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Thin-section photomicrograph under a polarizing microscope of a quartz sandstone, showing the grains (white) and pore space (blue epoxy that was injected into the pore space prior to cutting the thin section). Note that individual grains are cemented together by quartz overgrowths (quartz cement that has precipitated around the original grain). If it were not for these overgrowths, fluids could move freely between pores. Thus the quartz overgrowth cement has reduced the permeability, as well as the porosity, of the sandstone.
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