epidote amphibolite subfacies to at least the medium or upper part of the amphibolite facies. Chemical analyses of hornblende, garnet, and clinopyroxene from two mafic gneiss specimens supplement data from the Mampong specimen (Knorring and Kennedy, 1958) and indicate that mineral compositions are largely controlled by rock compositions.

Xenoliths of eucritic gabbros in an alpine-serpentinite body at Wairere have been altered to rodingites and xonotlite about their margins by hydrothermal metasomatism at temperatures between 430–470°C A chemical analysis and the optical properties of the xonotlite are reported, the textures of the rocks are described, and the metasomatic process is discussed.

Pale-blue cordierite (complete analysis given; shows total iron oxides only 0·98 %) occurs in phlogopite schist near White Well (24° 28′ 30″ S. 116° 2′ E.) 20 miles north-west of Yinnietharra, Western Australia. Associated minerals are corundum, dravite, and sillimanite, accessory zircon and rutile. The cordierite is in translucent masses to 8 ×8×4 cm with vitreous lustre, D 2·57, α 1.536, β 1·540, γ 1·543, all colourless, 2Vα80° Unit cell a 17·055, b 9·724, c 9·350 Å. The host phlogopite (complete analysis given) is in brown flakes to 1 cm, D 2·88, α 1·558, β 1·585, γ 1·59, 2Vα 5–20° 1 M polymorph.