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40 hits

  • Thumbnail for SS-6
    SS-6

    Most noteworthy in this sample are the millimeter-scale, euhedral, augite crystals with compositional zoning, twinning, more magnesian rims. Biotite is the subhedral phenocrystic phase in this sample and both are surrounded by interstitial nephaline. Several nephaline grains display micrographic-like intergrowths with a cloudy, difficult to identify phase. Equant olivine grains are frequently found as inclusions within pyroxene and biotite phenocrysts. Euhedral apatite are found throughout.

  • Thumbnail for BB31
    BB31

    This sample has a granitic texture and contains roughly equal amounts of both feldspar phases. The k-feldspars are more sericitized than the plagioclase grains, many of which are concentrically zoned. The hornblende grains are anhedral and a times, form almost granular aggregates of equant grains. Hornblende is altering to chlorite, which also interfingers with the relatively subhedral biotite phase. Apatite and epidote are present in trace amounts.

  • Thumbnail for BB1
    BB1

    The two feldspar phases in this medium-grained, hypidiomorphic syenite are found in roughly equal abundance, though the plagioclase grains are subhedral to the k-feldspar's anhedral shape, and typically displays concentric zoning. Minor myrmekitic intergrowths between quartz and plagioclase are found between feldspar contacts. Both biotite and hornblende have highly irregular grain shapes.

  • Thumbnail for ERD2
    ERD2

    This seems to be a porphyritic basalt though the phenocrystic phases, presumably augite, are plucked enough as to be virtually nonexistent. The remainder of the thin section is a glassy groundmass with unoriented microlites. The microlitic phases are probably pyroxenes and plagioclase, though color, relief, birefringence, and extinction angles seem incongruent with these minerals.

  • 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 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 BB2
    BB2

    This hypidiomorphic, equigranular monazite has a classic granitic texture with a mosaic of grains all crystallizing simultaneously and impinging on one another's growth. Hornblende is particularly abundant and generally in contact with some combination of biotite, chlorite, and opaques. Some plagioclase grains are concentrically zoned.

  • Thumbnail for BB30
    BB30

    The phenocrysts in this porphyritic granite are inclusion-rich, clay-altered, subhedral plagioclase blocks that do not appear to be in equilibrium with the surrounding groundmass. The groundmass is a mottled smattering of quartzofeldspathic minerals that have no discernable crystal habit. Amidst the background blur of first-order whites and grays are petrographically messy clusters of chlorite, hornblende, and iddingsite, with growth of chlorite and iddingsite at the expense of hornblende. Large, prismatic and hexagonal apatite crystals are readily apparent in this thin section. A few prismatic crystals of monazite, zircon, or xenotime are present (though difficult to definitively identify).

  • Thumbnail for ERD1A
    ERD1A

    This coarse-grained dacite has a granitic texture of hypidiomorphic inequigranular plagioclase, hornblende, and augite. Secondary zeolites and calcite are found randomly throughout. This thin section is both too thick and plucking is disruptive of many hornblende grains.

  • 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 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 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
    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 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 BB12
    BB12

    This medium-grained phaneritic rock contains an equal ratio of plagioclase to clinopyroxene grains. Much of the plagioclase is strongly sericitized and several grains are concentrically-zoned. The clinopyroxenes are frequently twinned, heavily-fractured, riddled with inclusions, and show low degrees of alteration to chlorite and serpentine. The pyroxene cleavage is prominent in this sample. Chlorite, serpentine, iddingsite, biotite, and opaques tend to cluster in randomly oriented mats, where chlorite and serpentine growth are often at the expense of biotite.

  • 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 102
    102

    This fine-grained dacite has a strongly cloudy appearance throughout, as well as a film on the surface of the slide that prevents clear focusing. Dusty plagioclase, plucked hornblende, and equant augite are the dominant phases. Euhedral apatite crosscuts most samples. Zeolites and calcite fill most voids or interstices.

  • Thumbnail for 44-4207 relief-scale
    44-4207 relief-scale

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

  • Thumbnail for 22
    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 867
    867

    This mineral phases in this gabbro are highly fractured and frequently embayed. The pyroxenes are roughly euhedral in shape and several equant grains are twinned. The opaques in this sample are typically equant in shape and contain inclusions of olivine or a fiery orange-red mineral that appears the same in plane- and cross-polarized light.

  • Thumbnail for SS-10
    SS-10

    Phenocrysts in this thin section are of augite, biotite, and opaques. Uralitization of augite is observed in one region of the sample and is absent elsewhere. One augite crystal appears dendritically intergrown with opaques. Nephaline, which is the dominant groundmass mineral, appears intergrown with another mineral though it is unclear which. These intergrowths are spherical in shape and observable at the macroscopic scale. Prismatic and radiating fibers of zeolites are found throughout. Apatite crystals span a great range of sizes with the longest crystals reaching more than a millimeter in length. Apatite crystals of a smaller size pervade the nephaline phase.

  • Thumbnail for 23-3A
    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 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 881
    881

    This gabbro is altered to the point of being nearly unrecongnizable. All primary phases are obsecured secondary mineralization. Clays have replaced the plagioclase grains and mats of chlorite and iddingsite, with some minor biotite are found clustered around the euhedral opaques. Overprinting all of this are euhderal apatite grains that range up to nearly a centimeter in length.

  • Thumbnail for SS-3
    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.