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4 hits

  • Thumbnail for CF1A
    CF1A

    The olivine phenocrysts in this vesicular basalt are unaltered, in contrast to the second phenocrystic phase, plagioclase, which has a speckled, mottled alteration texture encroaching concentrically inwards from the grain boundaries. Only the cores and a thin corona of plagioclase on the outermost edges of the phenocrysts remains unaltered by this opaque phase. The glassy groundmass contains of unoriented plagioclase microlites and tiny olivine crystals. Olivine phenocrysts are larger and more abundant in CF1A than CF1B. The plucking in this sample is extensive, thus olivine phenocrysts are quite incomplete.

  • Thumbnail for CF1B
    CF1B

    The olivine phenocrysts in this vesicular basalt are unaltered, in contrast to the second phenocrystic phase, plagioclase, which has a speckled, mottled alteration texture encroaching concentrically inwards from the grain boundaries. Only the cores and a thin corona of plagioclase on the outermost edges of the phenocrysts remains unaltered by this opaque phase. The glassy groundmass contains of unoriented plagioclase microlites and tiny olivine crystals. Olivine phenocrysts are smaller and less abundant in CF1B than CF1A. Plucking of olivine crystals is extensive.

  • Thumbnail for BB5
    BB5

    The groundmass of this thin section is predominantly glass with a roughly-aligned mesh-work of plagioclase, much of which has been replaced by calcite. Pseudomorphism of euhedral plagioclase phenocrysts by calcite and serpentine is pervasive; calcite grains of variable crystallographic orientations preserve the Carlsbad and albite twins characteristic of plagioclase. Throughout the sample, ellipsoids of chalcedony-rimmed serpentine masses are common; the smallest and most spherical of these have only chalcedony. The largest, amoeboid vugs of this basalt are mantled first by a rim of chalcedony, and inwards of that, a rim of serpentine. The core of the vugs contain radiating masses of an unidentified zeolite.

  • Thumbnail for JPN-9
    JPN-9

    The crystals in this thin section look somewhat out of equilibrium with the melt. The groundmass is microcrystalline and riddled with plagioclase and pyroxenes, as well as glass. Crystals of the phenocryst population are generally subhedral, with rounded corners and irregular twins. The pyroxenes are frequently rimmed by pyroxenes of differing composition. Plagioclase phenocrysts are of differing generations; the first has cloudy, inclusion-rich cores with fresh rims and the second lacks this core. Both may be concentrically zoned. The pyroxenes in particular tend to cluster in glomeroporphyroclasts. The occasional granular aggregate of calcite can be observed in this sample.