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Geology Thin Slides

286 hits

  • Thumbnail for 0620-1B
    0620-1B

    Strongly aligned, microcrenulated glaucophane is the primary constituent of this blueschist. Sphene and muscovite are much less abundant.

  • Thumbnail for 0621-4
    0621-4

    This fine grained sample is fairly monomineralic. The small, feathery crystals are aligned to form a weak foliation. Opaques are plentiful though sphene much less so.

  • 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 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 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 174
    174

    This coarse-grained sample is dominated by anhedral and undulatory quartz and feldspar grains. Subgrains are observed in both phases. The birefringence of both quartz and feldspar are somewhat higher than usual, with both phases exhibiting lower first-order yellows and oranges. An isolated occurance of riebeckite is found at the far edge of one thin section and displays excellent amphibole cleavage and characteristic blue to yellow-green pleochroism. The other copies have highly altered purple-blue pleochroic minerals with indistinct cleavage.

  • Thumbnail for 18-9
    18-9

    The layering in this fine-grained amphibolites is defined by bands of amphibole alternating with granoblastic quartzofeldspathic layers. Granular aggregates of epidote are dispersed randomly throughout the rock. Fractures cross-cutting and off-setting the layering are more visible with the naked eye then under the microscope, though one fracture (with no offset) is filled with a seam of epidote. A couple of porphyroclastic and poikiloblastic chlorite grains are clearly truncated by the fractures.

  • Thumbnail for 21-1B
    21-1B

    The foliation in this sample is due in part to aligned hornblende crystals and also to a slight compositional layering. The hornblende grains concentrate in layers of larger grains, with smaller, less abundant layers dominated more by granoblastic quartzofeldspathic phases. Lenticular layers of inclusions within the quartzofeldspathic layers lend a cloudy appearance to portion of the rock. Larger grains of epidote are evident by their classic yellowy-green pleochroism. Small, rounded, colorless titanite crystals are found throughout.

  • 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 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 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 23-3B
    23-3B

    This unfoliated sample contains randomly oriented, subhedral hornblende, with large poikiloblastic and deformed chlorite clasts. The matrix of this thin section is predominately poikiloblastic feldspar with occasional calcite crystals. Embayed chlorite grains are found with aligned inclusions of epidote, calcite, and feldspar.

  • Thumbnail for 23-5A
    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 23-6
    23-6

    This amphibolite has a strong, continuous foliation defined by the alignment of hornblende. Elongate, granular aggregates of epidote are present.

  • 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 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 25-2
    25-2

    The hornblende crystals in this thin section are prismatic and subhedral to euhedral in shape. They are strongly aligned. Filling in between the amphiboles are altered feldspars, granular epidote crystals and euhedral apatites.

  • Thumbnail for 27-2A
    27-2A

    A continuous foliation in this sample is defined by aligned hornblende grains. Altered biotites are poikiloblastic and have kink bands causing undulose extinction. The aligned hornblende grains are juxtaposed against equigranular polygonal-interlobate quartz and aggregates of altered and decussate muscovite and clinozoisite.

  • Thumbnail for 2V-84
    2V-84

    These two oriented andalusite crystals are aligned to show BXA and BXO interference figures.

  • Thumbnail for 32
    32

    Grain shapes in this thin section tend to be subhedral to anhedral with a great variety in grain size. Lath-shaped plagioclase grains have both albite and Carlsbad twins, while larger, more equant grains show concentric zoning. Other phases in this sample include strongly pleochroic biotite, clinopyroxene, nephaline, orthoclase, and opaques. The clinopyroxene, biotite, and opaques tend to cluster together. Fairly large, euhedral apatite crystals are found throughout.

  • Thumbnail for 323
    323

    The feldspars in this sample show a range of solid solution substitution textures including exsolution lamellae and flame lamellae, although birefringence colors are in the lower first order yellows to pinks. The dark mineral throughout the sample is subhedral, embayed ferrohastingsite which displays excellent amphibole cleavage and retains its originally euhedral crystal structure. A hematite stain pervades this rock, particularly along fractures.

  • Thumbnail for 33-1
    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 44-4205 interference figures
    44-4205 interference figures

    Ward's samples with oriented crystals to aid in finding interference figures.

  • Thumbnail for 44-4206 2V slide
    44-4206 2V slide

    Wards' samples with oriented crystals to show varying 2V angles.

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

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