The main minerals (garnet, pyroxene, hornblende, and scapolite) in certain Dahomeyan (Archaean) garnet-hornblende-pyroxene-scapolite gneisses from Ghana have been separated and analysed. Both the garnet and the pyroxene show marked eclogitic affinities, and the rocks are considered to occupy a transitional position between the amphibolite and eclogite facies. The scapolite is a sulphatic variety containing more than 4 % SO3 and is believed to be a product of autometasomatism. A revised scheme for the distribution of mineral facies within the temperature-pressure field of metamorphism is proposed.

Specimens from some forty different localities over the composition range from about 20 % An to about 70 % An have been examined by X-ray single-crystal methods. The majority of these specimens show the normal intermediate plagioclase pattern characterized by weak pairs of subsidiary layer-lines of variable separation. It has been found that the separation of these layer-lines is a linear function of composition over the whole composition range. Further, this range is found to extend to more soda-rich compositions than was formerly believed, whilst the limits of the range are approximately defined by the compositions at which the separations of the subsidiary layer-lines about either the a- or the b-axes become zero.

The anomalous patterns shown by some specimens are discussed, and it is shown that most of these may be interpreted in terms of the previous geological history of the specimens.

A comparison of the present results with those obtained by previous workers in this field is made, and it is shown how the principal points of difference may be resolved. An account of the phase relationships of the low-temperature plagioclase series is given, and the problems arising from the present work are discussed.

Pyrosmalite is a widespread, though very limited, constituent of the Broken Hill lode, and is formed by the hydration and chlorination of the manganeseiron silicates. It is altered in contact with galena and eventually disintegrates into a mat of very fine fibres. The pyrosmalite varies substantially in ratio of Mn/Fe, and two specimens from the rare vughs and fractures gave on analysis ratios of 1·96 and 0·71. The refractive indices decrease slightly with increasing amounts of manganese and water, with a simultaneous small increase in the unit-cell dimensions. The name pyrosmalite is retained for the mineral species in accord with its earlier usage and its recent limitation to the iron-rich variety of an isomorphous series is not endorsed.

The position of the rhombic section of plagioclase felspars, as calculated from the measured cell dimensions, has been shown to vary with the structural state, the variation from high- to low-temperature forms being most pronounced for albite. The theoretical composition plane of pericline twins is the rhombic section. If the structural state or the composition of the felspar changes after twinning has occurred, the cell dimensions and hence the rhombic section will change; however, the composition plane of the pericline twin will probably remain unchanged unless recrystallization occurs, or perhaps if the felspar is subjected to strong stress. Thus the observed pericline composition plane need not coincide with the rhombic section. Comparison of observed composition planes with the corresponding calculated rhombie sections shows that some of them do indeed differ and the differences have been used to provide information on the changes occurring subsequent to the establishment of the pericline twins. The interpretation is complicated by the occurrence of the acline-A twin, whose composition plane is {001}, for the rhombie section of plagioclase felspars from An0 to An60 may coincide with {001} if the structural state of inversion has a particular value. When the observed composition would agree with both pericline and acline-A twinning, the new term b-axis twin has been used.

A closer understanding of the causes for the variation of refractive index with change of composition has been sought in terms of certain properties of ions. The study is confined to isostructural ionic crystals. It is assumed that polarization of the anions (usually O2− or F−) makes the principal contribution to the total refractive index effect, and that variation of refractive index is determined mainly by variable anion polarization. Polarization of the anion is controlled largely by the field in which it finds itself which, in turn, depends on the effective force of attraction of the cation: ionization potentials are used for the purpose of comparing such forces. For compounds involving pairs of 8- and 18-electron cations (Mg2+ and Zn2+; Al3+ and Ga3+; Si4+ and Ge4+; Ca2+ and Cd2+) or pairs of transition cations (Zr4+ and Hf4+; Nb5+ and Ta5+; Mo6+ and W6+) refractive indices are higher in the crystal that contains the cation of the element with the higher ionization potential. For larger suites of cations of various types (for example, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+, radii from 0·65 to 0·80 Å.) the predicted relationship between ionization potential and refractive index holds precisely for the compounds A2SiF6.6H2O (A2+, etc.), but only in part for other compounds; this may be due to the effect of the crystal field on certain 3d transition cations.

The occurrence of dickite as a rock-forming mineral in sandstones of the Lower Carboniferous Series, the Millstone Grit, and the Middle Jurassic has been reported previously by the authors. In view of the paucity of data on this mineral, six samples were purified and examined optically, by staining tests, and by the X-ray powder method. In addition, a chemical analysis, a differential thermal analysis, and a thermal weight-loss curve were made on the purest sample. The investigations confirm the identity of the material.

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 Eskdalemuir tholeiite has the following modal composition: glass 42·2 %, felspar 24·4 %, pyroxene 30·6 %), magnetite 2 %, chlorite 1%. The glass has been separated and analysed. It has 70% SiO2 and is of granitic composition. On a selected triangular diagram the plots of the Eskdalemuir rock, its residual glass, and its crystallinc portion are spread along a narrow belt marking the compositions of the Tertiary tholeiites. Because the differences in the chemical compositions of the phrases of the Eskdalemuir tholeiite are produced by crystallization, it is concluded that the variety of composition of the other Tertiary tholeiites is consistent with a genesis controlled by crystallization differentiation.

