Mechanistic Principles of Barite Formation: From Nanoparticles to Micron-Sized Crystals

Ruiz-Agudo C., Ruiz-Agudo E., Putnis C. V., Putnis, A.

Zusammenfassung

This study reports evidence for barite (BaSO4) formation from aqueous solution via non-classical pathways. Our observations support the occurrence of a liquid-liquid separation in the absence of any additive as the initial stage of the crystallization process. The first solid (primary) particles or nuclei seem to form within the initially formed liquid-like precursor phase. TEM and SEM observations of the nanostructure evolution of samples quenched at successive stages of crystallization indicate two levels of oriented aggregation of nanosized solid particles. The first is the aggregation of crystalline primary nanoparticles of ca. 2-10 nm length to give larger but still nanometer sized particles (ca. 20-100 nm length). For the first time, clear evidence of crystallographically oriented aggregation of secondary, nanometer-sized particles to form a barite single crystal is reported. During the second aggregation step of these secondary nanoparticles, most of the porosity in the largest, micron-sized aggregates is annealed, resulting in perfect single crystals. Once an amount of 50 ppm of additive, in our case a maleic acid/allyl sulfonic acid copolymer with phosphonate groups, is present in the solution the dense liquid precursor phase seems to be stabilized, forming a PILP (polymer induced liquid precursor) and secondary nanoparticles are temporarily stabilized against recrystallization. Growth by classical monomer addition, ripening processes or nanoparticle attachment also seems to contribute to barite formation during the latest stages of the processes.