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92 hits

  • Thumbnail for 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 6

    The three essential felsic species occur in two sizes. The larger plaioclases are euhedral and zoned and are pheocrysts, in places glomeroporphritic. A few large subhedral kaolinized orthoclase grains are present as are large quartz anhedra. These are set in a poorly defined groundmass of variable grained quartz, orthoclase, plagioclase, biotite, and hornblende. Of these the mafics are subhedral, the plagioclase is euhedral, and the others anhedral. Hornblende, more strongly altered than biotite, is variably colored in pale green, bluish green and buff. It is replaced by chlorite and epidote. Accessories are magnetite, apatite, sphene and zircon.

  • Thumbnail for ERB-08

    This thin section contains two main zones. One half contains subhedral grains of k-feldspar, quartz, orthopyroxene, biotite and opaques. Kink bands, deformation twins, and core-mantle structures (fine-grained felspar rimming the older grain) in the the feldspars show evidence of strain accomodation. Aggregates of finer-grained serieate-lobate quartz grains with thickened grain boundaries and uniform extinction show evidence of grain boundary migration. Larger quartz grains with undulose extinction and deformation lamellae also indicate strain accomodation. The opaque mineral(s) have rounded, anhedral grain shapes and tend to cluster with biotite and orthopyroxene. Biotite is subhedral, with kink bands and undulose extinction; it is generally found in the intersticies between grains, most commonly adjacent to orthopyroxene grains. Orthopyroxene is subhedral and occasionally embayed. The other half of the thin section is dominated by two porphyroclasts of feldspar. This portion of the thin section abounds with symplectic intergrowths of quartz and feldspar as well as much subgrain development in both mineral. Growing normal to the rims of opaques and biotite grains is a poplulation of acicular aluminosilicate.

  • Thumbnail for 174

    This coarse-grained sample is dominated by anhedral and undulatory quartz and feldspar grains. Subgrains are observed in both phases. The birefringence of both quartz and feldspar are somewhat higher than usual, with both phases exhibiting lower first-order yellows and oranges. An isolated occurance of riebeckite is found at the far edge of one thin section and displays excellent amphibole cleavage and characteristic blue to yellow-green pleochroism. The other copies have highly altered purple-blue pleochroic minerals with indistinct cleavage.

  • 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 497

    Though dominated by k-feldspar, albite and quartz, this thin section contains biotite pseudomorphs after hornblende, and trace monzaite (distinguished by its high relief, high birefringence colors, and square to diamond shape).

  • Thumbnail for 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 875

    The quartz and feldspar grains in this thin section have rather amoeboid, though elongated shapes. These elongated grains are roughly aligned giving this syenite a somewhat layered appearance. Feldspar intergrowths are primarily of the flame lamellae variety. Rare biotite grains are observed.

  • Thumbnail for ERB-07a

    This coarse-grained sample is dominated by subhedral to anhedral k-feldpsar and subhedral garnet, with pockets of quartz aggregates clustering near the feldspar grains and less abundant biotite and orthopyroxene grouped with the garnet phase. Symplectic growth concentrates along boundaries between garnet and k-feldspar. Contacts between garnet are typically host to a mixture of chlorite and biotite. The orthopyroxene grains are strongly fractured and embayed.

  • Thumbnail for 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 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 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 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 KBR17

    Western Minerals Inc. calls this sample as 'metasomatised pyroclastics, baked and well indurated, slightly metasomatised.' It is fine-grained and dominated by anhedral quartzofeldspathic phases which vary in size and are peppered with small, equant pyroxenes. Rounded, partially chloritized clusters are scattered throughout.

  • Thumbnail for GR-07

    Aligned muscovite crystals form a foliation with anhedral, interlobate, though elongated grains of quartz and k-feldspar. Grain boundaries are irregularly shaped.

  • Thumbnail for 22

    Most phases in this thin section are subhedral to anhedral in shape. The k-feldspar have patchy exsolution textures. Most other phases are sparsely distributed and generally subhedral to anhedral.

  • Thumbnail for 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 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 23-3A

    This strongly foliated amphibolite shows evidence of mimetic growth of k-feldspar into elongate crystals due to restriction of a preexisting fabric defined by amphibole and micas. The dominant mica phase, chlorite, has anhedral (almost skeletal) grain shapes. Hornblende is the same. Both phases are highly embayed. Several hornblende grains contain euhedral feldspar 'inclusions' and the classic amphibole grain shape is lost along its margins due to encroaching feldspar grains. Chlorite tends to be associated with elongate epidote crystals or granular aggregates. Dusty fracture zones trace across the fabric of this sample and are filled with cataclastic breccia and glass. Euhedral apatite and small, rounded to larger, anhedral titanite grains are dispersed throughout the thin section, typically oriented with the foliation.

  • Thumbnail for BB18

    This fine-grained granite contains abundant feldspars, many of which display concentric zoning from plagioclase cores to k-feldspar rims. Biotite grains are altering to and interfingered with chlorite. Rounded hornblende crystals are rare in this sample.

  • Thumbnail for BB14

    Phenocrysts of resorbed plagioclase and k-feldspar give this otherwise glassy rock its porphyritic texture. A couple of resorbed pyroxene phenocrysts are present. Aligned needles of an opaque phase give the glassy groundmass a trachytic texture.

  • Thumbnail for 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 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-3674-5

    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 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.