Opalization

Opalization is a diagenetic process in which opaline silica (opal) precipitates from silica-rich fluids within the pore spaces of sedimentary rocks or replaces existing material, forming opal fabrics or opalized fossils. It commonly occurs when groundwater or hydrothermal fluids percolate through silica-bearing rocks and the chemistry favors opal over other forms of silica. Opal can fill voids and vesicles, producing nodules or cement, or it can replace organic matter, yielding opalized wood or fossils.

Silica that forms opal exists in several structural varieties, most commonly opal-A, opal-CT, and opal-C, with differing hydration and crystal organization. Precious opal contains submicroscopic spheres that diffract light to give play-of-color, whereas common opal lacks strong iridescence. Opalization is often associated with volcanic ash beds, silica-rich sands and clays, and arid to semi-arid environments where evaporation concentrates silica.

Opalized rocks are notable in many regions; Australia is especially famous for opal-bearing rocks and opalized fossils, as well as opalized wood. Opalization also preserves paleontological material and can create commercially valuable gemstones from the host rock. The study of opalization sheds light on diagenetic pathways and past environmental conditions.