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53 hits

  • Thumbnail for 484
    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 SS-13
    SS-13

    This porphyroclastic thin section has a groundmass dominated by k-feldspar and zeolites, though other phases may be present in lesser amounts. The phenocrysts are euhedral augite and olivine, which are frequently found clustered together in glomeroporphyroclasts. The olivine is consistently rimmed by iddingsite and the augite frequently displays concentric zoning. Tiny crystals of equant apatite and rounded opaques thoroughly pepper the thin section.

  • Thumbnail for JPN-8
    JPN-8

    The groundmass of this porphyritic basalt is comprised chiefly of glass, plagioclase, and clinopyroxene, in decreasing proportions respectively. Some plagioclase phenocrysts have sieve textures, some inclusions of glass blebs, others are concentrically zoned, and still more are relatively fresh. Clinopyroxene phenocrysts frequently form glomeroporphyroclasts, sometimes with olivine; several grains have coronas of differing composition growing around them. Both olivine and clinopyroxene are heavily fractured and irregularly shaped.

  • Thumbnail for CM3
    CM3

    Augite and olivine are the two crystalline phases in this 'cloudy' vesicular basalt. Iddingsite replacement of olivine is almost comprehensive; commonly, iddingsite nodes are the only indication of olivine's presence. The groundmass is microcrystalline and altered enough as to be unidentifiable.

  • 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 JPN-15
    JPN-15

    This inequigranular mosaic of pyroxenes and olivine is highly fractured. Subparallel swarms of high-density fractures are visible throughout and contain the highest concentrations of serpentine. The infilled minerals of one conspicuous vein have a nice comb texture.

  • 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 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 CF1B
    CF1B

    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 smaller and less abundant in CF1B than CF1A. Plucking of olivine crystals is extensive.

  • Thumbnail for SS-3674-3
    SS-3674-3

    The largest crystals in this sample are augite, in which twinning is common, as is alteration to a mixture of chlorite and biotite, and the formation of glomeroporphyroclasts. Chlorite and biotite also are found rimming olivine grains, which are themselves clustered with augite and biotite crystals. The mafic phases are typically subhedral in shape, in contrast to the nephaline, k-feldspar, and zeolites which are anhedrally intergrown together in the interstices between augite crystals.

  • Thumbnail for SS-3674-4
    SS-3674-4

    The groundmass of this thin section comprises an intergrowth of k-feldspar and nephaline, as well as small euhedral diopsidic-augite crystals, biotite, and apatite. The phenocrystic phases are dominated by augite, many with partial rims of more diopsidic composition, and which are frequently zoned and twinned. Olivine is present adjacent to the pyroxenes and is commonly altered to biotite.

  • Thumbnail for CM2A
    CM2A

    Augite and olivine are the two predominant crystalline phases in this vesicular basalt and define a microporphyritic fabric. Iddingsite replacement of olivine is minimal. As smaller crystals, both phases, along with opaques, a feldspar, and glass, comprise the groundmass.

  • Thumbnail for CM2A
    CM2A

    Augite and olivine are the two predominant crystalline phases in this vesicular basalt and define a microporphyritic fabric. Iddingsite replacement of olivine is minimal. As smaller crystals, both phases, along with opaques, a feldspar, and glass, comprise the groundmass.

  • Thumbnail for PS-1b-bs
    PS-1b-bs

    The groundmass of this basalt is composed of plagioclase microlites with tiny interstitial clinopyroxenes. Vesicles are infilled with chlorite and quartz. The few larger plagioclase phenocrysts have been partially altered to calcite.

  • Thumbnail for BATUR 94 CR
    BATUR 94 CR

    Sieve textured plagioclase blocks and laths dominate the phenocrystic population in this microporphyritic vesicular basalt. Clinopyroxene phenocrysts are less abundant and olivine less abundant still. All three phases are found in the groundmass. Groundmass clinopyroxene and olivine are more abundant than in BATUR 94 FL.

  • Thumbnail for JPN-10
    JPN-10

    The plagioclase lath in this aphanitic basalt are generally all the same size and are strongly aligned to form a classic trachytic texture with interstitial olivine. A few larger crystals of both phases are observed.

  • Thumbnail for JPN-7
    JPN-7

    Although labeled an andesite on the sample record sheet, the presence of olivine and absence of hydrous mafic phases in this thin section indicate it is more basaltic in composition. Texturally, it is characteristic of a basalt as well, with a glassy and plagioclase-rich groundmass and phenocrysts of plagioclase, both pyroxenes and olivine. All phenocrysts are subhedral and plagioclase contains inclusions of glass.

  • Thumbnail for AGUNG PMC
    AGUNG PMC

    The high quantity of vesicles in this thin section keep the phenocrysts of plagioclase, clinopyroxene, and olivine well separated. While the clinopyroxene is euhedral in shape, the other two phases are more subhedral. Plagioclase phenocrysts have a sieve texture. The groundmass is mostly glass, with a few, moderately sized grains of plagioclase.

  • Thumbnail for dunite
    dunite

    Granular olivine crystals with virtually no alteration and few fractures. Great for obtaining interference figures.

  • Thumbnail for SS-3674-6
    SS-3674-6

    This porphyritic sample contains phenocrysts of augite, olivine, and pseudoleucite. Olivine has an iddingsite rim when in contact with the groundmass. The pseudoleucite phenocrysts are a colorless and cloudy, roundish eight-sided crystals with first-order grey birefringence and extinction occurs in amoeboid-shaped sections within the phenocryst. These same pseudoleucite crystals make up the majority of the groundmass, along with calcite, k-feldspar and other alteration products. Three copies of this thin section contain the contact zone itself, with a calcite and clay-rich, layered Eagle sandstone.

  • Thumbnail for SS-3674-2
    SS-3674-2

    The bulk of this thin section is augite. In plane polarized light, these grains have pleochroic rims of differing composition, which translates to concentric zoning in cross-polarized light. Twinning is common, as is alteration to a mixture of chlorite and biotite, and the formation of glomeroporphyroclasts. Chlorite and biotite also are found rimming olivine grains, which are themselves clustered with augite and biotite crystals. The mafic phases are typically subhedral in shape, in contrast to the nephaline, k-feldspar, and zeolites which are anhedrally intergrown together in the interstices between augite crystals.

  • Thumbnail for SS-5
    SS-5

    Biotite and olivine dominate this sample. The olivine is frequently twinned and highly fractured. In the interstices between olivine grains grows large, anhedral biotite grains. Both phases have a sieve-like, pock-marked appearance. Rutile and opaques, all equant in shape, are plentiful.

  • Thumbnail for 483
    483

    The k-feldspars in this granite have prominent perthitic intergrowths, which overprint obvious tartan and Carlsbad twins. Feldspars growth is more euhedral than that of quartz or biotite. A single, tiny, fractured olivine crystal is present along the margin of the thin section.

  • Thumbnail for CM-incl
    CM-incl

    The layering in this thin section is defined by an opaque phase. Vesicles are pervasive throughout the rock and due to their abundance, the phenocrysts of olivine, pyroxene and feldspar appear to float freely on the slide. Epidote crystals are found among the opaque layers. The rock has a overall cloudy appearance.

  • Thumbnail for SG2A
    SG2A

    Plagioclase microlites and small rounded crystals of augite and olivine define this thin section. The interstices between crystals are filled with reddish-brown glass.