DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
A geode observed in thin section, photographed with transmitted, polarized light, reveals the details of the precipitation and crystallization of silica in a rock void. Geodes form when chalcedony (silica, SiO2) precipitates from liquids percolating inside an empty void or crack in a host rock. The peripheral layers here formed are amorphous to sub-crystalline silica, while the core is made up of crystalline quartz. Actual width is 44 mm.
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