Colorado College Logo


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

5 hits

  • Thumbnail for 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 SS-5

    Biotite and olivine dominate this sample. The olivine is frequently twinned and highly fractured. In the interstices between olivine grains grows large, anhedral biotite grains. Both phases have a sieve-like, pock-marked appearance. Rutile and opaques, all equant in shape, are plentiful.

  • Thumbnail for SS-2

    This hypocrystalline thin section contains euhedral to subhedral phenocrysts of augite and eight-sided, rounded, phenocrysts of a colorless, moderate-relief, very weakly birefringent mineral, perhaps leucite, sodalite, or analcite. The groundmass of indistinct, microlitic k-feldspar and pyroxene is rich with equant opaques and cloudy in appearance.

  • Thumbnail for 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-3674-2

    The bulk of this thin section is augite. In plane polarized light, these grains have pleochroic rims of differing composition, which translates to concentric zoning in cross-polarized light. Twinning is common, as is alteration to a mixture of chlorite and biotite, and the formation of glomeroporphyroclasts. Chlorite and biotite also are found rimming olivine grains, which are themselves clustered with augite and biotite crystals. The mafic phases are typically subhedral in shape, in contrast to the nephaline, k-feldspar, and zeolites which are anhedrally intergrown together in the interstices between augite crystals.