The investigation which has led to this description of a new mineral began because of a suggestion by Dr. W. F. Foshag, when on a Visit in 1951 to the Mineral Gallery of the British Museum (Natural History), that a brown cut gemstone exhibited as olivine had perhaps been incorrectly determined. More recently, Dr. Foshag has stated that the idea came from Dr. George Switzer, who as a result of an X-ray powder photograph taken in June 1950 of a similar specimen in the collection of the United States National Museum in Washington concluded that his material was not olivine and was likely to be a new species. The present work would not have been pursued had it been realized at the time that Dr. Switzer intended to continue the study when he had suitable material for analysis.

Ejected blocks of piutonic igneous facies have long been known in the trachytic and basaltic tufts of Green Mountain and Middleton Peak on Ascension Island, and a petrographic account of them has been recorded by Professor R. A. Daly in his memoir on the geology of Ascension Island.

In the last twenty years quartz has become of considerable importance to the telecommunications industry, one of its most important uses being the control of the frequency of oscillation of high-frequency transmitters. In this country alone the peak war-time demand for oscillator crystals was well over a million a year.

Although quartz is very abundant in the earth's crust it is usually found in the polycrystalline state or as small crystals. Large crystals, reasonably free from twinning and flaws, are relatively rare and most of the supply for piezoelectric purposes comes from Brazil.

The X-ray work described below has been done on crystals of bredigite obtained from two different sources, the natural mineral from Scawt Hill, County Antrim, and the synthetic mineral from spiegeleisen slags. Both types of material have been fully deseribed by Tilley and Vincent (1948). Their analysis shows that the slag mineral is a calcium orthosilicate in which Ca is partly replaced by Mg, Mn, and Ba, the extent of the replacement being represented by the formula (Cal.59Ba0.08Mg0.31Mn0.09)SiO4. Tilley and Vincent conclude that this orthorhombic (pseudo-hexagonal) phase is identical with the high-temperature form α' of pure Ca2SiO4, the existence of which was suggested by Bredig (1943a), and confirmed by TrSmel (1949) using a high-temperature powder camera. Bredig (1943a) also suggests, on the basis of a comparison of X-ray powder patterns, that the crystal structure of α'-Ca2SiO4 is similar to that of β-K2SO4. The present single-crystal work was originally undertaken in order to investigate the structure of α'-Ca2SiO4 more fully.

Some specimens of baryte, recently obtained from Westmorland, show up to six changes of habit during growth. This was elucidated with the aid of the colour-zoning and fluorescent zones. A change of habit is also recorded in one specimen from County Durham.

The tholeiite of Kinkell (11 km. north of Glasgow G.P.O.) was described by the first author (Walker, 1930) and an analysis of its residual glass was given in a later publication (Walker, 1935). Since unaltered and separable residual glasses of basaltic rocks are rare, the Kinkell example has been referred to frequently in petrological literature. E. A. Vincent (1950) has made a detailed comparison between the Kinkell glass and a similar one from a tholeiite from Kap Daussy, east Greenland. In view of this and of the important part played by such glasses in discussions on basaltic differentiation, all the major constituents of the Kinkell rock have been isolated and analysed chemically and spectrographically. They are described below together with a fuller account of the field relations of the rock and some comments upon the significance of the new data.