Thin sections have a finite thickness (around 30 micrometers) and no thin section is 'perfect'.  It is important for petrographers to remember that the thin section is not exactly 2D and to be aware of the visual impact of the 3D distribution of rock components within the slice.  They must also be familiar with the appearance of common thin section flaws.  Air bubbles in epoxy, imperfectly penetrated impregnation resin, scratches, plucked crystals, scratched up carbon coats: all of these things you can simply tune out if you know what they look like.




Click the thumbnails below to open a larger view of the image. Open image windows can be re-sized and scrolled.

example of Shrinkage crack 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).
example of incomplete impregnation 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.
example of bubbles in epoxy 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.
example of bubbles in index medium Bubbles in index medium - The three dark circles you see in this slide are air bubbles in the index medium beneath the cover slip.
plucking 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.
example of dye crystals Dye crystals - Undissolved crystals in the dye add texture to the blue epoxy.