Experimental synthesis of ThSiO4 by fluid-induced alteration of chevkinite-(Ce): Evidence for element mobility and variable crystal structure

During a series of high P-T experiments aimed at understanding the hydrothermal alteration of the REE,Ti silicate chevkinite-(Ce), three ThSiO 4 phases (thorite, huttonite, and an amorphous ThSiO 4 variety) were formed, showing a wide range of textures from small discrete grains to large-scale replacement of the chevkinite-(Ce). All three phases have compositions close to ThSiO 4 , except that the amorphous type contains small amounts of LREE (≤ 0.025 apfu) and inferred H 2 O. The three different forms have been confirmed by electron backscatter diffraction, which has also provided information on the microstructure of the crystals. The earliest phase to form was huttonite, composed of many randomly oriented microcrystals. This was later replaced by thorite via fluid-aided coupled dissolution-reprecipitation. The amorphous type formed both by apparent hydrous alteration of the original crystalline thorite and huttonite. The inferred sequence of events is compatible with thermodynamic constraints on the relative stability of thorite and huttonite. • Thorium is highly soluble and mobile in high pH, transported as hydroxyl or F complexes. • Huttonite to thorite conversion: at constant P and T, due to Ostwald's kinetic effects. • ThSiO 4 formation sequence: huttonite, thorite, then an amorphous phase from hydrated thorite or huttonite. • ThSiO 4 rapidly turns amorphous, revealing a fluid-driven geochemical process distinct from metamictization. • REE arrangement in chevkinite influences thorite crystal orientation.