The Skaergaard intrusion of Kangerdlugssuak, East Greenland (68° 10′ N., 31° 40′ W.) contains a series of gabbros and related rocks—the layered series—which has been interpreted as the result of fractional crystallization of a fairly normal gabbro magma (Wager and Deer, 1939). Analysis of these rocks and of some of the constituent minerals by Dr. W. A. Deer has shown a considerable and systematic variation in the major constituents (1939, table XXXVI and pp. 67–84) and it was considered therefore that it would be of value to determine the variation in some of the rarer elements.

The work of Goldschmidt and his collaborators on the distribution of trace elements in minerals (summarized in Goldschmidt, 1937) combined with our recently acquired knowledge of many crystal structures has made clearer the factors controlling the distribution of the trace constituents in minerals.

During a study of the sources of the dusts in South Wales coal mines, carried out on behalf of the Industrial Pulmonary Disease Committee of the Medical Research Council, a mineral of the mica group was found as the main constituent of the shales overlying the coal in several mines. Of six shales investigated four were associated with anthracite, one with semi-bituminous, and one with bituminous coal. Between the coal seam itself and the shale which forms its roof there usually occurs a more coaly shale referred to locally as ‘clod’. Five of the samples investigated were isolated from ‘clods’ and one from a roof.

During a study of the petrography and mineral constituents separated from various shales overlying the coal-measures of South Wales, Dr. Alfred Brammall noticed on several hand-specimens from Llandebie a white infilling to fissures or a coating on slickensided surfaces which could be readily detached from the matrix. Chemical examination showed that this coating, unlike the shales themselves, contained more sodium than potassium. The present note gives an account of subsequent chemical, X-ray, and optical work, which confirms Dr. Brammall's suggestion that the white incrustation contains a new sodium-rich mineral allied to mica. It is not, I hope, an unfitting tribute to his work in this field as well as to his interest in recent advances in mineralogy that I have named this mineral brammallite. A short account of its nature and relationship to illite, which is the chief constituent of the shale itself (see preceding paper, p. 297), will now be given.