Colorado College Logo

  DigitalCC

Use AND (in capitals) to search multiple keywords.
Example: harmonica AND cobos

60 hits

  • 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 LS-04a
    LS-04a

    Poorly-sorted, angular, clast-supported, arkosic conglomerate with large clasts of deformed quartz and microcline. Many feldspar grains are altering to clays. Deformed micas are interspersed throughout. Perthite and scotch-plaid twins are extensive. The cement is comprised of microcrystalline quartz and locally, calcite.

  • Thumbnail for BB19
    BB19

    This hypocrystalline thin section contains a trachytic groundmass of plagioclase laths, biotite needles, elongate strings of quartz, and serpentine pseudomorphs. The phenocrystic population consists of rounded quartz grains, often with a calcite rim, and concentrically-zoned, euhedral to subhedral plagioclase blocks. The plagioclase is being replaced in places by calcite, which is also found in fractures within the sample.

  • Thumbnail for BB3
    BB3

    Large, euhedral to subhedral plagioclase blocks are in far greater abundance than either k-feldspar or quartz, both of which are anhedral in shape. All the ferromagnesian phases are anhedral in shape and tend to cluster together. Hornblende growth is at the expense of clinopyroxene. Opaques cluster with the ferromagnesian phases. Quartz displays first-order yellows in this slightly thickened sample.

  • Thumbnail for CM2A
    CM2A

    Augite and olivine are the two predominant crystalline phases in this vesicular basalt and define a microporphyritic fabric. Iddingsite replacement of olivine is minimal. As smaller crystals, both phases, along with opaques, a feldspar, and glass, comprise the 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 JPN-22
    JPN-22

    This thin section is a jumbled mess of serpentine, chlorite, calcite, quartz, and other unidentifiable minerals. Texturally, it looks reminiscent of a basalt.

  • Thumbnail for J-26-11C
    J-26-11C

    This strongly foliated sample contains mostly hornblende crystals, the alignement of which defines the schistoscity. A granoblastic texture consisting of quartz, and highly poikiolblastic k-feldspar is interspersed among the hornblende grains. Occasional subhedral epidote and calcite grains crop up through the thin section. Of special note are a series of fractures that obliquely cross-cut the foliation; cataclastic textures can be observed along these zones. Small, rounded, colorless to light brown titanite crystals are present.

  • Thumbnail for 33-1
    33-1

    This sample has a granoblastic texture defined by feldspars, garnet, and pyroxene. Crystals of all phases are roughly equant in shape. Alteration is concentrated along the rare hornblende grains and more commonly throughout the pyroxenes. Calcite and apatite are accessory phases and chlorite is found as an alteration product.

  • Thumbnail for Marble C
    Marble C

    Large, interlocking calcite crystals comprise this marble. This thin section is useful for finding interference figures.

  • Thumbnail for LS-01
    LS-01

    A moderately-sorted, subrounded quartz sandstone with a calcite cement. Microcline and albite are present in smaller amounts than the quartz clasts. A luster-mottling texture is present, in which the calcite cement forms crystals larger than the clasts it cements, as indicated by the optical continuity and parallel twins across large portions of the thin section's cement. Layering is observed in thin section by zones of abundant cement alternating with more clast-supported, calcite-poor zones. Most quartz clasts show undulatory extinction and subgrain development.

  • Thumbnail for LS-02a
    LS-02a

    The bulk of this sample is composed of microcrystalline limestone interspersed with calcite rhombs and transected by veinlets of clay and coarse-grained calcite. According to the collectors, this sample was collected because of its replacement textures.

  • Thumbnail for BB6
    BB6

    Amidst the groundmass of glass, randomly oriented plagioclase laths and equant to anhedral patches of secondary serpentine, are strewn subhedral phenocrysts of plagioclase and twinned clinopyroxene.

  • Thumbnail for SS-4
    SS-4

    Randomly oriented biotite phenocrysts dominate this thin section. The interstices between phenocrysts contain prisms of k-feldspar, anhedral masses of calcite and nephaline, and radial splays of muscovite. Opaques are equant and up to 0.5mm in diameter. The nephaline phase has a bluish cast in cross-polarized light. The groundmass phases are cross-cut by a mesh-work of clays and various alteration products. Slightly diamond-shaped rutile is pervasive throughout.

