| Wood must first be covered with such agents as volcanic ash, volcanic lava
flow, volcanic mud-flows, sediments in lakes and swamps or material washed in by violent
floods - by any means which would exclude oxygen and thus prevent decay. A number of
mineral substances (such as calcite, pyrite, marcasite) can cause petrification, but by
far the most common is silica. Solutions of silica dissolved in ground water infiltrate
the buried wood and through a complex chemical process are precipitated and left in the
individual plant cells. Here the silica may take a variety of forms; it may be agate,
jasper, chalcedony or opal. The beautiful and varied colors of petrified wood are caused
by the presence of other minerals that enter the wood in solution with the silica. Iron
oxide stains the wood orange, rust, red or yellow. Manganese oxide produces blues, blacks
or purple. |
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