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20 hits

  • Thumbnail for 990
    990

    The minerals in this sample all appear to intergrow with one another. Most grain boundaries are lobate, especially between feldspar phases and flame lamellae and perthitic intergrowths abound. Although a few hornblende crystals are euhdral in shape, the majority are found as rims around possible clinopyroxene or olivine grains and they tend to group with the other mafic phases. Euhedral apatite appear throughout.

  • Thumbnail for 877
    877

    This generally allotriomprhic-equigranular thin section is thicker than standar thin sections, thus the quartz and some k-feldspar grains display first order red to pink colors. The riebeckite is nearly opaque in both plane and cross-polarized light.

  • Thumbnail for 513
    513

    Square phenocrysts of quartz and feldspar disrupt this porphyritic granite's otherwise mosaic of equigranular grains. Subhedral and partially resorbed biotite is scattered throughout.

  • 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 unknown-71
    unknown-71

    Unaligned, prismatic crystals of schorl, with strongly mauve to indigo pleochroism and concentric compositional zoning are scattered throughout the sample. Basal sections remain a deep blue regardless of rotation of the stage. A corona of clear tourmaline (perhaps) rims most grains. The tourmaline are found in a sericitized feldspars.

  • Thumbnail for 869
    869

    Several glomeroporphyroblasts of clinopyroxene and opaques with hornblende, biotite, and chlorite rims are scattered throughout this syenite. The remainder consists of altered k-feldspars and rare quartz grains with rather sutured grain boundaries.

  • 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 671
    671

    This allotriomorphic granite shows evidence of weathering as most feldspars are partially obscured by a dusting of clay minerals. Tartan twinning of microcline is obvious throughout the sample. The curving of biotite cleavage and subhedral grain shapes indicates the biotite is not pristine. It is commonly found occurring with subhedral hornblende as well as large, clear to light brown, anhedral sphene crystals.

  • 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 LS-04b
    LS-04b

    This moderately-sorted, clast-supported, angular, micaceous sandstone is cemented with microcrystalline quartz and localized calcite. A red hematite stain pervades the sample and concentrates along weathered feldspars. The elongate micas are warped and decaying.

  • Thumbnail for BB32
    BB32

    This is a hypidiomorphic, generally equigranular syenite. Some grains of both feldspar phases have a sieve-like texture. Biotite is abundant, chlorite and muscovite much less so.

  • Thumbnail for 485
    485

    The k-feldspars in this granite have prominent perthitic intergrowths, which overprint obvious tartan and Carlsbad twins. Feldspars growth is more euhedral than that of other phases, though grain boundaries between feldspars are rather sutured. Anhedral riebeckite and biotite have seive-like textures. Olivine, if present, is found in trace amounts.

  • Thumbnail for 323
    323

    The feldspars in this sample show a range of solid solution substitution textures including exsolution lamellae and flame lamellae, although birefringence colors are in the lower first order yellows to pinks. The dark mineral throughout the sample is subhedral, embayed ferrohastingsite which displays excellent amphibole cleavage and retains its originally euhedral crystal structure. A hematite stain pervades this rock, particularly along fractures.

  • 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 876
    876

    The riebeckite grains in this sample are embayed in a skeletal fashion, with embayments strongly controlled by the amphibole cleavage. The feldspars have classic albite and tartan twins in addition to patchy lamellae.

  • 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 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 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 502
    502

    This fine-grained granite has a mosaic texture. The quartz seems undisturbed, however, most other phases display evidence of weathering and disequilibrium. The dusting of clays and presence of sericite inclusions in the feldspars indicates they are chemically weathered. Biotite and muscovite are skeletal in appearance and biotite is frequently interfingered with chlorite.

  • Thumbnail for 483
    483

    The k-feldspars in this granite have prominent perthitic intergrowths, which overprint obvious tartan and Carlsbad twins. Feldspars growth is more euhedral than that of quartz or biotite. A single, tiny, fractured olivine crystal is present along the margin of the thin section.