Societies and Academies

Abstract

LONDON. Mineralogical Society, June 19.—Dr. A. Hutchin-son, president, in the chair.—L. J. Spencer, with chemical analyses by E. D. Mountain: New copper-lead minerals from the Mendip Hills (Somerset). Mendipite (2PbO PbCl2), which occurs as crystalline nodules in manganese-ore, is recorded from new localities. Chloroxiphite (2PbO.Pb(OH)2.CuCl2) as green monoclinic blades resembling epidote, and diaboleite (2Pb(OH)2. CuCl2) as bright-blue tetragonal plates resembling boleite, both occur embedded in the mendipite. Hydrocerussite (2PbCO3.Pb(OH)2) is abundant, sometimes as large crystals (i.e. crystallised “white lead”). Crednerite (CuO Mn2O3) forms fan-like aggregates of thin plates. Pyromorphite was, some centuries ago, evidently an important ore of lead in the Mendips. Wulfenite and mimetite have been found at Higher Pitts near Priddy. The various minerals show progressive stages of alteration with some well-marked pseudomorphs: mendipite→hydrocerussite→cerussite; chloroxiphite→hydrocerussite + malachite I→cerussite + crednerite→malachite II.—W. F. P. McLintock: On a petalite-bearing rock from Devonshire. The well-known aplite from Meldon in Devonshire develops in certain parts of the intrusion a soda-lithia phase rich in the rare lithium-aluminium silicate, petalite. This mineral, not previously known from Britain, occurs as one of the final products of consolidation of the aplite either in coarse-grained veins of pegmatite associated with quartz, orthoclase, albite, a lithia-bearing mica, tourmaline, and apatite, or as irregularly shaped masses throughout the rock itself. The petalite gives rise by decomposition to the pink clay, montmorillonite, so well known from this locality. Certain other veins, free from petalite, are also present, the most interesting constituents of the rarer types being prehnite, axinite, and a pleochroic cordierite. The apatite in the aplite is a pneumatolytic mineral occurring not only in the rock as ophitic patches enclosing quartz and felspar, but also impregnating certain-xenoliths of peculiar type.—A. Brammall and H. F. Harwood: The accessory minerals of the Dartmoor granite. Tourmaline, which is one of the minerals identified, originated at two stages in the cooling history of the intrusion: (1) pre-solidincation-primary and secondary; (2) post-solidification-secondary. The more severe and widespread pneumatorysis and the lodes are referred to a post-solidification stage.— Seitaro Tsuboi: (1) Optical dispersion of three intermediate plagioclases. The principal refractive indices, α β, γ, of (a) oligoclase from Hawke mine, Bakersville, North Carolina, (b) andesine from Maeyama, Shinano, Japan, and (c) labradorite from County Down, Ireland, for light of 9 different wavelengths, and lie optical orientations of the first and the third of these felspars, for light of 5 different wave-lengths (700, 644, 589.3, 535, 508.5μμ) were determined. (2) A dispersion method of determining plagioclases in cleavage-flakes. The method is based on the principle of H. E. Merwin's improved immersion method. By means of a diagram a quick and exact determination of plagioclases is possible. It is applicable to small crystals such as are common in rocks.—C. S. Garnett: The “toadstone-clays” of Derbyshire. The olivine-dolerites (“toadstones”) of Derbyshire undergo two types of alteration: (1) by ordinary weathering to limonite or ochreous deposits; (2) in the absence of oxidising agents under a limestone covering, they may pass through dolerite-greenstone and “green-earth” to a greenish-white or almost white clay (“toadstone-clay”) with the composition 2Al2O3. 6SiO2. 3H2O.—G. T. Prior: (1) The meteoric stone which was seen to fall at Ashdon, near Saffron Walden, Essex, on March 9, 1923. The stone, which weighed about 1300 grams, is a white chondrite showing on one face well-marked radiating lines of flow of the fused crust. (2) The Sinai meteorite. The meteoric stone of 1455 grams which was seen to fall near Kantara in the north of the Sinai Peninsula in July 1916 is an intermediate hypersthene-chondrite, having a percentage of nickel-iferous iron of about 8.6 in which the nickel amounts to about 15 per cent.—G. Greenwood: Communications from the Crystallographic Laboratory of the University of Manchester. No. 1. The detection of rotatory polarisation in an orthorhombic crystal exhibiting crossed axial dispersion. A plate perpendicular to the acute bisectrix of a crystal of tri-phenyl-bismuthine dichloride when in the extinction position transmits a brilliant green monochromatic light due to circular polarisation.—A. F. Hallimond: The chemical classification of the mica group. I. The acid micas.