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121 hits

  • 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 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 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 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 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 Bm
    Bm

    The outer rim of this bomb is glassy with vesicles and augite and olivine phenocrysts. Small, widely dispersed plagioclase microlites are also present. The core of the bomb contains a porous aggregate of fragmental quartz held together by glass.

  • Thumbnail for CF1A
    CF1A

    The olivine phenocrysts in this vesicular basalt are unaltered, in contrast to the second phenocrystic phase, plagioclase, which has a speckled, mottled alteration texture encroaching concentrically inwards from the grain boundaries. Only the cores and a thin corona of plagioclase on the outermost edges of the phenocrysts remains unaltered by this opaque phase. The glassy groundmass contains of unoriented plagioclase microlites and tiny olivine crystals. Olivine phenocrysts are larger and more abundant in CF1A than CF1B. The plucking in this sample is extensive, thus olivine phenocrysts are quite incomplete.

  • Thumbnail for PS-2a-qd
    PS-2a-qd

    Hypidiomorphic, inequigranular k-feldspar and plagioclase with large clasts of chlorite which are likely pseudomorphs of hornblende. Radiating splays of chlorite are distributed randomly throughout, in addition to radiating zeolites, though the latter are confined to void spaces.

  • Thumbnail for PS-2a-pgr
    PS-2a-pgr

    A granitic inequigranular diorite with graphically intergrown, sericitized k-feldspar and quartz, and chloritized hornblende. Clinozoisite and sphene are accessory minerals found throughout the thin section.

  • Thumbnail for KBR6
    KBR6

    Though clinopyroxene and plagioclase were undoubtedly once more abundant in this gabbro, subsequent alteration has left radiating zeolites, a cloudy film of clays, and veinlets and pseudomorphs of iddingsite the dominant minerals in this thin section.

  • Thumbnail for KBR13
    KBR13

    Described by Western Minerals Inc. as 'porphyroblastic metasediments/hybrid rocks, interpreted by Eales and his coworkers as metamorphosed 'Red Beds' Formation sediments but conceivably magmatic in origin. The specimens illustrate the range in textures and compositions.' Texturally, this fine-grained thin section is an allotriomorphic, junky intergrowth of larger, roughly rectangular feldspars with granular calcite and chlorite. Opaques are abundant.

  • Thumbnail for SC-5-BL
    SC-5-BL

    The groundmass of glass and plagioclase microlites is host to euhedral phenocrysts of three euhedral mineral phases, plagioclase, hornblende, and biotite.

  • Thumbnail for KBR3
    KBR3

    The clinopyroxene abundance in this thin section is lower than other gabbros from Birds River, lending more of a subophitic texture to the rock. Plagioclase is the most dominant mineral. Iddingsite is less abundant.

  • Thumbnail for SC-11
    SC-11

    Phenocrysts in this thin section are subhedral k-feldspar with strong Carlsbad twins, subhedral plagioclase, anhedral, granular quartz aggregates, and skeletal biotite with granular hematite rims. The groundmass is a granular, amorphous quartzofeldspathic mixture.

  • Thumbnail for SC-5
    SC-5

    There is no glass in this fine-grained thin section. Rather, allotriomorphic, randomly-oriented plagioclase is peppered with fine-grained granules of clinopyroxene and opaques. Few clinopyroxene phenocrysts cluster together to form glomeroporphyroclasts. The feldspars show signs of kaolinitization and sericitization.

  • Thumbnail for JPN-9
    JPN-9

    The crystals in this thin section look somewhat out of equilibrium with the melt. The groundmass is microcrystalline and riddled with plagioclase and pyroxenes, as well as glass. Crystals of the phenocryst population are generally subhedral, with rounded corners and irregular twins. The pyroxenes are frequently rimmed by pyroxenes of differing composition. Plagioclase phenocrysts are of differing generations; the first has cloudy, inclusion-rich cores with fresh rims and the second lacks this core. Both may be concentrically zoned. The pyroxenes in particular tend to cluster in glomeroporphyroclasts. The occasional granular aggregate of calcite can be observed in this sample.

  • Thumbnail for KBR2A
    KBR2A

    This gabbro has a subophitic texture. Alteration to chlorite and iddingsite occurs mostly between grain boundaries.

  • Thumbnail for BATUR 1918
    BATUR 1918

    Subhedral phenocrysts of plagioclase, clinopyroxene, and olivine are scattered randomly throughout this basalt. Plagioclase has a sieve texture and is frequently zoned. The corners of most grains are rounded. Both clinopyroxene and olivine are generally equant in shape and olivine is more abundant than the pyroxene. All three phases show some degree of resorption. The groundmass contains moderately-sized crystals of all three phases. This sample is very fresh and unaltered.

  • Thumbnail for 59-K-91
    59-K-91

    This vesicular basalt contains microphenocrysts of olivine and orthopyroxene, both of which are also lesser components of the groundmass which consists mainly of uniformly-sized plagioclase laths. The olivine phenocryst have weak reaction rims. Some plagioclase is zoned and sieved.

  • Thumbnail for SYH-1EE
    SYH-1EE

    Allotriomorphic omphacite and clinozoisite are intergrown with one another, with clinozoisite occurring frequently as inclusions within the omphacite crystals. The occurrence of muscovite is typically contained to polycrystalline nodes with radiating crystals. The conspicuous rutile grain in the center with radiating veinlets has a rim of plagioclase buffering it from the surrounding material. It contains inclusions of chlorite and sphene. Sphene is abundant as large, subhedral clasts.

  • Thumbnail for SC-7
    SC-7

    The groundmass of this microporphyritic basalt consists of glass and plagioclase microlites. Subhedral plagioclase phenocrysts are roughly aligned and some are concentrically zoned. Subhedral, plucked, clinopyroxene phenocrysts of variable size are less abundant than those of plagioclase.

  • Thumbnail for 515
    515

    This hypidiomorphic inequigranular granite has rather scalloped grain boundaries, particularly among the quartzofeldspathic phases, though the biotite grains exhibit similar textures with the feldspars. One anhedral biotite grain has bulbs protruding into the neighboring k-feldspar grain. Quartz extinction is uniform. Olivine is present in trace amounts and the highly-fractured phase is typically surrounded by a thick rim of iddingsite.

  • Thumbnail for 6
    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-23
    ERB-23

    Intergrown, subhedral two-pyroxene gabbro with hornblende and poikilitic plagiclase with deformation twins and abundant pyroxene inclusions.

  • 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.