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    Abstract

    LONDON Mineralogical Society, January 25. JAMES PHEM-ISTER: Zoning in plagioclase feldspar. The paper describes various types of zoning in plagioclase feldspar in the calciferous sandstone basalt lavas in one district of Scotland. The zoning is classified as (a) normal, (b) simple reverse, (c) oscillatory. Simple reverse zoning is associated with other differences in the zones which point to important time intervals between the growth of the zones. Oscillatory zoning is classified as oscillatory-normal and oscillatory-reverse and attention is directed to the occurrence of oscillatory-zoned crystals which show no general tendency towards either more calcic or more sodic plagioclase. Distinction is drawn between the main zones and the thin shells of alternately more and less calcic composition within the main zones. The alternating composition of the thin shells is possibly the result of lack of balance between rate of growth of the crystal and rate of diffusion from the surrounding magma. Recurrence of calcic plagioclase in the inner part of main zones is explained as the result of eruption of hot magma into the crystallising liquid, probably consequent on eruption of lava at higher levels. H. H. READ: On zoned associations of anti-gorite, talc, actinolite, chlorite and biotite in Unst, Shetland Islands. In an injection-zone within the staurolite-kyanite-garnet-gneisses of western Unst occur spherical or ellipsoidal bodies, up to 20 ft. in diameter, composed of an interior of antigorite, followed outwards by an orderly sequence of zones made up entirely of talc, of actinolite, of chlorite, and of biotite. It is considered that the zoned bodies result from the fragmentation of peridotite sills during the staurolite-kyanite-garnet metamorphism, followed by the entry of fluids into the masses during injection-metamorphism and the formation of the zonally arranged layers. At the same time, material displaced from the masses reacted with the country-rock to give the biotite-zone. Transitions to the country-rock were mostly pared away during the later chloritoid and chlorite-producing meta-morphisms that have affected the staurolite-kyanite-garnet-gneisses. M. H. HEY and F. A. BANNISTER: Studies on the zeolites (7). Clinoptilolite, a silica-rich variety of heulandite. Rotation photographs of a single crystal from the original specimen of ‘clinoptilo-lite’ (so-called ‘crystallized mordenite’ of L. V. Pirsson) show that it is a silica-rich variety of heulandite. The chemical composition and optical properties are in agreement with this interpretation. The mineral bears no relation to ptilolite. B. RAMO RAO and A. BBAMMALL: Notes on cordierite in the Dartmoor granite. Two groups of associated, but as yet unrelated, facts were recorded concerning the sector-twinned cordierite in the garnetiferous granite of Sweltor: (1) an aggregate of cordierite grains is separable into fractions varying in composition; in particular, the molecular ratio FeO/MgO varies from 0-37 to 1-28 in six intermediate fractions analysed, the ratio for the aggregate being 1-52. (2) all sectors are optically negative, but the value of 2V varies between 56° and 72°. Centrally paired sectors give the same 2V value, whereas adjacent sectors often give different values, the maximum difference observed being 12°.

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