A small specimen at first thought to be natrolite has proved to be a new mineral, for which the name rhodesite is proposed; it is fibrous, with α 1·502, β 1·506, γ 1·515, elongation γ, extinction straight, sp. gr. 2·36. The composition is near 4(Ca,Na2,K2)O. 10SiO2. 7H2O, and the water is all zeolitic.

Rhodesite, from Kimberley, S. Africa, has been examined by electron-microscope, X-ray, and other methods. The acicular crystals are orthorhombic, with a 23·8, b 6·54, c 7·05 Å.; elongation c, cleavage (100); unit-cell contents approximately (Ca,Na2,K2)8Si16O40.11H2O; optical properties α 1·502, β 1·505, γ 1·515; α = b, β = α, γ = c. With it occurs a similar but distinct fibrous mineral, which appears to be a new species for which the name mountainite is proposed; it is monoclinic, with a 13·51, b 13·10, c 13·51 Å., β 104° elongation b, cleavage (001); unit-cell contents approximately (Ca,Na2,K2)16Si32O80.24H2O; optical properties α 1·504, β 1·510, γ 1·519; β = b (elongation). Both rhodesite and mountainite show close similarities to the fibrous zeolites, mountainite in particular resembling thomsonite. The possible structural basis of these similarities is discussed.

Chemically analysed samples of magnetites carrying exsolved ulvöspinel and ilmenite have been heated in vacuo at various temperatures without change in overall chemical composition and the changes occurring studied microscopically, magnetically, and by X-rays.

Magnetite with exsolution bodies of ulvöncl can be homogenized probably below 600°C.; thermomagnetic curves and microscopic textures suggest that the solvus curve is very steep at the Fe3O4 end of the Fe3O4-Fe2TiO4 series.

On heating magnetite containing ilmenite lamellae up to 950°C., some interchange of ions between the phases occurs (Fe3O4+FeTiO3 → Fe2TiO4+Fe2O3, the ilmenite lamellae taking Fe2O3 into solid solution. The accompanying host magnetite not only gains Fe2TiO4, but at the same time dissolves some FeTiO3. Above 950°C. extensive decomposition takes place, with the production of pseudobrookite, and homogenization has not been achieved. It is suggested that under magmatic conditions extensive solid solution exists in which the FeTiO3 component may be regarded as having a spinel (γ) structure; after exsolution, monotropic inversion may occur to yield a rhombohedral ferri-ilmenite (Chevallier et al., 1955), which on slow cooling changes in composition towards FeTiO3.

Magnetite with exsolution bodies of both ulvöspinel and il menite shows on heating the features of both the above binary types.

The optical properties and specific gravity, and the variations in these properties, of analysed plagioclases separated from the meteorite (eucrite) from Juvinas, France, are compared with the corresponding values for a terrestrial plagioclase of almost identical composition from an allivalite from Rhum. The range in properties for the Juvinas plagioclase indicates a compositional range from about Ab35An65 to Ab15An85, but the great majority of grains are bytownite with approximately 83 % (molecular) anorthite.

The frequency of occurrence of the twin-laws of the Juvinas plagioclase resembles that of plagioclase from basalts and allied rocks.

The variable changes and, in some grains, absence of change in the optical properties shown by the Juvinas plagioclase crystals, after heating to 1100°C. for 48 hours followed by rapid cooling, contrasts with the small but constant optical changes shown by the allivalite plagioclase (for which geological evidence indicates slow cooling) after identical heat treatment. It is inferred that the meteoritic plagioclase grains have had differing thermal histories, and that some grains have been subjected to previous natural quenching.

This saponite, which is closely associated with zeolites such as stillbirth, heulandite, thomsonite, and gonnardite, occurs in vesicles in basalt. It has the composition: 1·02M+ (Mg5·84Mn0·01Al0·03Fe0·08) (Si6·99Al1·01)O20(OH)4, according to a new method for calculating the ionic formulae of smectites: this method also enables determination of the temperature at which all sorbed moisture is removed—in this instance about 800°C. The exchangeable cations are largely Ca2+. The chemical, thermal, and X-ray data are discussed with reference to other saponite samples. Electron-microscope examination showed a tendency to elongation in the direction of the α-axis. It is concluded that the saponite is of excellent purity and free from interstratification.

An illite separated from an Upper Old Red Sandstone sediment at Denholm Hill Quarry, Roxburghshire, when examined by optical, X-ray, and chemical methods, appeared to be closely similar to Fithian illite. Its composition was , where M+ for the Ca-saturated sample was [(H3O)0·58Ca0·10Na0·06K1·12]. The differential thermal curve showed two peaks: one at 550° C. and the other at 600° C. (i.e. somewhat intermediate between the Fithian and Ballater illites). The dehydration curve was also intermediate. Electron-microscope examination showed rather better crystallization than in Fithian illite. The possible reasons for variation of the illite peak temperature are discussed.

A small isolated outcrop in a large area of charnockites in the Mawson area, MacRobertson Land, Antarctica, consists of enstatite-sapphirine-cordierite rocks. Other highly magnesian outcrops occur in the district and these rocks may be completely recrystallized xenoliths. Analyses of the country rock, the two sapphirine-bearing rocks, and the sapphirine are given; the country rock is very rich in iron and has a high FeO : MgO ratio, while the sapphirine-bearing rocks are very low in iron and high in MgO, approximating to ternary mixtures of MgO, Al2O3, and SiO2.