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115 hits

  • Thumbnail for JN-01
    JN-01

    Moderately aligned hornblende prisms dominate this rock and give it its green color. Interspersed among the hornblende is a granular matrix of quartz and k-feldspar. Biotite and epidote appear periodically. The layering in this thin section is controlled by variation in grain size of the hornblende minerals.

  • 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 25
    25

    Highly sericitized subhedral feldspars comprise the bulk of this thin section. Anhedral quartz fills in interstices between feldspar grains. Chlorite pseudomorphism of hornblende is prevalent and biotite alteration, chloritization, and embayment indicates of it too, was out of equilibrium with the melt. Euhedral titanite is found throughout.

  • Thumbnail for 497
    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 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.

  • 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 ERB-08
    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 872
    872

    The feldspar grains in this thin section span a broad range of sizes, with larger grains displaying strong lamellae of either the blebby or flame variety. The mafic phases are anhedral to skeletal in shape and are much less abundant than the more felsic phases. The minerals in this thin section are highly fractured lending the appearance of porousness to this intrusive igneous rock.

  • 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 544
    544

    Although the bulk of this thin section has an aplitic texture of quartz and feldspar, several larger feldspar phenocrysts punctuate the mosaic texture of the smaller population of quartzofeldspathic phases. Biotite is sparse and where present, skeletal in shape.

  • 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 15-1
    15-1

    The k-feldspar in this thin section is unaltered and displays albite and tartan twins and beautiful flame lamellae. The grain boundaries between feldspars are often sutured. Interstitial calcite is present. Anhedral and altered hornblende, clinopyroxene, and biotite are distributed randomly throughout.

  • Thumbnail for 871
    871

    The feldspar grains in this thin section span a broad range of sizes, with larger grains displaying strong lamellae of either the blebby or flame variety. The mafic phases are anhedral to skeletal in shape and are much less abundant than the more felsic phases. Small zircon crystals are clustered with a hornblende aggregate.

  • Thumbnail for BB33
    BB33

    This sample has a classic, medium-grained, granitic texture. Grains are generally comparable in size and shape is typically subhedral to anhedral. Biotite is anhedral and weakly pleochroic.

  • 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 KBR9
    KBR9

    Plagioclase and clinopyroxene form subhedral and anhedral grains, respectively, though unlike the other ferrotholeiites in this suite, both are cross-cut by or being pseudomorphed by iddingsite, a very dominant mineral in this thin section. Interstitial, undeformed quartz is found in the groundmass and inclusions of apatite crystals lend an almost graphic texture to this phase. Described by Western Minerals Inc. as an 'interstitial ferrotholeiite, very highly fractionated…textures are similar to those associated with rapidly frozen or highly viscous melts.'

  • 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-14
    SC-14

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

  • Thumbnail for 51
    51

    A very poorly sorted rock with variably sized and angular quartz and feldspar pieces (both plagioclase and orthoclase; some fresh, others sericitized) set in a finer-grained matrix of chlorite, sericite, carbonate, silky quartz, leucoxene, magnetite and pyrite. Matrix chlorite corrodes quartz grains marginally. A few microfragments of rock (quartzite, granite, gneiss) also are present. Accessory detritals include muscovite, sphene, tourmaline and epidote. Most of the quartz pieces are markedly undulatory. A very low degree of incipient metamorphism is indicated by abundant secondary epidote in the matrix which also locally contains minute platelets of stilpnomelane.

  • 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 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 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 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 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 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.