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6 hits

  • Thumbnail for =56
    =56

    Fine-grained rounded aggregates of chalcedonic quartz set in a matrix of chalcedony and hematite. Veinlets of chalcedony hematite crosscut the sample.

  • 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of this thin section is predominantly glass with a roughly-aligned mesh-work of plagioclase, much of which has been replaced by calcite.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 57
    57

    Chalcedonic quartz forms pseudomorphs after carbonate oolites. Both concentric and radial internal structures are preserved. Where and oolite is sectioned centrally the sandgrain nucleus appears. Both the oolites and scattered quartz sand grains are set in a matrix of fine-grained chalcedony.

  • Thumbnail for 56
    56

    A very fine-grained brownish-gray aggregate of chalcedonic quartz, scattered specks and aggregates of slightly coarser quartz and irregular stringers and blebs of translucent to opaque organic material. Local patches and grains of fossiliferous calcite appear.

  • Thumbnail for BB22
    BB22

    The micas in this hypocrystalline rhyolite form euhedral needles and are generally aligned. Phenocrysts of plagioclase have been replaced by calcite and chalcedony. The groundmass of glass contains indiscernible crystals. This thin section is strongly altered and due to the fine grain size, difficult to identify minerals in.

  • Thumbnail for SC-7A-BL
    SC-7A-BL

    The groundmass of this porphyritic basalt is predominantly glass with less abundant plagioclase microlites. The most readily-identifiable mineral of the phenocryst population is plagioclase. Of greater abundance are euhedral, opaque pseudomorphs. Rare inclusions of pyroxene are observed within these opaques. Pockets of chalcedony disrupt the otherwise uniform groundmass.