Abstract
LONDON. Mineralogical Society, Nov. 6.—C. E. Tilley: On scawtite, a new mineral from Scawt Hill, Co. Antrim (with chemical analysis by Mr. M. H. Hey). This new monoclinic mineral, with composition 6CaO.4SiO2. 3C02, occurs in the contact zone between the chalk and tertiary dolerite, from which another new mineral, larnite, was recently described (Min. Mag., vol. 22, p. 77)—F. Coles Phillips: On the composition-plane of [010]-twins in the acid plagioclases. In the true pericline twin, the inclination of the variable composition-plane for different composition is correctly given by Wulfing's curve. T. Barth's conclusion that there is no regular variation is not justifiable, and results partly from confusion with other twinlaws such as that of acline A. The pericline twin should be of frequent occurrence in the crystalline schists.—M. H. Hey: On the variation of optical properties with chemical composition in the rhodonitehustamite series. A complete optical study of three analysed members of the rhodonite-bustamite series, with the data available from the literature, shows regular variation in the optical properties and specific gravity with change in lime content.—F. Coles Phillips: A preliminary account of some mineralogical and chemical changes induced by progressive metamorphism in the Green Bed group of the Scottish Jalradian. Analyses prove the Green Beds to be a truly isochemical series in respect of the constituents significant in progressive metamorphism. The earliest-formed plagioclase is pure albite, but a progressive entry of the anorthite molecule can be traced. The adjustment to equilibrium is apparently close, all the reconstituted plagioclase of a given rock having the same composition. In the highest grades the felspar is a medium andesine. Similar variations with increasing grade are found in the associated epidiorites. The earliest-formed greenish micaeous mineral is a true potash mica, which undergoes increase in FeO in higher grades. Hornblende appears in the chlorite-zone only in rocks low in potash.
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