Wood varies in chemical composition, is not crystalline in structure, and is organic. By definition, these three things rule it out as a mineral. read more
Usually, mineral-laden water infiltrates the air spaces in the wood, and over a period of wet/dry cycles, more and more minerals are deposited as the water evaporates. At the same time, the biological material decays, leaving space for even more minerals. read more
In some cases, the wood fibers and cellulose dissolve and some minerals replace them. Sometimes the mineral substance of the fossils will completely dissolve and some other minerals replace them. The common minerals that form this kind of fossils are calcite, iron, and silica. read more