Mineralization of Fossil Wood with Macrocrystalline Quartz: A Microscopic Investigation

Abstract

Optical microscopy and SEM imaging are powerful tools for evaluating the origins of quartz in fossil wood. Silicification is the most common mechanism for wood petrifaction, but this silica is commonly in cryptocrystalline or microcrystalline form (e.g., chalcedony or agate). Two essential requirements for the formation of macrocrystalline quartz is a concentration of dissolved silica low enough to allow the development of well-ordered lattices, and sufficient open space to allow euhedral or subhedral crystals to grow. Macrocrystalline quartz commonly occurs as a late-stage precipitate in open spaces that remained after initial mineralization had occurred. These spaces include vessels in angiosperm wood, and vugs and fractures in all types of wood. Exterior surfaces may also be suitable sites for quartz crystal growth. In some occurrences, crystalline quartz has directly encrusted or replaced wood cells. Diagenetic transformation of opal can produce cryptocrystalline or microcrystalline forms of quartz, but this process is not likely to produce macrocrystals.