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115 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 LS-06
    LS-06

    Well-sorted, rounded, clast-supported quartz sandstone with aggregates of chert filling in pore spaces. Quartz is undulatory and has early subgrain wall development.

  • 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 44-4207 relief-scale
    44-4207 relief-scale

    These six minerals are mounted on one slide to show comparative relief.

  • 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 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 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 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 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 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 Bs
    Bs

    The outer rim of this bomb is glassy with vesicles and augite and olivine phenocrysts. Small, widely dispersed plagioclase microlites are also present. The core of the bomb contains a porous aggregate of fragmental quartz held together by glass.

  • Thumbnail for PS-1b-bs-ch
    PS-1b-bs-ch

    The predominant mineral in this thin section is opaque red chert which is cross-cut by veins infilled with chlorite and serpentine. Towards the margins of the thin section is a metamorphosed basalt with a groundmass plagioclase mircolites and quartz, calcite, and chlorite in-filled vesicles and veins. According to collector David Sawyer, the interpillow red chert is an important indicator of deep sea origins.

  • Thumbnail for B2
    B2

    The outer rim phenocrysts of augite and olivine, though the red stain, which concentrates in the glass and olivine phases, obscures the characteristic birefringence of the olivine. Plagioclase microlites are present in the glass and calcite has infilled some pore spaces. The core of this bomb appears to be hematite and glass cemented tiny quartz crystals.

  • Thumbnail for KBR19
    KBR19

    Fragmental quartz chunks in glass with occasional glass-surrounded plagioclase clusters. Western Minerals Inc. indicates this sample has partially melted to glass with tridymite developed.

  • 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 SC-14
    SC-14

    But for the anhedral quartz microphenocryss in this vesicular rhyolite, the rest is a clay-altered glass.

  • Thumbnail for SC-10-BL
    SC-10-BL

    Indiscernable, amorphous quartzofeldspathic crystals, opaques, and glass comprise over 95% of this thin section. The remainder are subhedral quartz and feldspar phenocrysts. The handsample contains biotite flakes and feldspar phenocrysts up to 8 mm in length, none of which were captured in thin section.

  • 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 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 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 Quartz A-E
    Quartz A-E

    This thin section consists of two quartz crystals, one of which is oriented perpendicular to the optic axis and provides a perfect crystal with which to obtain an interference figure.