Colorado College Logo

  DigitalCC

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

123 hits

  • Thumbnail for 174
    174

    This coarse-grained sample is dominated by anhedral and undulatory quartz and feldspar grains. Subgrains are observed in both phases. The birefringence of both quartz and feldspar are somewhat higher than usual, with both phases exhibiting lower first-order yellows and oranges. An isolated occurance of riebeckite is found at the far edge of one thin section and displays excellent amphibole cleavage and characteristic blue to yellow-green pleochroism. The other copies have highly altered purple-blue pleochroic minerals with indistinct cleavage.

  • Thumbnail for BB31
    BB31

    This sample has a granitic texture and contains roughly equal amounts of both feldspar phases. The k-feldspars are more sericitized than the plagioclase grains, many of which are concentrically zoned. The hornblende grains are anhedral and a times, form almost granular aggregates of equant grains. Hornblende is altering to chlorite, which also interfingers with the relatively subhedral biotite phase. Apatite and epidote are present in trace amounts.

  • 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 BB18
    BB18

    This fine-grained granite contains abundant feldspars, many of which display concentric zoning from plagioclase cores to k-feldspar rims. Biotite grains are altering to and interfingered with chlorite. Rounded hornblende crystals are rare in this sample.

  • 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 Phenocrysts of plagioclase have been replaced by calcite and chalcedony.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 871
    871

    The feldspar grains in this thin section span a broad range of sizes, with larger grains displaying strong lamellae of either the blebby or flame variety. The mafic phases are anhedral to skeletal in shape and are much less abundant than the more felsic phases. Small zircon crystals are clustered with a hornblende aggregate.

  • Thumbnail for ERD1A
    ERD1A

    This coarse-grained dacite has a granitic texture of hypidiomorphic inequigranular plagioclase, hornblende, and augite. Secondary zeolites and calcite are found randomly throughout. This thin section is both too thick and plucking is disruptive of many hornblende grains.

  • Thumbnail for SG1A
    SG1A

    Plagioclase laths of varying sizes and orientations represent the bulk of this thin section. Augite is present, though in noticeably smaller quantities and olivine only in trace amounts. Tiny opaques pervade the sample.

  • Thumbnail for SS-3674-6
    SS-3674-6

    This porphyritic sample contains phenocrysts of augite, olivine, and pseudoleucite. Olivine has an iddingsite rim when in contact with the groundmass. The pseudoleucite phenocrysts are a colorless and cloudy, roundish eight-sided crystals with first-order grey birefringence and extinction occurs in amoeboid-shaped sections within the phenocryst. These same pseudoleucite crystals make up the majority of the groundmass, along with calcite, k-feldspar and other alteration products. Three copies of this thin section contain the contact zone itself, with a calcite and clay-rich, layered Eagle sandstone.

  • Thumbnail for SS-6
    SS-6

    Most noteworthy in this sample are the millimeter-scale, euhedral, augite crystals with compositional zoning, twinning, more magnesian rims. Biotite is the subhedral phenocrystic phase in this sample and both are surrounded by interstitial nephaline. Several nephaline grains display micrographic-like intergrowths with a cloudy, difficult to identify phase. Equant olivine grains are frequently found as inclusions within pyroxene and biotite phenocrysts. Euhedral apatite are found throughout.

  • 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 PS-2d-troc
    PS-2d-troc

    Allotriomorphic plagioclase and highly-fractured olivine are the dominant minerals in this thin section. Clinopyroxene, when present, is found adjacent to olivine. Fractured zones in this rock follow olivine clusters and frequently cross-cut plagioclase crystals in swarms between nodes of granular olivine.

  • Thumbnail for PS-2c-lg
    PS-2c-lg

    Granular, inequigranular gabbro containing occasional olivine crystals. Serpentine is localized to fractures. Grain shapes are very rounded and amorphous.

