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92 hits

  • Thumbnail for LS-03
    LS-03

    Moderately sorted, subangular, matrix-supported, quartz and glauconite sandstone with calcite and hematite matrix. Quartz is undulatory with subgrain walls developing and potassium feldspar twins are deformed. Pockets of finer-grained material contain rounded epidote grains.

  • Thumbnail for ERB-09a
    ERB-09a

    Grain boundaries between the quartz and feldspar phases in this sample are consistently lobate, often with finer-grained subgrains rimming each crystal. Quartz in undulatory. The mica phases are subhedral and often resorbed, with kink bands causing undulose extinction along the length of the grain. Along one edge of the thin section is a seam dominated by clinozoisite. A hematite stain gives a dusty orange appearance to the feldspars and fractures within the sample contain higher concentrations of the phase.

  • Thumbnail for ERB-09b
    ERB-09b

    This metamorphosed granite has a weak spaced foliation defined by moderately aligned elongate minerals and slight compositional layering. The opaque phases are segregated into two discrete bands, along which, large quartz grains show a crystallographically preferred orientation, causing them to appear nearly isotropic. Alternatively, this nearly isotropic phase could be untwinned leucite, however, an the lack of twins and association with the remaining phases is unlikely. The chlorite and muscovite phases appear to be collectively pseudomorphing a preexisting phase, as evidenced by dark, elongate inclusions within the chlorite grains.

  • Thumbnail for 884
    884

    The flame lamellae that pervade this thin section are cut thicker than ususal and thus display first order yellows and oranges, though the tartan twinning of the microcline retains the ususal first order grey birefringence. Grain boundaries between feldspars are lined with smaller feldspar grains, which gives the larger grains a scalloped appearance. The hornblende grains are highly irregular in shape and contain many inclusions of feldspar, opaques, biotite, and other unidentifiable phases.

  • Thumbnail for 874
    874

    This thin section is divided in half diagonally by texture and grain size. The fine-grained side consists of roughly equigranular quartz, microcline, albite, biotite, and riebeckite. More quartzofeldspathic-rich portions have sutured grain boundaries whereas in more mafic patches the grains are typically equant and subhedral. The mineral phases in the coarse-grained half of the thin section are the same though the grain size increases several fold and clear grain boundaries are rarer.

  • Thumbnail for 504; 505
    504; 505

    The feldspars in this thin section are partially sericitized and somewhat poikilitic and the biotite is substantially altered. Grains are generally anhedral in shape.

  • Thumbnail for ERB-0X3
    ERB-0X3

    In this sample, a spaced, anastamosing schistoscity defined by biotite, garnet, hornblende and opaques wraps around deformed feldspar lenses which display undulose extinction and bent growth as well as deformation twins. Polyganized quartz and feldspar mantles the feldspar porphyroclasts. Quartz ribbons are prevalent throughout this sample. The sense of shear is ambiguous.

  • Thumbnail for SS-3674-1
    SS-3674-1

    Subhedral biotite and uralitized augite crystals are the prominent mafic phases in this sample, though olivine is also present in lesser amounts and as an inclusion in one of the two other mafic phases. K-feldspar, zeolites, and apatite are the primary leucocratic constituents. There is a good deal of intergrowth between and alteration of minerals in this thin section.

  • Thumbnail for BB8
    BB8

    Large k-feldspar and quartz crystals provide a backdrop for the smaller, less well-preserved mafic phases in this sample. Decomposing biotite is frequently found interfingered with chlorite. Hornblende grains are rather skeletal. Plagioclase crystals have relatively rounded corners and tend to cluster together. Large anhedral aggregates of sphene are interspersed in the interstices between k-feldspar grains.

  • Thumbnail for SS-5
    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 484
    484

    Perthitic intergrowths of k-feldspar and albite are prevalent throughout this coarse-grained, nearly allotriomorphic granite. Riebeckite and biotite are found intergrown together in anhedral masses. The rare tiny, equant olivine crystal is observable.

  • Thumbnail for SS-3
    SS-3

    This coarse-grained sample contains phenocrysts of a wide variety of size and composition. The largest, visible in hand sample as whitish spheres with obvious rims, are intergrown, wedge-shaped k-feldspars and radiating blades of natrolite. A dusting of sericite throughout the k-feldspar gives a cloudy appearance to the these mineral aggregates. Randomly oriented grains of these same phases comprise the bulk of the 'groundmass' for this sample. Augite phenocrysts are the dominant mafic phase and display a variable degree of alteration to iddingsite?. A few grains of arfvedsonite can be seen associated with the pyroxenes.

  • 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 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 679
    679

    This allotriomorphic granite is dominated by feldspar phases which are altering to clays. The mica phases are anhedral, emabyed, and filled with unaligned opaque inclusions.

  • Thumbnail for ERB-02
    ERB-02

    This sample is a strongly mylonitized pelitic schist with a continuous schistoscity defined by strongly alligned biotite, fibrous sillimanite, and opaques and interspersed with discontinuous quartz and k-feldspar ribbons. K-feldspar is strongly sericitized. The sense of shear is ambiguous.

  • Thumbnail for 21-1B
    21-1B

    The foliation in this sample is due in part to aligned hornblende crystals and also to a slight compositional layering. The hornblende grains concentrate in layers of larger grains, with smaller, less abundant layers dominated more by granoblastic quartzofeldspathic phases. Lenticular layers of inclusions within the quartzofeldspathic layers lend a cloudy appearance to portion of the rock. Larger grains of epidote are evident by their classic yellowy-green pleochroism. Small, rounded, colorless titanite crystals are found throughout.

  • Thumbnail for 507
    507

    Most crystals in this thin section are highly embayed, including the distinctive riebeckite grains.

  • 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 ERB-03
    ERB-03

    In this mylonitized sample, biotite, sillimanite, and the opaques define a spaced schistoscity overprinting a background matrix of seriate-interlobate quartz and feldspar. The foliation wraps around polymineralic porphyroclasts with fish-like shapes. The sense of shear is ambiguous. A lineation is defined by aligned sillimanite and biotite.

  • Thumbnail for 23-6K
    23-6K

    A foliation defined by aligned hornblende is cross-cut by a seam running from the upper-left to lower-right of the thin section. The seam is coarse-grained and unfoliated with a quartzofeldspathic margin and grains of resorbed, subhedral clinopyroxene in the middle.

  • Thumbnail for 21-4
    21-4

    The foliation in this coarse-grained rock is the result of compositional layering; discontinuous hornblende-rich zones separate poikiloblastic plagioclase-rich regions. Albite and Carlsbad twins in the feldspars are almost entirely obscured by randomly-oriented inclusions of sericite.

  • 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 SS-3674-5
    SS-3674-5

    This is a very hypidiomorphic coarse-grained sample consisting of large grains of augite, frequently intergrown with opaques, and intergrown k-feldspar and nephaline. Augite is found altering to biotite. Radiating splays of zeolite are present to varying degrees within the three copies of this thin section.

  • Thumbnail for BB1
    BB1

    The two feldspar phases in this medium-grained, hypidiomorphic syenite are found in roughly equal abundance, though the plagioclase grains are subhedral to the k-feldspar's anhedral shape, and typically displays concentric zoning. Minor myrmekitic intergrowths between quartz and plagioclase are found between feldspar contacts. Both biotite and hornblende have highly irregular grain shapes.