The cell parameters of 35 members of the arfvedsonite-eckermannite series and catophorites (17 from the literature, 18 newly determined) are presented and an attempt made to relate them to chemical composition. In general, all parameters (including β, obtuse) decrease from arfvedsonite to eckermannite, but for different reasons: a, b, and β (very slightly) as the Mg: Fe2+ ratio and F increase; c as Fe3+ decreases. The apparent decrease in a and b with high Li is coincidental, due to its strong correlation with F, which varies inversely With (OH). In addition, b is increased by high Ca and slightly reduced by high A1.

Also, the empirical and calculated curves relating MgO and total iron as Fe2O3 (weight %) with the Mg ratio, 100 Mg/(Mg + Fe2+ Fe3+ + Mn), for all amphibole groups are given, as a rapid method of obtaining an estimate of the Mg ratio.

The full lists of possible hkl values for an arfvedsonite and an eckermannite are enumerated up to 45° 2θ, Cu-Kα, to illustrate the difficulty in indexing X-ray photographs and diffractometer charts of this group.

Electron probe analyses have been made of eight kaersutites from San Carlos, Arizona, and one from Kakanui, New Zealand; also of olivine, clinopyroxene, ilmenite, and spinel from San Carlos. There is not as yet adequate evidence that the kaersutite is an upper mantle phase.

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.

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.

Confusion over the status of crichtonite is due to the fact that de Bournon included two different minerals in his species: thin hexagonal plates are indeed ilmenite, as has been generally assumed, but the much rarer steep rhombohedra with basal plane are a separate species, with a distinctive X-ray powder pattern (three strongest lines: 3·39 Å,s; 2·875,s; 2·131,S) and a composition (Fe2+,Fe3+,Ti)1·71O3 with Fe2+:Fe3+:Ti near 8:7:33. There is a rhombohedral pseudo-cell containing three oxygens, with α 23° 19′, a 7·117 Å but there is some evidence of twinning, and the true symmetry is probably orthorhombic or lower.

Surface areas of ‘amorphous’-type opals measured by nitrogen-absorption techniques were not consistent either with the surface area of the spheres of about 2000 Å diameter observable by electron microscopy or with the thermal dehydration data. This apparent inconsistency can be explained in terms of a smaller particulate structure of the order of 100–200 Å shown by higher resolution electron micrographs. This structure also accounts for a marked shrinkage shown in dilatometer curves of this type of opal.

Chlorine is estimated in nine hornblendes (0·13–1·12%) and nine biotites (0·19–0·62%) from the Kondapalli charnockites for the first time, using the electron microprobe. The data obtained suggest that these Kondapalli minerals are unexpectedly and unusually rich in Cl, which is well equilibrated in the five available coexisting hornblende-biotite pairs.

It is argued on crystal-chemical grounds that Si deficiency is compensated by the entering of Al, Fe3+, or Ti4+, in this order of preference, into tetrahedral sites. It is pointed out that Ti4+ occurs in tetrahedral coordination in exceptional circumstances only, while Fe3+ relatively easily takes a tetrahedral coordination.

A trachyte plug near Jos contains a suite of inclusions, commonly rounded and of large size, including olivine nodules, pyroxenites, and a variety of basic to intermediate plutonic rocks, as well as albite megacrysts.

The olivine nodules are typical of those found elsewhere in alkaline basic lavas and widely interpreted as upper mantle fragments. Most of the other inclusions contain evidence of crystallization at high pressures, even the syenites, which are interpreted as plutonic equivalents of the trachyte; for the feldspar in them is sanidine-microperthite.

Discovery of this inclusion assemblage in a trachyte provides further support for the thesis that alkaline salic lavas can be generated at sub-crustal depths, by partial melting beneath domically uplifted continental shield areas (Bailey, 1964).

The Nigerian Cenozoic volcanic province lies in an area of regional uplift, and is characterized by scattered small volcanic centres rather than by major eruptive foci. High pressure inclusions are being found at an increasing proportion of these centres, suggesting that the magmas rose directly and rapidly to the surface from the upper mantle levels at which they were all (including the trachyte) generated.

Chemical, optical, and X-ray data are presented for forsteritic olivine, bronzite-enstatite, diopsidic augite, anorthite, and spinel from ultramafic nodules at seven localities in Auckland province. Olivine has a composition range from Fo79–93 for large primary crystals to Fo80–100 for smaller interstitial grains. Orthopyroxenes (En83–96) carry exsolution lamellae of clinopyroxene parallel to (100); clinopyroxenes are mainly diopsidic augites with exsolved bronzite. The plagioclase (An97) has high-temperature characteristics and a lattice structure probably transitional between primitive and body-centred anorthite. Spinels include chromite, picotite, ceylonite, hercynite, and magnetite. Element distribution indicates that the ultramafic assemblages attained equilibrium at temperatures near 1250 °C and at moderate pressures, corresponding to the melting interval of anhydrous basaltic magmas within the uppermost mantle.

The biotites in the pegmatites are Fe2+ type. X-ray analyses indicated that they are 1M or 3T polymorphs. In the colour of the biotite iron plays an important role. From the Fe/(Fe+Mg) ratios, the analysed biotites are assumed to have crystallized between 760 and 500 °C.

Optical, d.t.a., X-ray, and chemical analysis investigations were carried out on serpenttinites from various Egyptian localities and on their associated talc-carbonates. The serpentinites were proved to consist of pure antigorite, lizardite, or clinochrysotile, or of mixtures of any pair of these serpentine minerals. Lizardite, which is reported for the first time in Egypt, is found to be of common occurrence. Talc, tremolite, carbonates, chlorite, olivine, biotite, chromite, magnetite, goethite, and stevensite are the main admixtures. The talc-carbonates consist mainly of talc and magnesite, or magnesite, dolomite, and talc together with minor amounts of antigorite, magnetite, goethite, and occasionally a little chlorite or calcite. Spectrographic analyses of some of the magnetically separated serpentines have been carried out and the origin of the Egyptian serpentinites and talc-carbonates is briefly discussed.

A rapid fusion technique is described in which the Li2B4O7 bead is polished, not crushed. The reliability of the technique is illustrated by a series of precision tests and analyses of standard rocks, using a 2:1 Li2B4O7 : rock-powder dilution. It is also pointed out that such a low-dilution fusion method may be used for Na2O and trace element analyses with only a small loss in sensitivity relative to the unfused-powder method. Any fusion method has the advantages of greatly reducing absorption effects and eliminating particle-size effects.

Previous investigations have been confined to individual members or to small compositional ranges of the amblygonite-montebrasite series. The present study is a survey based on twenty-two specimens in the range Amb14Mon86 to Amb92Mon8 (1·8–11·8 % fluorine), and indicates that the fluorine content may be related both qualitatively and quantitatively to differences in the X-ray powder patterns. Fully indexed powder data to d = 1·68 Å and derived cell parameters are given for four members of the series. There is no evidence of a break in the continuity of the series. The first British occurrence of both amblygonite and montebrasite is reported from a pegmatite vein in aplite at Meldon, Okehampton, Devonshire.