Descriptions, with chemicaland mineralogical data, are given of zoned wollastonite-grossular-clinozoisite skarns found at the contact of limestone and granite at Puyvalador, near Quérigut, France. It is shown that the silica and alumina necessary for the formation of the skarn were supplied by the adjacent granite, which in return received lime from the marble replaced by the skarn zones. The modified facies of the granite in contact with the skarn is also described; it shows evidence of migration of silica towards the contact and of accumulation of lime derived from the neighbouring marble.

Similar, but not identical, changes are found at other localities along the Quérigut granite contact with limestone. The features of the Puyvalador granite-limestone contacts are compared and contrasted with those described from some other regions. The contact rocks dealt with here are characterized by the presence of abundant clinozoisite, indicative of relatively low temperatures and abundance of water at the contact in the later stages of evolution of the granite.

A new analysis gives the formula Ca4Si3O12F2H6; the specimen was from Alter Berg, Leitmeritz, Bohemia, and also contained apophyllite. The unit cell is triclinic, but all reflections can be indexed on a geometrically monoclinic cell which is c-face centred and has a 16·2, b 9·34, c 13·2 Å., β 113°, Z = 6, principal cleavage (001). This cell could also be described as having a hexagonal base of side 9·34 Å., but with c tilted through 23° from the vertical in the plane of one of the sides. A dehydration curve showed a step at 300–400°C., corresponding to loss of two-thirds of the water. The remaining water is lost between 400° and 940° C., together with some fluorine, the latter probably as HF. Crystals heated at 450–600° C. retain the hexagonal-based cell, but d001 shrinks from 12·1 to 11·1 Å, and there is evidence of much stacking disorder of layers normal to c. Single crystals heated at 700° C. show ordered transformation, giving cuspidine twinned in three orientations as the only recognizable product. At 940°C. a mixture is obtained, mainly β-CaSiO3 with some β-Ca2SiO4.

The high-temperature phase equilibrium relationships in the system diopside-anorthite—åkermanite have been investigated by the quenching method. This system may be regarded as essentially a simple eutectic system. The eutectic is located at the composition 9 diopside:44 anorthite:47 åkermanite (by weight) and at a temperature of 1226° C. Åkermanite is a stable phase at least as low as this temperature. Triclinic anorthite is the only form of CaAl2Si2O8 encountered in the liquidus range of temperatures.

The angular separations (2θ (131)+2θ (220)−4θ and (2θ for 111 analysed plagioclase felspars have been determined from diffractometer records. From An0 to An70 the petrological environment has a strong influence on these separations, and if the An-content is independently known an accurate estimate of the structural state can he made from the first of these angular separations. From An70 to An100 there are only small variations in both separations with An-content and structural state. From An85 to An100 inconsistencies in the data suggest that other factors can influence any interpretation so seriously that only qualitative or semi-quantitative deductions may be made. Consideration of the angular separations and subsidiary reflections of sodie plagioclases from a wide range of geological environments has led to the novel hypothesis that in this composition region even plutonic specimens are in a metastable state with slightly disordered structures.

The value to the petrographer of the available methods for the estimation of An- content and structural state is critically discussed. Although it is thought that the standard optical curves in present use are in need of some revision, the determination of optical orientation by universal-stage methods is probably adequate for routine studies. For more detailed investigations, this may be supplemented by X-ray powder data and refractive index measurements, whilst a complete description of the felspar requires the addition both of chemical analysis and single-crystal X-ray work. In particular, the efficacy of the various X-ray methods that have been proposed is discussed, and recommendations of the most suitable method for various composition ranges are made.

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.

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.