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20 hits

  • Thumbnail for AGUNG FL
    AGUNG FL

    The plagioclase crystals in this vesicular basalt display a continuous range of sized from groundmass microlite to sieve-textured, sometimes zoned blocky phenocrysts. The groundmass contains more blocky plagioclase than other lavas from the Bali suite. The pyroxenes are generally subhedral and form glomeroporphyroclasts.

  • Thumbnail for JPN-7
    JPN-7

    Although labeled an andesite on the sample record sheet, the presence of olivine and absence of hydrous mafic phases in this thin section indicate it is more basaltic in composition. Texturally, it is characteristic of a basalt as well, with a glassy and plagioclase-rich groundmass and phenocrysts of plagioclase, both pyroxenes and olivine. All phenocrysts are subhedral and plagioclase contains inclusions of glass.

  • Thumbnail for ERB-12
    ERB-12

    This two-pyroxene, hornblende-bearing gabbro is highly fractured and has a much finer-grained population of pyroxenes growing in the interstices between larger pyroxene grains. A thin band of finer-grained pyroxne and plagioclase cross-cuts the sample.

  • 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-19
    JPN-19

    Most of this thin section is serpentine. Scattered throughout are optically-continuous, serpentine-framed fragments of olivine and somewhat larger, texturally-similar orthopyroxene fragments.

  • 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 57-K-91
    57-K-91

    This vesicular basalt contains microphenocrysts of olivine and orthopyroxene, both of which are also lesser components of the groundmass which consists mainly of uniformly-sized plagioclase laths. The olivine phenocryst have weak reaction rims. Some plagioclase is zoned and sieved.

  • Thumbnail for orthopyroxenite
    orthopyroxenite

    This course-grained orthopyroxenite is ideal for finding interference figures for orthopyroxene.

  • Thumbnail for SW1A
    SW1A

    This coarse-grained ophitic norite contains intergrown plagioclase and orthopyroxene. Chlorite alteration of pyroxene is pervasive.

  • Thumbnail for 58-K-91
    58-K-91

    Unoriented plagioclase laths and interstitial glass and clinopyroxene make up the groundmass of this vesicular basalt. Olivine and orthopyroxene microphenocrysts are scattered throughout.

  • 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 1059
    1059

    This sample consists of two distinct populations distinguished by size and composition. The groundmass of elongate subhedral crystals of plagioclase, orthopyroxene and opaques is interspersed with vesicles infilled with calcite. The calcite amygdules are rimmed with quartz and occasionally chlorite.

  • Thumbnail for ERB-08
    ERB-08

    This thin section contains two main zones. One half contains subhedral grains of k-feldspar, quartz, orthopyroxene, biotite and opaques. Kink bands, deformation twins, and core-mantle structures (fine-grained felspar rimming the older grain) in the the feldspars show evidence of strain accomodation. Aggregates of finer-grained serieate-lobate quartz grains with thickened grain boundaries and uniform extinction show evidence of grain boundary migration. Larger quartz grains with undulose extinction and deformation lamellae also indicate strain accomodation. The opaque mineral(s) have rounded, anhedral grain shapes and tend to cluster with biotite and orthopyroxene. Biotite is subhedral, with kink bands and undulose extinction; it is generally found in the intersticies between grains, most commonly adjacent to orthopyroxene grains. Orthopyroxene is subhedral and occasionally embayed. The other half of the thin section is dominated by two porphyroclasts of feldspar. This portion of the thin section abounds with symplectic intergrowths of quartz and feldspar as well as much subgrain development in both mineral. Growing normal to the rims of opaques and biotite grains is a poplulation of acicular aluminosilicate.

  • Thumbnail for ERB-07a
    ERB-07a

    This coarse-grained sample is dominated by subhedral to anhedral k-feldpsar and subhedral garnet, with pockets of quartz aggregates clustering near the feldspar grains and less abundant biotite and orthopyroxene grouped with the garnet phase. Symplectic growth concentrates along boundaries between garnet and k-feldspar. Contacts between garnet are typically host to a mixture of chlorite and biotite. The orthopyroxene grains are strongly fractured and embayed.

  • 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 PS-4-th
    PS-4-th

    Tectonized harzburgite from fault slice along Lionshead Fault. No harzburgite exposed in continuous section. This sample is a strongly serpentinized harzburgite. Serpentine and opaques are the dominant phases in this thin section. Fractured, resorbed, and inclusion-rich orthopyroxenes are also present.

  • 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 JPN-15
    JPN-15

    This inequigranular mosaic of pyroxenes and olivine is highly fractured. Subparallel swarms of high-density fractures are visible throughout and contain the highest concentrations of serpentine. The infilled minerals of one conspicuous vein have a nice comb texture.

  • Thumbnail for 59-K-91
    59-K-91

    This vesicular basalt contains microphenocrysts of olivine and orthopyroxene, both of which are also lesser components of the groundmass which consists mainly of uniformly-sized plagioclase laths. The olivine phenocryst have weak reaction rims. Some plagioclase is zoned and sieved.