These two minerals are analogous silver compounds with similar pseudo-hexagonal monoclinic form and similar compositions: polybasite, 8(Ag,Cu)2.(Sb,As)2S3, and pearceite, 8(Ag,Cu)2S.As2S3. Cu replaces Ag in considerable and varying amounts in both minerals, and in polybasite there may also be substantial replacement of Sb by As. However, the rather numerous existing analyses do not indicate continuous chemical variation from arsenian polybasite to pearceite, and therefore it is proper to recognize the two minerals as distinct species rather than end-members of a series.

Extinction is the increase in absorption that is to be expected when Bragg reflection of X-rays takes place in single crystals, and the corresponding decrease in intensity of reflection observed.

The increase of absorption was directly observed in 1914 in two ways. Using the ionization spectrometer, W. H. Bragg found that the amount of radiation transmitted by a crystal section was notably decreased when the transmitted beam had suffered reflection from a set of crystal planes.

Pinnoite, first described by Staute in 1884, is straw-yellow, granular to faintly fibrous. It occurs in the upper kainite layers at Stassfurt, and is found associated with earthy boracite from which it is freed by washing with water. The analysis corresponds to the formula MgO.B2O3.3H2O, and Staute assumes the mineral to be an alteration product of boracite, 6MgO.MgCl2.8B2O3. Groth classes pinnoite together with CaO.B2O3 as a hydrated metaborate. This assumption does not seem justified, as will be pointed out later.

During the course of recent work on minyulite (KAl2(PO4)2 (OH,F).4H2O) X-ray, optical, and density data were obtained for several hydrous K-Al phosphate minerals. These included one specimen of a soft white compact mineral labelled by P. Pallary as minervite from Sebdou, Orau, Algeria, which was presented in 1940 to the Trustees of the British Museum by Major M. Connolly. Minervite was originally described and analysed in 1895 by A. Carnot from Minerva grotto, Fauzan, dép. Hérault, France. In the same year Carnot also described the same species from Tour-Combes grotto, Misserghin, Oran. Sebdou is some 80 miles SSW. of Oran and geological maps show there a patch of Pliocene on late Jurassic. No doubts were entertained that the specimen under discussion belonged to the same species as minervite from Misserghin, since the geological formations of both localities are closely related.

The abundance of the well-known potassium aluminium silicate minerals, orthoclase, microcline, and muscovite, stands in striking contrast with the rarity of the little-known potassium aluminium phosphate minerals. The only one of these which occurs as distinct crystals and which has yielded a satisfactory formula is minyulite, KAl2(PO4)2(OH,F).4H2O. This mineral was first described by E. S. Simpson and LeMesurier from Western Australia in 1933 and an examination of well-crystallized specimens from South Australia by L. J. Spencer et alii was published in 1943. Spencer's paper included X-ray single-crystal and powder data for minyulite. X-ray, optical, and specific gravity data were also determined at that time for a few other hydrous potassium aluminium phosphate minerals from the British Museum collections. These included palmerite from guano deposits in a large cavern, Monte Alburno, near Controne in Salerno, Italy, presented by E. Casoria who described the mineral in 1904, and taranakite from the Sugarloaves, near New Plymouth, Taranaki, New Zealand.

The name ‘halloysite’, first used in 1826 by Berthier, is derived from that of Omalius d'Halloy, who found the mineral in Angleur, Liége, Belgium. In 1935, Mehmel discovered that there were two distinct substances included under the name halloysite, one of which contains loosely bound water and shows a 10 Å. basal reflection on an X-ray diagram, the other none and shows a reflection at about 7 Å. Mehmel preserved the name halloysite for the hydrous mineral, and for the product of its dehydration he proposed the name ‘metahalloysite’. Both substances may occur naturally (though this may not have been known to Mehmel at the time). Previous to this work it had been suggested by Hofmann, Endell, and Wilm that the 10 Å. material on dehydration at a low temperature gave kaolinite. Mehmel showed that this was wrong; and that the error was due to the similarity of the X-ray diagrams of kaolinite and metahalloysite. Hofmann and co-workers, and Mehmel, all tacitly assumed that the name halloysite was correctly applied to the 10 Å. material rather than the 7 Å. material.

The decision of the Council to organize an excursion, the first since the Jubilee of the Society in 1926, arose from an invitation presented by Dr. A. E. J. Vickers, on behalf of Imperial Chemical Industries Ltd., to visit the anhydrite mine at Billingham. During the excursion the opportunity was also taken of visiting mineral localities in the northern Pennines (west Durham and Cumberland) under the leadership of Professor Wager and the writer. Accommodation was arranged in Durham at University College (by kind permission of the Master), and here, housed in the ancient castle, members enjoyed a comfortable stay. Transport each day was by private coach. The excursion was organized by Dr. G. F. Claringbull; the thanks of members are also due to Mr. E. Myers, secretary of the north-east section of the Society of Chemical Industry for his help in making local arrangements.