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60 hits

  • 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 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 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.

  • 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 BB27
    BB27

    A notable feature of this thin section are the large, equant and tabular, concentrically-zoned plagioclase phenocrysts with accumulations of k-feldspar around the margins. The quartzofeldspathic phases within the groundmass of the sample are subhedral in shape, often exhibiting rather cuspate-lobate grain boundaries. Subhedral biotite is scattered throughout the sample, though it frequently clusters with chlorite, calcite, and opaques in greenish aggregates, which appear in handsample as green phenocrysts.

  • 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 CM4
    CM4

    The vesicles in this microporphyritic vesicular basalt are substantially smaller than the other Carr Mountain samples, though composition remains similar. More calcite permeates the groundmass, in which plagioclase microlites are visible. Unaltered augite and olivine microphenocrysts are observed. An isotropic zeolite, presumably analcite, is seen infilling several vesicles.

  • 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 GR-01
    GR-01

    Anhedral, interlocking calcite crystals growing around aligned, acicular glauconite and muscovite crystals. This thin section is porous and pock-marked. Skeletal garnets with numerous relict calcite inclusions are variably shaped, from beautifully euhedral, to anhedral aggregates.

  • 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 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-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 LS-02b
    LS-02b

    The bulk of this sample is composed of microcrystalline limestone interspersed with calcite rhombs and transected by veinlets of clay and coarse-grained calcite.

  • Thumbnail for Ralph
    Ralph

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

  • Thumbnail for 878
    878

    Grain shapes in this thin section are consistantly anhedral. Weathering of the feldspars give them a dusty appearance. Because the thin section is cut too thick, the birefringence of quartz and some k-feldspars is as high as second order blues. Mnay hornblende crystals are nearly opaque and they are frequently found surrounding and likely replacing inclusions of pyroxene (perhaps). The biotite and hornblende are rather skeletal. Euhedral sphene and apatite are occasionally scattered throughout.

  • Thumbnail for Calcite A-E
    Calcite A-E

    This thin section consists of two oriented calcite crystals, both good for obtaining interference figures.

  • 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 Syros-unlabeled
    Syros-unlabeled

    Glaucophane, clinozoisite, and muscovite define a spaced foliation with undulose, polycrystalline patches and lenses of quartz. Quartz is relatively equigranular and grain boundaries sutured and showing evidence of bulging.

  • Thumbnail for GR-11
    GR-11

    Stacked, elongate, monocrystalline calcite layers.

  • 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 Marble C
    Marble C

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

  • 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 BB5
    BB5

    The groundmass of this thin section is predominantly glass with a roughly-aligned mesh-work of plagioclase, much of which has been replaced by calcite. Pseudomorphism of euhedral plagioclase phenocrysts by calcite and serpentine is pervasive; calcite grains of variable crystallographic orientations preserve the Carlsbad and albite twins characteristic of plagioclase. Throughout the sample, ellipsoids of chalcedony-rimmed serpentine masses are common; the smallest and most spherical of these have only chalcedony. The largest, amoeboid vugs of this basalt are mantled first by a rim of chalcedony, and inwards of that, a rim of serpentine. The core of the vugs contain radiating masses of an unidentified zeolite.

  • 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 JPN-21
    JPN-21

    According to the sample information sheet, JPN-21 is a crossite-epidote-amphibole-bearing blueschist. Though mineral identification is somewhat complicated by the fine-grained nature of this foliated green rock, the dominant minerals appear to be quartz, chlorite, and calcite or aragonite, all of which are elongated and strongly aligned.