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40 hits

  • Thumbnail for 865

    A noteworthy characteristic of this allotriomorphic granite is the presence of feldspar laths with albite twinning in the core and anhedral k-feldspar rims. The remainder of the rock is comprised of equigranular and subhedral biotite, hornblende, and k-feldspar grains. Randomly oriented prismatic apatite grains are abundant in this sample.

  • Thumbnail for SS-4

    Randomly oriented biotite phenocrysts dominate this thin section. The interstices between phenocrysts contain prisms of k-feldspar, anhedral masses of calcite and nephaline, and radial splays of muscovite. Opaques are equant and up to 0.5mm in diameter. The nephaline phase has a bluish cast in cross-polarized light. The groundmass phases are cross-cut by a mesh-work of clays and various alteration products. Slightly diamond-shaped rutile is pervasive throughout.

  • Thumbnail for 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 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 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 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 33-1

    This sample has a granoblastic texture defined by feldspars, garnet, and pyroxene. Crystals of all phases are roughly equant in shape. Alteration is concentrated along the rare hornblende grains and more commonly throughout the pyroxenes. Calcite and apatite are accessory phases and chlorite is found as an alteration product.

  • 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 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-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 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 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 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 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 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 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 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 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 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 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 23-5A

    A continuous foliation defined by aligned euhedral biotite characterizes this fine-grained sample. Granoblastic quartz and feldspars fill the voids between biotite crystals. Small, equant opaqes are distributed evenly throughout.

  • 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.

  • 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 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.