  • Thumbnail for FT-1
    FT-1

    The foliation in this rock is defined by aligned lawsonite, chlorite, glaucophane, and calcite. Lenses of quartz are rare. Euhedral though skeletal garnets have overgrown the foliation, and inclusions within the garnet are often continuous with though at an angle to the dominant foliation indicating garnet growth during deformation. This thin section has a greasy-looking film on the surface that makes it difficult to focus clearly.

  • Thumbnail for SS-1
    SS-1

    The dominant mafic phase in this thin section is biotite, followed by augite. These two phases comprise the phenocryst population. Due to secondary alteration, the once euhedral grains of augite are now 'patchy' in appearance. The groundmass consists of a cloudy matrix of biotite microlites set amongst a indistinguishable mixture of sanidine and zeolites. Interstitial calcite is found in the groundmass, and can be seen infilling voids in the augite phenocrysts.

  • 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-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 60
    60

    Interlocking microcrystalline calcite crystals and microspar with veinlets of hematite. (Angular fragments of turbid, fine-grained, distinctly layered limestone are cemented and veined by clear, much coarser grained calcite. The limestone pieces contain uniformly fine-grained calcite, veinlets and patches of hematite, small aggregates of chalcedony, grains of sand and silt-sized quartz and disseminated shreds of opaque carbonaceous debris. Some pieces are so fine-grained that they approach the texture of lithographic limestone.)

  • 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 52
    52

    Microscopically mainly cryptocrystalline and merely translucent in shades of gray-brown. Subordinate silty quartz grains, feldspar pieces, sericite flakes, calcite rhombs, and a few particles of pyrite, zircon, magnetite and leucoxene. The dark matrix is composed mainly of illitic clay material.

  • Thumbnail for 49
    49

    A uniformly fine-grained parallel-oriented aggregate of angular quartz grains, feldspar fragments, chlorite flakes, sericite flakes and shreds, aggregates of submicroscopic clay minerals (chiefly kaolinite), and carbonaceous shreds. Accessory detrital species recognizable are biotite, apatite, zircon, epidote, and tourmaline.

  • Thumbnail for BB29
    BB29

    The four phenocrystic phases in this thin section all display disequilibrium textures. Tabular plagioclase pheoncrysts are oscillatorily-zoned at the core and surrounded by as many as three distinct, cloudy, inclusion-rich rims. Quartz phenocrysts are rounded by resorbtion; thick coronas of calcite and hematite pepper, though do not entirely replace, their rims. Relict biotite phenocrysts, pseudomorphed by an opaque phase, are recognized more readily by the consistant rectancular and hexagonal shape than by the rare inclusion of the residual 'host'. These grains, frequently clustered together, have thick black rims and are sometimes infilled with a calcite-like mineral. This calcite-like mineral, frequently found replacing the biotite and as part of the quartz phenocryst coronas, is also observed pseudomorphing a tabular-shaped mineral found both as a phenocryst and part of the groundmass. The calcite seems to be replacing rectangular serpentine phenocrysts, presumably, pyroxene pseudomorphs themselves. The trachytic groundmass consists predominantly of microlites of plagioclase, prismatic opaques, glass, and the periodic rectangular calcite pseudomorph. Granular hematite veins cross-cut the thin section.

  • Thumbnail for LS-05
    LS-05

    This porous, fine-grained, well-sorted, well-rounded, clast-supported quartz sandstone is cemented together with hematite. Clasts of microcline, hornblende, calcite, and chert are dispersed in minor amounts throughout the sample. The layering which is visible to the naked eye is unnoticeable in thin section.

  • Thumbnail for LS-07a
    LS-07a

    This well-sorted, subrounded, chert-cemented quartz sandstone contains alternating clay-rich and poor layers. Accessory minerals and microcrystalline calcite are concentrated along the clay or hematite-rich layers. Hematite concretions and veinlets are found in this sample.