This paper describes a development for the universal stage of the extinction curve method for determining the orientation of the optical indicatrix in biaxial crystals. The extinction curve may be defined as the locus of the poles of the extinction directions when the crystal is rotated between crossed polarizers about an arbitrary but fixed axis perpendicular to the microscope axis.

When two or more extinction curves, or parts of them, are plotted on a stereographic projection, their intersections give the directions of the axes of the indicatrix, of which at least two are directly accessible. The main application of this new orthoscopic method is to provide a means of refining the orientation of the indicatrix obtained by this or any other method. The accuracy that can be obtained for minerals of low birefringence such as the felspars is considerably greater than with the conventional orthoscopic method.

A complete account is given of the details of the plotting procedure.

Cross-hatched twinning is characteristic of two alkali-felspar species, microclines and anorthoclases. Because of the difference in the crystallographic angles of these two species, the cross-hatched twinning is most clearly seen in (001) sections of microcline and in sections perpendicular to the a crystallographic axis in anorthoclases. If pericline twinning is present it is generally seen in (010) sections of microclines, but if the microcline is microperthitic the twin lamellae may not be seen. Strings or lamellae of perthitic albite can be distinguished from pericline twin lamellae by refractive index differences and also by the angle that their trace makes with the trace of (001) in (010) sections.

Chloritoids from the Saxa Vord schists in the Shetland Isles and the Dalradian schists north of Stonehaven have been studied optically and chemically. In both eases monoclinic and triclinic crystals occur with α = b in the monoclinic varieties. Twinning is common with twin axes [110] and [230]. The formula of chloritoid is considered in relation to the structure recently suggested by Brindley and Harrison (1952).

Previous work has established that natural ‘low-temperature’ intermediate plagioclases show single-crystal diffraction patterns in which the subsidiary reflections are split into two; the separation of these split reflections appears to be dependent on the composition of the felspar. Several of these specimens have been subjected to varying heat treatments and their diffraction patterns examined.

It is found that over the whole composition range the split subsidiary reflections have disappeared after treatment at high temperatures, and only the principal felspar reflections, which are characteristic of an albite-like structure, remain. Natural specimens initially showing anomalous patterns can also be homogenized in this way by suitable heat treatments. A careful study of the mode of disappearance shows that the separation of the subsidiary reflections is unchanged as long as they remain visible.

The structural and petrological implications of this work are discussed.

Rare-earth distribution has been determined for fourteen niobatetantalates containing essential rare earths. Those minerals with more than about 15 % TiO2 (and corresponding to either priorites or members of the euxenite-polycrase series) have Yt as the dominant rare earth with the heavier lanthanons in excess of the lighter lanthanons. Those minerals with less than about 4 % TiO2 and less than about 30 % (Yt,Ln)2O3 (and corresponding to samarskites) also have Yt as the dominant rare earth but they show a marked concentration of Gd + Tb + Dy over the other lanthanons. It is tentatively suggested that this power of selective lanthanon enrichment may be characteristic of samarskite among the niobate-tantalates examined.

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.

Isokite (CaMgPO4F), a new mineral homologous with sphene, was described late in 1955 from a Rhodesian carbonatite plug. A fine-grained variety is now reported from Bohemia, where it occurs as a replacement of (probably a hydrothermal alteration product of) coarse medium brown ‘later’ triplites. It is intimately associated with fine-grained apatite that along with quartz formed post-isokite.

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.

Phase relationships in the section CaSiO3-MgSiO3-Al2O3 through the quaternary system CaO-MgO-Al2O3-SiO2 have been determined. The bearing of the results on the genesis of aluminous pyroxenes and associated minerals in metamorphic rocks is discussed.

Geiger-counter powder records of soda-rich plagioclases have been carefully measured and indexed. The lattice parameters of the synthetic specimens are almost independent of the lime content, the largest variation (that in β*) amounting to only 0·2°. After strong heating the powder patterns of the natural specimens closely approached those of the synthetic specimens.

Eight out of the nine natural plagioclases gave lattice parameters which fall within experimental error on a continuous line. There is a fairly large variation in lattice parameters, especially for γ*, which changes by 2°. As the lime content increases the lattice parameters of the natural plagioclases approach those for the high-temperature synthetic and heated natural specimens until at An50 the separation is only one-fifth of that at An0. The ninth specimen (from a dacite) gives parameters midway between the curves, thus exhibiting a state of partial inversion.

The partially heated natural plagioclases and the plagioclase from the dacite give parameters indistinguishable within experimental error from the parameters of unheated plagioclases with a higher lime content. There is, therefore, no reliable powder X-ray method for the determination of composition in the region An0-An50. If the composition is known the powder method may be used for the determination of the thermal state. If the plagioclase can be judged to be in the low-temperature state from geological evidence the powder method can be used to estimate the composition with an accuracy of 2 % An.

The layer structure of the kaolin mineral dickite has been shown to differ significantly from the idealized arrangement of regular tetrahedra and octahedra. Since the structure of kaolinite can be studied only by X-ray powder methods, it cannot be refined beyond the idealized arrangement. By applying the dickite type of layer structure to the ease of kaolinite, an explanation for the triclinic symmetry of kaolinite has been obtained, and also an improved agreement with the observed X-ray intensities.

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.