FORMATION OF PETRIFIED WOODS - AN ORGANIC MOLECULE-TEMPLATED SILICA MINERAL PRECIPITATION IN NATURE

Material scientists working on the synthesis of self-assembled nano-structured materials are usually fascinated with the ability of living organisms such as diatoms in using organic molecules to template the synthesis of their body skeletons. In this communication, we will present our Transmission Electron Microscopy (TEM) study on the formation of petrified woods - another example of supramolecular templating process in nature, in which inorganic silica mineral precipitation is templated by dead wood tissues (i.e., cellulose molecules). During petrification, silica minerals gradually replace wood tissues; cellular textures of tissues are commonly preserved even when the wood is completely silicified. The entire process involves silicate mineral dissolution in percolating groundwater and the subsequent precipitation of silica minerals within wood tissues. The TEM study was conducted on a petrified wood sample collected from a late Tertiary (5 - 20 million years ago) siliceous geologic formation in eastern Texas. Our TEM observations reveal that the sample is made of nano-scale silica particles and textural details of original wood tissues are well preserved as variations in the orientation and bulk density of silica nano-crystals. The preserved textures, including cell membranes and nucleus, range from nanometer to sub-micrometers in scale. The preservation of detailed cellular textures suggests that silica precipitation during petrification was templated by organic molecules. Based on a geochemical consideration, this templating process probably took place under alkaline to neutral pH conditions. A mechanistic understanding of the templating process based on the interaction of dissolved silica species with functional groups of tissues will be presented, and its implication to the synthesis of mesoporous silica materials will be also discussed.

Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Co., for the US Department of Energy under Contract DE-AC04-94AL85000.