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37 hits

  • 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-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 JPN-1
    JPN-1

    The phenocrysts in this basalt are plagioclase, orthopyroxene and clinopyroxene. Most pyroxenes are gathered in glomeroporphyroclasts with plagioclase, though the latter phase is most commonly found as isolated, irregularly twinned and shaped phenocrysts. The groundmass is glass and plagioclase.

  • Thumbnail for JPN-6
    JPN-6

    Plagioclase is a much more abundant member of the groundmass in this basal than many other basalts in this suite. It, along with two pyroxene phases, comprise the poikilitic, subhedral phenocryst population. Plucking of both phases disrupts the quality of the thin section.

  • Thumbnail for BATUR 1936
    BATUR 1936

    Subhedral phenocrysts of plagioclase, clinopyroxene, and olivine are squeezed into the fine web of rock that rims the massive vesicles of this scoria. Plagioclase has a sieve texture and is frequently zoned. The corners of most grains are rounded. Both clinopyroxene and olivine are generally equant in shape and olivine is more abundant than the pyroxene. All three phases show some degree of resorption. The groundmass contains moderately-sized crystals of all three phases. This sample is very fresh and unaltered.

  • 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 JPN-8
    JPN-8

    The groundmass of this porphyritic basalt is comprised chiefly of glass, plagioclase, and clinopyroxene, in decreasing proportions respectively. Some plagioclase phenocrysts have sieve textures, some inclusions of glass blebs, others are concentrically zoned, and still more are relatively fresh. Clinopyroxene phenocrysts frequently form glomeroporphyroclasts, sometimes with olivine; several grains have coronas of differing composition growing around them. Both olivine and clinopyroxene are heavily fractured and irregularly shaped.

  • 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 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.

  • 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-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 BATUR 1918
    BATUR 1918

    Subhedral phenocrysts of plagioclase, clinopyroxene, and olivine are scattered randomly throughout this basalt. Plagioclase has a sieve texture and is frequently zoned. The corners of most grains are rounded. Both clinopyroxene and olivine are generally equant in shape and olivine is more abundant than the pyroxene. All three phases show some degree of resorption. The groundmass contains moderately-sized crystals of all three phases. This sample is very fresh and unaltered.

  • 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 JPN-12
    JPN-12

    This cryptocrystalline basalt consists of a groundmass of plagioclase microlites, equant opaques, glass, and interstitial, anhedral phlogopite. Microphenocrysts of subhedral clinopyroxene comprise the remainder of the sample.

  • Thumbnail for BATUR 94 FL
    BATUR 94 FL

    Sieve textured plagioclase blocks and laths dominate the phenocrystic population in this microporphyritic vesicular basalt. Clinopyroxene phenocrysts are less abundant. Both phases are found with glass as constituents of the groundmass.

  • Thumbnail for JPN-3
    JPN-3

    The olivine that is so conspicuous in the JPN-3 handsample is conspicuously absent in thin section and represented by a mere 1 or 2 grains. Rather, phenocrysts of plagioclase and pyroxenes dominate the phaneritic phases. Both phases are subhedral and the plagioclase in particular indicates several different generations of growth. Some plagioclase is concentrically zoned, though most grains contain alternating inclusion-rich and poor zones. Inclusions are of pyroxenes and glass blebs. The groundmass contains glass, plagioclase microlites, and tiny pyroxenes.

  • 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-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 JPN-5
    JPN-5

    Based on grain size, two populations of pyroxenes and plagioclase exist in this sample; the first are the fine-grained, equant crystals of the glassy groundmass, and the second, the subhedral to anhedral phenocrysts visible in handsample. The inconsistently-shaped plagioclase grains are sometimes zoned and frequently contain inclusions of glass.

  • Thumbnail for JPN-2
    JPN-2

    Phenocrystic phases in this thin section are subhedral plagioclase, clinopyroxene, and orthopyroxene, found both as isolated crystals and as glomeroporphyroclasts. Some plagioclase phenocrysts have inclusions of equant pyroxenes and blebs of glass. Glass and plagioclase comprise the vesicular groundmass.

  • Thumbnail for JPN-11
    JPN-11

    This aphanitic basalt is comprised entirely of aligned, microcrystalline plagioclase laths with interstitial olivine. The olivine is being replaced by iddingsite and while larger olivine phenocrysts are merely rimmed with iddingsite, the smaller, interstitial grains have been entirely altered to iddingsite.