Calcite-aragonite transformation in eggshell: A crucial role of organics and assessment of the impact of milling conditions on its extent using Taguchi design

Abstract

Phase transformations during high-energy ball milling, being a tool of mechanochemistry, are interesting due to the possible production of metastable phases without the need of artificially introducing high temperatures and pressures. In our work, the transformation of calcite to aragonite in eggshell is studied in detail. The presence of organic material, either naturally present eggshell membrane, or artificially supplied L-cysteine, was found to be crucial for the phase transformation to occur. The degree of the transformation in eggshell was strongly dependent on the used milling conditions. The optimization was assesed using Taguchi method, namely using 44 orthogonal array. The optimized parameters encompassed milling time, sample mass, milling speed and duration of breaks and it is shown that sample mass has the decisive effect on the amount of obtained aragonite. Under the most efficient conditions, 73.7 % of aragonite was obtained. Prolonging milling until 4 hours furhter boosts the transformation, reaching 89.9% of aragonite, but further milling leads to the collapse of aragonite structure and pure calcite was observed after 5 hours.