Chemical, optical, and X-ray data are presented for pyroxenes from a representative selection of chondrites. The Mg-Fe pyroxenes originally crystallized above 1000-1200 °C as the protoenstatite-protohypersthene polymorph, and inverted on cooling to the polysynthetically twinned clinoenstatite-clinohypersthene polymorph. In most chondrites the latter became converted to orthopyroxene during recrystallization at temperatures ranging up to 900 to 950 °C. The contents of certain minor elements changed during the clino-ortho inversion. Ancestral protoenstatiteprotohypersthene may have been slightly non-stoichiometric. Accessory Ca-rich pyroxenes are discussed briefly.

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.

Glass-ceramics based on the mineral cordierite have good mechanical and thermal properties. They can be easily prepared from glasses whose compositions lie in the primary phase field of cordierite on the 5 % CaO plane of the CaO-MgO-SiO2-Al2O3 system. For uniform crystallization in the bulk, addition of a suitable nucleating agent is essential. The addition of 10–12 wt % of TiO2 is considered to be suitable for both ternary and quaternary glasses. During controlled heat treatment of these glasses extensive phase separation occurs. The nucleation and crystallization processes that follow have been studied by differential thermal analysis, X-ray diffraction, and light and electron microscopy. Some of the physical and mechanical properties of these glass-ceramics have been measured.

The macroscopic features of a number of Gibeon meteorites have been reviewed and selected specimens have been examined in more detail by metallographic, microprobe, and X-ray diffraction techniques.

A number of specimens appear to have been reheated by man and in these cases reheating temperatures are indicated. Specimens that show no signs of artificial reheating are discussed in relation to the form, distribution, and structure of the kamacite, daubréelite, and troilite. X-ray examination of the kamacite reveals partly annealed shock damage. Small monomineralic inclusions of daubréelite appear to be concentrated in the non-octahedral kamacite and show no signs of alteration By contrast all the troilite has been remelted and has dissolved contiguous plessite and daubréelite. The remelting of troilite appears to have been induced by shock at temperatures below about 450°C.

The principal data are collected about the fall and the distribution of the fragments of the Siena, Italy, meteorite (11·6°E., 43·1°N.). A complete individual, weighing 110·55 g, is described in some detail. Crust morphology, mineralogical composition, and structure were studied. Optical data were established by microscopical analysis and both thin and polished sections were observed. Compared with available electron-probe analysis they are found in good agreement. The chemical analysis is Fe 12·93, Ni 1·39, Co 0·09, FeS 5·46, SiO2 37·10, TiO2 0·14, Al2O3, 3·91 FeO 11·46, MnO 0·35, MgO 23·81, CaO 1·63, Na2O 0·90, K2O 0·16, P2O5 0·44, H2O+ 0·10, H2O− 0·00, Cr2O3 0·56, total 100·43. Siena is an ordinary chondrite, with relatively ambiguous characteristics and evident features of recrystallization and metamorphism.

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 distribution of kamacite, taenite, and plessite suggests that kamacite nucleated at the sulphides, which appear to have been plentifully dispersed in the parent taenite. Such a process could take place without the undercooling that is usually assumed in normal octahedrites. The plessite fields show internal boundaries that are probably relics of grain boundaries in the parent taenitesulphide aggregate. Microprobe investigations have been made of nickel distributions in and between kamacite, taenite, plessite, and schreibersite, and particular attention has been paid to the nickel distribution at the internal grain boundaries in the plessite. Evidence is presented for the diffusion of nickel along these boundaries and for their migration during the later stages of formation of the microstructure.

Harmotome, associated with analcime, is described from andesite dykes at Rehia and Maungarahu in the Tokatoka district, New Zealand. The Maungarahu harmotome has the composition: (Na1·03K0·23)(Ba1·54Ca0·18)Al4·66Si11·31 12·39 H2O; physical properties: α 1·498, β 1·503, γ 1·506, 2Vγ 75–82° β:[001] 28° D 2·378; and cell constants: a 9· 906±0·006, b 14·119 ±0·003, c 8·712±0·007 Å, β 124° 57′ ± 3′. The harmotome is Ba-poor, and 2 Na+ has substituted for Ba2+ on a valency basis. Analcime associated with the Maungarahu harmotome has chemical composition: (Na15·16 K0·03Ca0·09)Al15·52Si32·47 15·93 H2O and physical properties: n = 1·483, D = 2·257, a = 13·708 Å. The zeolites are believed to have crystallized from Na and Ba-rich alkaline solutions at approximately 200°C The source of the Ba is baryte concretions in adjacent limestones.

The kamacite contains a general precipitation of micrometre-sized particles of exsolved taenite. Larger particles are found decorating Neumann lines and at kamacite grain boundaries. The coexisting kamacite and taenite appear to have nickel contents of 6·4 % and about 53%, which are consistent with a temperature of about 330–60 °C on the binary iron-nickel equilibrium diagram.

Electrostatic calculations estimated the energy of hypothetical ion clustering in intermediate plagioclase feldspars. The calculated energy appears too great to allow segregation of Ca and Na atoms into domains upon annealing without a corresponding movement of Al and Si from an original disordered structure. The calculated energy may be sufficiently small that Ca and Na atoms can disorder partially without movement of Al and Si upon heating a low-plagioclase to high temperatures.

A specimen from Långban, Sweden, was found to consist largely of sjögrenite in which segregation and ordering of the metal cations had occurred, yielding an intergrowth of regions of two main types with compositions probably of or near Mg2/3Fe1/3(OH)2(CO3)1/6(H2O)0·4 and Mg12/13Fe1/13(OH)2(CO3)1/26(H2O)0·8. To a smaller extent segregation had proceeded further, giving regions probably approximating to the end-member Mg(OH)2.H2O. It is suggested that the names sjögrenite and pyroaurite be used for the 2H- and 3R- polytypes of this structure irrespective of Mg:Fe ratio, segregation, or cation ordering.

