It has long been known that if a diamond is heated in pure oxygen, or even in air, to a temperature of about 700° C. or higher (depending on the initial quality of the diamond), it will be wholly or partially oxidized to CO or CO2, but that if the diamond is heated in the absence of oxygen—preferably in vacuo—to a temperature of about 1500° C. or higher (again depending on the initial quality of the diamond), it will be wholly or partially converted to graphite. For complete conversion the graphite is always poly crystalline. The partial conversion of diamond to graphite is, however, of considerable interest, because when graphite forms on a single crystal of diamond it exhibits a strong preferred orientation.

As long ago as 1906 I had noticed a yellowish clay-like mineral occurring somewhat abundantly at both Cligga mine, Perranzabuloe, and at Bunny mine, St. Austell. From its appearance the mineral did not look particularly interesting, neither did it suggest being one containing a metal, and for these reasons no attempt was made to determine its character. In a paper read before this Society in 1937 I mentioned this yellowish clay-like substance as occurring associated with rashleighite at the Bunny mine. Soon after the publication of this paper in 1948 I received a letter from Monsieur N. Varlamoff in the Belgian Congo saying he had seen my reference to the 'yellowish clay-like substance' (p. 356) and suggested it might possibly be identical with the mineral varlamoffite which he had recently discovered in the Belgian Congo and which had been named after him, his description of the mineral with analysis, mode of occurrence, and localities having been published in 1948.

In the Geological Survey memoir a dike is recorded cutting the Lewisian gneiss near Achmelvich, north-west of Lochinver, Sutherland. At the time the mapping was carried out no thin section was prepared of the rock, and the dike was presumed to be a continuation of the Canisp porphyry dike which is exposed in the river Inver near Lochinver and farther south-east.

During an investigation of the post-Cambrian sills and dikes of Assynt and the adjoining districts of north-west Scotland, the writer examined the Achmelvich dike in some detail and recognized its affinities with the nepheline-syenites. It is the purpose of the present note to describe the field occurrence and petrography of the dike.

It is a well-known method in the measurement of birefringence to utilize a crystal's 'bevelled edge', on which appears a sequence of polarization colours, from that colour representing small relative retardation where the edge is thin, to a higher colour corresponding to the full thickness of the mineral in the microsection. In dolerites these bevelled edges on augites are extremely common and are very apparent between crossed nicols. In ordinary light, however, the majority of them are undiscernible and a grain appears to have uniform thickness over its entire area. The reason for the non-appearance of the bevelled edge in ordinary light is straightforward: the augites have excellent relief, that of the adjacent felspars is low; relief is a surface phenomena and independent of thickness, so that a thin edge of augite overlying felspar has the relief of augite, and the felspar is unnoticed.

The mineral afwillite is one of the hydrated calcium silicates which are found in set cements. It has the empirical formula 3CaO.2SiO2.3H20; a recent determination of the structure by Megaw (in press) has shown that the structural formula is Cas(SiO3OH)2. 2H2O, the hydroxyl groups being linked directly to silicon atoms. When afwillite is heated it loses water. Parry and Wright observed that practically all the water could be driven off by heating for prolonged periods at 300° C., or for shorter periods at higher temperatures. They also found that a further reaction took place on heating to about 1300° C. The present work was undertaken to investigate these changes, and to identify the products formed, using-X-ray powder methods.