  • Thumbnail for BB13
    BB13

    In thin section, this flow-banded aphanitic rock displays a few fractured and embayed plagioclase and clinopyroxene phenocrysts in a groundmass dominated by aligned microlites of the same two phases. One or two phenocrysts of orthoclase are also present. The microlites define a trachytic texture, which is observable in handsample as flow bands.

  • Thumbnail for 544
    544

    Although the bulk of this thin section has an aplitic texture of quartz and feldspar, several larger feldspar phenocrysts punctuate the mosaic texture of the smaller population of quartzofeldspathic phases. Biotite is sparse and where present, skeletal in shape.

  • Thumbnail for KBR12
    KBR12

    Described by Western Minerals Inc. as 'porphyroblastic metasediments/hybrid rocks, interpreted by Eales and his coworkers as metamorphosed 'Red Beds' Formation sediments but conceivably magmatic in origin. The specimens illustrate the range in textures and compositions.' If so, textures in this sample, though difficult to describe, better resemble those of igneous than sedimentary rocks. This sample is jumbled mess of bladed, graphically intergrown k-feldspar and clinopyroxene, subhedral plagioclase altering to a dendritic, cloudy mineral, patches of anhedral, granular quartz, and elongate, poikilitic opaques.

  • Thumbnail for KBR9
    KBR9

    Plagioclase and clinopyroxene form subhedral and anhedral grains, respectively, though unlike the other ferrotholeiites in this suite, both are cross-cut by or being pseudomorphed by iddingsite, a very dominant mineral in this thin section. Interstitial, undeformed quartz is found in the groundmass and inclusions of apatite crystals lend an almost graphic texture to this phase. Described by Western Minerals Inc. as an 'interstitial ferrotholeiite, very highly fractionated…textures are similar to those associated with rapidly frozen or highly viscous melts.'

  • 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 KBR18
    KBR18

    Elongate plagioclase laths with interstitial clinopyroxene and minor interstitial hornblende and biotite. Classic diabasic texture. Said by Western Minerals Inc. to be the youngest dolerite body in the area and was emplaced at much deeper crustal levels than the gabbros.

  • Thumbnail for 866
    866

    The feldspars in this granite are altering to clays and thus have a consistantly cloudy appearance. The indented grain boundaries in this sample indicate the mutual interference of all phases during growth. The anhedral shape of muscovite grains indicates it grew in the interstices between grains.

  • Thumbnail for 497
    497

    Though dominated by k-feldspar, albite and quartz, this thin section contains biotite pseudomorphs after hornblende, and trace monzaite (distinguished by its high relief, high birefringence colors, and square to diamond shape).

  • Thumbnail for J-19-10
    J-19-10

    Although stored with a series of amphibolites, this hornblende-bearing granodiorite preserves more igneous textures than metamorphic. This hypidoimorphic sample consists of equigranular feldspars, randomly oriented, euhedral biotites, and rather anhedral hornblendes grains. Small, rounded, colorless but dusty crystals of titanite are distrubuted throughout the specimen.

  • Thumbnail for 32
    32

    Grain shapes in this thin section tend to be subhedral to anhedral with a great variety in grain size. Lath-shaped plagioclase grains have both albite and Carlsbad twins, while larger, more equant grains show concentric zoning. Other phases in this sample include strongly pleochroic biotite, clinopyroxene, nephaline, orthoclase, and opaques. The clinopyroxene, biotite, and opaques tend to cluster together. Fairly large, euhedral apatite crystals are found throughout.

  • Thumbnail for 865
    865

    A noteworthy characteristic of this allotriomorphic granite is the presence of feldspar laths with albite twinning in the core and anhedral k-feldspar rims. The remainder of the rock is comprised of equigranular and subhedral biotite, hornblende, and k-feldspar grains. Randomly oriented prismatic apatite grains are abundant in this sample.

  • Thumbnail for BB14
    BB14

    Phenocrysts of resorbed plagioclase and k-feldspar give this otherwise glassy rock its porphyritic texture. A couple of resorbed pyroxene phenocrysts are present. Aligned needles of an opaque phase give the glassy groundmass a trachytic texture.