The X-ray powder diffraction pattern of synthetic anorthite crystallized from a CaO-Al2O3-SiO2 glass having the anorthite (1:1:2) molar ratio is identical with that reported in the literature, and also with that of a natural Japanese anorthite specimen. Increasing the CaO or SiO2 content of the parent glasses used for crystallization studies has no measurable effect on that portion of the powder pattern attributable to anorthite. However, glasses containing an excess of Al2O3 ranging from 5 to 10 mol % gave, after crystallization at temperatures below c. 1150 °C distinctively different powder pattern. Several powder reflections that are normally strong, such as , etc., were found to be virtually absent. Moreover, the chemical excess of Al2O3 did not appear as a separate alumina-bearing phase. Upon reheating these anorthites to temperatures above 1200–50 °C or upon crystallizing a fresh portion of alumina-rich glass above 1200-50 °C, only the normal anorthite powder X-ray pattern was obtained; the pattern also contained some reflections due to corundum (α-Al2O3). Examination of the anomalous low-temperature anorthite by electron diffraction shows that the apparent absence of some powder lines is caused by both albite and Carlsbad twinning, which occur on an intimate scale not exceeding a few unit cell repeats. From the unique manner of its occurrence, the twinning is believed to be associated with the inclusion of an excess of Al3+ in the anorthite.

Results of an electron microprobe study of the metal phase and perryite inclusions in the metal of Horse Creek, Mount Egerton, and Norton County enstatitic meteorites are given. The origin of the ‘pseudo-octahedral’ structures observed in the metal phase of the Horse Creek and Mount Egerton ‘anomalous enstatitic meteorites’ is discussed.

The xenoliths, which are gabbroic in character, consist of kaersutite, titanaugite, plagioclase, and iron ore in variable amounts. Biotite and olivine are also occasionally present. New analyses and trace element data are given for two xenoliths and their coexisting kaersutite, titanaugite, and plagioclase. After discussing several possible origins with the aid of a principal latent vector variation diagram, it is concluded that they represent the accumulation of phases that crystallized from the Tristan magma at depths between 5 and 25 km. Evidence is also presented to show that the parental Tristan magma may be of trachybasalt composition.

An X-ray powder pattern of idocrase (vesuvianite) from Canzoccoli, Italy, has been indexed by the method described by Nedoma (1968a, b). The improved X-ray data collected in the paper can be useful in identifying this mineral and in discussing the influence of diadochic substitutions on the lattice parameters of idocrase.

The system sanbornite-celsian has been investigated by the quenching method, and the results compared with earlier studies. The system is essentially a simple binary eutectic system with little or no solid solution. The eutectic is located at a composition of 69% sanbornite and 31% celsian (by weight) and at a temperature of 1227±3°C. No evidence for substantial solid solution of celsian in sanbornite, as reported in previous work, was found. Paracelsian, a naturally occurring polymorph of celsian, was not encountered in the study.

Iron-rich olivines have been oxidized in air in the laboratory and the mechanism of their breakdown has been elucidated using X-ray diffraction and electron microscopy. Low-temperature oxidation (500–800 °C) produces well-oriented hematite- and magnetite-like precipitates together with amorphous silica. The reaction is a cellular one in which thin needles of oxide about 50–100 Å apart grow into the matrix separated by regions of amorphous silica. Nucleation of spherical colonies of the iron oxide and silica occurs on dislocations.

Although the hematite or magnetite always shows the same topotactic relationship with the matrix, the direction in which the needle-like precipitates grow is determined by the orientation of the nucleating dislocation. The small size and highly distorted nature of these precipitates accounts for the diffuseness of their X-ray reflections.

Oxidation at 1000 °C produces undistorted equiaxed grains of the oxides about 0·2 μm in size. They are surrounded by silica, which produces a disordered electron diffraction pattern. As the temperature is raised, the silica achieves more structural order and the oxide grains increase in size.

Chemical analyses, optical properties, and specific gravities are presented for nine axinites. A reciprocal substitution relationship is demonstrated between Mg and Mn2+, and a linear variation diagram is constructed between refractive indices and the (Fe2++Fe3++Mn2++Ti) ions calculated on the basis of 16(O,OH). The composition of these axinites is independent of the composition of their host rocks; their colour appears to be independent of the Fe2+/Mn2+ ratio for the range of iron and manganese values of the specimens investigated.

A review is given of the metallurgical literature relating to the formation of metal rodeutectics, which are texturally similar to myrmekite. Since no satisfactory explanation for rodeutectics has been advanced, a comparison is made with myrmekite. It is proposed that this intergrowth develops as a result of the constriction of quartz during its recrystallization and inclusion in an expanding blastic growth of plagioclase, and the forces involved lead to the prolongation along the growth directions of quartz in the form of rods with a relatively small surface area (i.e. with circular cross-sections). Possible forces that could produce the same geometry in metal intergrowths are those that result from the relative contractions of the two components during freezing; in all cases for which quantitative data are known, the metallic rods have a greater contraction on freezing than the host substance. Examples of myrmekite and myrmekite-like intergrowths in the Constant Gneiss are described in relation to their particular origin. Two factors, the lack of proportionality of qtartz to feldspar and the intimate association of myrmekite with myrmekite-like intergrowths, support the proposed mechanism of constriction of pre-existing quartz.

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.