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Shrinkage crack - The crescent-shaped
crack here formed by shrinkage during curing of the blue epoxy and was
filled with clear expoxy when the section was glued to the glass. Notice
how both clear and blue epoxy are black in cross-polar (mouse-over). |
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Incomplete impregnation - It is very
difficult to achieve perfect penetration of colored impregation media into
the complex pore systems of limestones. In this image you see numerous air
bubbles that prevented the full intrusion of the blue epoxy. |
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Bubble in epoxy - A closer view of an air
bubble within the impregnation medium. Round white patches within the
colored epoxy are trapped air bubbles. Such bubbles can
be perplexing the first few times you encounter them. They are
round, isotropic or nearly so, have very low relief compared to mineral
grains, and sometimes trap grains of abrasive (dark blob), as you see
here. In this image there are also thin dark lines that outline air
bubbles within the index medium beneath the cover slip. |
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Bubbles in index medium - The
three dark circles you see in this slide are air bubbles in the index
medium beneath the cover slip. |
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Plucking - It happens sometimes that a
sample is not firmly glued to the glass slide, perhaps because it was not
polished sufficiently flat or because large air bubbles became trapped
within the epoxy that was used to glue the slide to the glass. In thinning
the sample to the standard thin section thickness, a consequence of this
type of flaw is plucking whereby pieces of the rock are completely removed
by the grinding/polishing process. In this case, the benefit of colored
impregnation media becomes very obvious: pores that were present before
sectioning are blue whereas those created by plucking are white in
transmitted light. Without this contrast it would be very difficult to
discriminate secondary pores (natural dissolution of minerals) from
plucked regions of the slide. In this image large regions of an oyster
(which additionally contains secondary pores) have been plucked. Bubbles
within the index medium have settled into two of these holes. |
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Dye crystals - Undissolved crystals in the
dye add texture to the blue epoxy. |