Nano-crystal formation of TiO (sub 2) polymorphs brookite and anatase due to organic-inorganic rock-fluid interactions

The occurrence of the titania polymorphs brookite and anatase as nano-crystals in organic matter-rich sediments of differing age and thermal maturity has been investigated by means of a multidisciplinary analytical approach (FIB-TEM, organic geochemistry, and petrography). It was the aim of the study to analyze the formation mechanisms, fate and behavior of the titania nano-crystals as a result of organic-inorganic rock-fluid interactions. Brookite nano-crystals have been detected in immature Mediterranean sapropels of Quaternary age, but anatase also occurs in deeper and older black shales (Furongian Alum Shale, Sweden; Devonian to Carboniferous Bakken Shale, Williston Basin, USA). Whereas anatase prevails as single crystals, brookite nano-crystals often are agglomerated. Single brookite nano-crystals from Posidonia Shale (Lower Jurassic, Northern Germany) have increasing crystal diameters with increasing thermal maturity. Exclusively anatase nano-crystals both as single crystals or as agglomerates have been detected at oil-water contacts in oilfields, and along fractures with fluid flow enriched in dissolved organic carbon. Titania nano-crystal precipitation, growth (and agglomeration) takes place in the pore water of micro-environments at low to high temperatures and where low pH is coupled to the occurrence of dissolved organic components. Low sedimentation rates preserving a critical geochemical environment or higher temperatures seem major controls for the precipitation of anatase and its tendency not to agglomerate.