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21 hits

  • Thumbnail for SC-6
    SC-6

    The range in grain size for the plagioclase laths in this thin section is continual from the groundmass euhedral microlites to the subhedral phenocrysts. Concentric zoning and sieve textures are common in the plagioclase phenocrysts. Clinopyroxene phenocrysts are also subhedral, commonly twinned, and infrequently clustered in glomeroporhpyroclasts. In addition to plagioclase, the groundmass contains clinopyroxene, opaques, and serpentine.

  • 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.

  • Thumbnail for SC-7
    SC-7

    The groundmass of this microporphyritic basalt consists of glass and plagioclase microlites. Subhedral plagioclase phenocrysts are roughly aligned and some are concentrically zoned. Subhedral, plucked, clinopyroxene phenocrysts of variable size are less abundant than those of plagioclase.

  • Thumbnail for SC-15
    SC-15

    This fine-grained, hypidiomorphic, inequigranular plutonic rock is intermediate in composition, with plagioclase as the dominant feldspar. K-feldspar is present in much lower abundance, and quartz is scarce. Mafic phases include anhedral biotite and hornblende, the latter of which is strongly chloritized.

  • Thumbnail for SC-19
    SC-19

    The grain size of plagioclase laths in this porphyritic basalt ranges continuously from the groundmass microlites to milimetric phenocrysts. Glass and sparse equant pyroxenes comprise the remainder of the groundmass. Clinopyroxene phenocrysts are patchy due to plucking.

  • Thumbnail for SC-5
    SC-5

    There is no glass in this fine-grained thin section. Rather, allotriomorphic, randomly-oriented plagioclase is peppered with fine-grained granules of clinopyroxene and opaques. Few clinopyroxene phenocrysts cluster together to form glomeroporphyroclasts. The feldspars show signs of kaolinitization and sericitization.

  • Thumbnail for SC-22
    SC-22

    The plagioclase laths in this microporphyritic basalt are separated by size into seperate groundmass and phenocryst populations. Glass and opaques are the other groundmass phases. Clinopyroxene and its periodic pseudomorph calcite are additional phenocrysts.

  • Thumbnail for SC-16
    SC-16

    The groundmass in this thin section is a salt-n-pepper mixture of fine-grained, amorphous quartzofeldspathic phases. All phenocryts are subhedral in shape, and the largest of these, the k-feldspar and plagioclase are host to abundant hematite-filled fractures. Biotite and hornblende phenocrysts are much smaller. A few spherulites, seen more clearly in plane than cross polarized light, are found in the groundmass.

  • Thumbnail for SC-17
    SC-17

    This vesicular rhyolite has a handful of anhedral quartz phenocrysts. The majority of the sample is sericitized groundmass.

  • Thumbnail for SC-13
    SC-13

    The least altered phase in this thin section is plagioclase, which forms roughly aligned, variably sized laths that make up both the groundmass, which is dominantly glass, and the phenocryst population. A second phenocryst, clinopyroxene, remains only as inclusions in the calcite pseudomorphs that replaced it. Vesicles, clays, and hematite pseudomorphs are abundant in this thin section.

  • Thumbnail for SC-12
    SC-12

    This groundmass heavy thin section is comprised of indistinguishable quartzofeldspathic phases. Round hematite nodules locally stain the groundmass around them. Anhedral phenocrysts of k-feldspar have been almost entirely replaced by sericite.

  • Thumbnail for SC-11
    SC-11

    Phenocrysts in this thin section are subhedral k-feldspar with strong Carlsbad twins, subhedral plagioclase, anhedral, granular quartz aggregates, and skeletal biotite with granular hematite rims. The groundmass is a granular, amorphous quartzofeldspathic mixture.

  • Thumbnail for SC-10
    SC-10

    The salt-n-pepper groundmass of opaques, pyroxenes, and feldspars appears almost intserstitial due to the high abundance of microphenocrysts. The phenocrystic population is dominated by subhedral plagioclase and, to a much lesser degree, clinopyroxene. Rounded, phenocryst-sized pockets of granular quartz are found throughout the groundmass.

  • Thumbnail for SC-6-BL
    SC-6-BL

    Plagioclase, the most abundant mineral in this thin section, grades smoothly in size from groundmass mircrolites to subhedral microphenocrysts. Tiny, equant opaques join the plag microlites in forming the groundmass. Clinopyroxene, the second most abundant mineral, is found as relatively euhedral microphenocrysts. Plucking of the pyroxenes is severe. A fiberous phase is frequently found infilling or perhaps pseudomorphing a rectangular phenocrystic phase.

  • Thumbnail for SC-4-BL
    SC-4-BL

    Phenocrysts of plagioclase, clinopyroxene, and iddingsite pseudomorphs after olivine–presumably–are surrounded by a groundmass of unoriented plagioclase microlites, glass, and localized calcite. Plucking of the phenocrysts is noticable and vesicles are present.

  • Thumbnail for SC-14
    SC-14

    But for the anhedral quartz microphenocryss in this vesicular rhyolite, the rest is a clay-altered glass.

  • Thumbnail for SC-18
    SC-18

    The fine-grained groundmass in this sample consists of indistinguishable quartzofeldspathic phases. Phenocrysts both feldspar phases are clustered together; the cores of most are absent, perhaps due to plucking during the making of the thin section or possibly due to resorption. Hornblende phenocrysts are very incomplete and anhedral. Biotite phenocrysts are much smaller and subhedral in shape.

  • Thumbnail for SC-20
    SC-20

    The glassy groundmass contains small plagiocase and clinopyroxene crystals, both of which are found as microphenocrysts. Plagioclase grains are variable in size and subhedral in shape. Clinopyroxene phenocrysts are substaniatlly larger, often due to the formation of glomeroporphyroclasts.

  • Thumbnail for SC-5-BL
    SC-5-BL

    The groundmass of glass and plagioclase microlites is host to euhedral phenocrysts of three euhedral mineral phases, plagioclase, hornblende, and biotite.

  • Thumbnail for SC-21
    SC-21

    This thin section is comprised entirely of glass and aligned plagioclase microlites. A few hematite nodules are found throughout. One weathered biotite phenocrysts can be seen.

  • Thumbnail for SC-10-BL
    SC-10-BL

    Indiscernable, amorphous quartzofeldspathic crystals, opaques, and glass comprise over 95% of this thin section. The remainder are subhedral quartz and feldspar phenocrysts. The handsample contains biotite flakes and feldspar phenocrysts up to 8 mm in length, none of which were captured in thin section.