Abstract
Micro-environments in black shale are reactors for geochemical reactions that differ from the bulk scale. They occur in small isolated pores of several 10 s to 100 s of nanometers without or with limited ionic exchange by diffusion to the surrounding matrix. The example of the formation of titania polymorphs brookite (and anatase) in black shale demonstrates that pH < 4 of the pore waters or lower must prevail to enable dissolution of Ti-bearing precursors followed by the precipitation of these metastable solids. Comparably low pH is applied during the industrial production of nanometer-sized brookite or anatase by sol-gel methods. The process parameters during industrial production such as low pH, negative Eh, or low ionic strength (to promote agglomeration) allow a comparison with parameters during geochemical processes leading to titania formation in black shale. Sol-gel processes are suggested herein as key geochemical processes in micro-environments of black shale in order to understand the formation of single brookite crystals or agglomerates on a nanometer scale.
Highlights
Shale is a fine-grained, siliciclastic sediment with high water content
Selected titania polymorphs in black shales will be presented in the form of an image collection highlighting micro-environments on a micrometer to nanometer scale, and their formation will be discussed as controls of organic matter conversion in micro-environments as analogues of industrial nano-titania production
Hydrogeochemical processes in micro-environments of black shale can completely differ from processes of the bulk rock
Summary
Shale is a fine-grained, siliciclastic sediment with high water content. Porosity-depth trends indicate a loss in porosity (and in water content) from initially up to 80% to less than 20% at a depth of around 3000 m, dependent on burial history and composition [1]. Much of the porosity can be filled with formation water (besides liquid and gaseous hydrocarbons) This aqueous phase is a matrix, reactant, and transport medium during rock–fluid interactions at various scales [7,8,9]. In this contribution, a characteristically exemplary micro-environment for the formation of nanometer-sized titania polymorphs in black shale is introduced—its significance for localized organic–inorganic interactions apart from the bulk scale. The selected example relates to the metastable titania (TiO2) polymorphs brookite and anatase, and how they form in black shale environments [10] This issue is reconsidered in this short communication, but with reference to the prenucleation concept and the so-called sol-gel process. Selected titania polymorphs (only brookite in this research) in black shales will be presented in the form of an image collection highlighting micro-environments on a micrometer to nanometer scale, and their formation will be discussed as controls of organic matter conversion in micro-environments as analogues of industrial nano-titania production
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