Banalsite, the new orthorhombic barium-felspar, was described from coarsely crystalline but massive material found as thin bands traversing manganese ore at the Benallt mine, Rhiw, Carnarvonshire. Up to the time of the reading of the paper which announced the discovery of the mineral no crystals had been found except two showing indications of faces seen in a thin section of one of the veins. The only other indications of crystal form were lines of black inclusions seen in thin section within the irregular boundaries of crystals forming the vein-material. Now I have been fortunate to find, in a specimen recently collected by Dr. A. W. Groves from no. 5 ore-body in the Benallt mine, a small cavity containing minute crystals of banalsite.

Antigorite is a lamellar variety of serpentine, and is supposed to be a dimorphous form of chrysotile, which is finely fibrous. Its chemical composition is approximately H4Mg3Si2O9, which is taken as the basis of calculations here.

This study was undertaken primarily because it was hoped that knowledge of the structure of antigorite would throw some light on that of chrysotile. Certain similarities between the two structures have been established, namely in the c(7·3kX or 14·6kX), and b(9·2kX) directions. There are two main differences, however. Firstly, imperfections which cause line broadening in the X-ray pattern of chrysotile, are absent in antigorite (apart from certain ‘streaks’). Secondly, the a(43·4kX) axis of antigorite is approximately eight times longer than the corresponding axis in chrysotile. A complete determination of the structure has not been achieved, but the X-ray pattern has been described, and some suggestions made as to the explanation of the peculiarities observed. A further study of the outstanding questions is in progress.