A study of phase equilibria among the orthosilicates on the Ca2SiO4—CaMgSiO4 join was undertaken with the use of hydrothermal techniques in the temperature range 600–900°C. at water pressures from 1000 to 15000 psi. Monticellite, merwinite, and Ca2SiO4 crystallize as pure phases throughout this temperature range, and intermediate compositions contain mixtures of the end members. Powder X-ray diffraction data obtained on the mixtures indicate no change in lattice dimensions from the end members. Hence, very little or no solid solution is indicated at these temperatures. Ca2SiO4 was crystallized as the γ-form below 675°C. (at 2000 psi) and the β-form above this temperature (presumably having inverted from the α′-form on cooling to room temperature). Infra-red absorption spectra obtained on the compounds γ-Ca2SiO4, β-Ca2SiO4, merwinite, and monticellite show characteristic differences in the pattern within the 9–13 micron region. All the data thus appear to confirm merwinite as a unique compound stable in the temperature range studied.

X-ray diffraction data on serpentine minerals are discussed. There arc three structural varieties of chrysotile: ortho-chrysotile, clino-chrysotile, and para-chrysotile. Methods are described for the estimation of the proportions of the first two of these in mixed specimens, and their distribution in nature is reviewed. The variations observed in powder photographs of serpentine minerals are interpreted in accordance with this classification. The criteria proposed by Selfridge for distinguishing between chrysotile and antigorite are shown to be unreliable, but valid criteria are presented. The powder photographs also reveal the existence of a third serpentine mineral, distinct from chrysotile and antigorite, for which the name lizardite is proposed. Four samples of bastite are shown to consist of chrysotile or lizardite rather than antigorite. All the serpentine minerals examined consist either of antigorite or of one or more of the group comprising lizardite and the chrysotile varieties.

Evidence from four recent boreholes in the Yorkshire potash field suggests that carnallitic was an important original constituent of the Middle Evaporite Bed, and that much of the sylvine of the potash zone is of secondary origin.

The carnallitie rocks have been affected by complex replacements, an earlier series involving sylvine, halite, and anhydrite, and a later series involving rinneite, halite, sylvine, and carnallitic.

The surface structure of blende crystals from Joplin, Jasper County, Missouri, U.S.A., has been explored by the application of phase-contrast microscopy and light-profile microscopy. Attention has been specially directed to the small-scale features with thicknesses (or depths) of molecular dimensions.

Some growth features of special interest were observed. A large number of beautifully developed growth spirals were observed on the tetrahedral face of a blende crystal. These are in accord with the dislocation theory of Burton, Cabrera, and Frank. The high visibility of these growth spirals is possibly due to the preferential deposition of some impurity, or etching, which also gives a mottled appearance to the entire surface. In addition, an extensive microscopic structure has been observed on some crystal faces. Triangular growth terraces and markings are seen, both orientated parallel to the edges of the crystal. Some triangles, inverted with respect to the latter, have been explained on similar lines to the well-known ‘growth trigons’ on the octahedral faces of diamond as arising from imperfect dislocations.

An attempt has been made to explain the growth features in the light of known theories of crystal growth.

The space group Ccmm of the accepted staurolite structure (7·82:16·52:5·63 kX 0·473:1:0·341, Náray-Szabó 1929) is a pseudo-space group; the true one has no c glide plane. Staurolite is monoclinie pseudo-orthorhombic, as shown optically by horizontal dispersion and morphologically by unequal developments of forms r(201) and r′(01). It has variable cell dimensions, 7·83–7·95:16·50–16·82:5·62 5·71 Å., constant β 90° + 3′, aspect C*, and probable space group C2/m (negative pyroelectric test). This leads to the prediction of a new type of twin, ‘by high order merohedry’, a penetration twin that simulates an orthorhombic single crystal. It is known (Friedel, 1922) that the 90°-cross can be accounted for by two different twin laws and the 60°-cross by five. The twin operations are respectively the pseudo-symmetry operations of a pseudo-tetragonal cell, obtained by transformation 010/003/100, and those of Mallard's pseudo-cube, resulting from transformation 013/03/300. By precession methods more than one twin law is established for each type of cross: both laws [100]90° and [013]180° are found for 90°-crosses: [313]180 and [102]120°, for 60°-crosses.

Small crystals (2 mg.) of plagioclase and potassium felspar were irradiated for four weeks in an atomic pile. Subsequent analysis of the radiation showed that this is a possible method for the determination of the sodium and potassium content, and it should be possible to develop the method further in this direction. It has not proved a suitable method for the determination of calcium.

Conclusions concerning the unit-cell contents and chemical formula of a mineral are all too often drawn from a small part of the available data. A procedure is outlined by which any chemical analysis for which a density is available can be utilized, provided X-ray data are available for a reasonable range of analysed specimens. The effects of possible errors in the determination of water, and in the assessment of essential and non-exsential water, are discussed both qualitatively and quantitatively.

A survey of all available data for anthophyllite has not disclosed any specimens in which the number of oxygen atoms per unit cell is significantly in excess of 96 with the possible exception of the Glen Urquhart gedrite and the Edwards (New York) material. But it is certain that the number of cations per unit cell is normally well in excess of 60 (partial occupation of the A lattice positions), and that, although ‘excess’ water in sonic fibrous anthophyllites may be adsorbed impurity, the number of hydroxyl groups in others is well in excess of 8 per unit cell; it is also fairly clear that the number of hydroxyl groups may fall below 8 per unit cell.