Mineralogy, geochemistry, and petrogenesis of the world's longest petrified wood

Fossilized wood is a key geoheritage feature found in many protected areas. Knowledge about its formation is also key to the understanding of such areas. This study investigates the mineralization processes of the world's longest petrified wood (Koompassioxylon elegans), BT1, located in Tak, Thailand, an extraordinary and rare specimen with significance for scientific, educational, geotourism, recreational, geopark, and geoheritage purposes. Multiple techniques including petrography, scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), and X-ray diffractometer (XRD) were employed. Petrographically, the wood structures exhibit different types of quartz crystals, with microcrystalline quartz and spherulitic chalcedony being prominent. The vessels contain chalcedony and crypto- to microcrystalline quartz, with vug-like precipitation occurring within them. Prismatic quartz crystals grow divergently around the vessels, while other cells such as tracheids, rays, and parenchyma (see ANNEX for definition of these terms) are permineralized by polycrystalline quartz, chalcedony, and opal, indicating degradation of these cells. The cells outside the vessels also contain the larger polycrystalline quartz grains cross-cutting the wood cell boundaries. Red to reddish-brown Fe-oxides coat some fissures and cracks. XRD analysis reveals dominant peaks of quartz, followed by analcite, feldspar, montmorillonite, and goethite. SEM-EDS analysis identifies major and trace elements in the wood, showing incomplete permineralization with remnants of carbon in the wood cells. Mineral replacement and permineralization occur in the cell walls, tracheid lumen, vessels, intercellular spaces, and void spaces. The weathering in BT1 is identified by the presence of iron stains, goethite inclusions, a spongy appearance, lenticular features, and fungal growth on the perforated texture.