DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Cross-section of tubular agate, close-up. Agate is an aggregate of different forms of silica (SiO2) within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. Thiscross-section shows tubules, thought to be the inlets of infiltration by which the mineral-laden aqueous solution permeated. Actual height: 26 centimetres.
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