Hydrothermal Synthesis of Pure Violarite
The synthesis of pure violarite was achieved by hydrothermal methods. Pure pentlandite was used as the starting material, which was synthesized using the conventional silica-tube method. The pure violarite was then synthesized from pentlandite by a hydrothermal method at 125 ºC in a 200 mM acetate buffer of pH 3.90. The reaction consists of the dissolution of pentlandite and the reprecipitation of violarite. A thermosyphon driven flow-though cell was employed to achieve the hydrothermal condition. The synthesis of violarite by this method takes much less time (10 days) compared to the conventional two-step silica-tube method (3 months), and the violarite synthesized by this method is much purer as characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The grain size of the violarite was calculated using Scherrer formula and is in the range 24-34 nm, much smaller than those of silica-tube synthetic violarite (80-125 nm). These small grains adhere to each other, creating a porous texture as indicated by the backscattered images. The violarite grain size increased to approximately 80 nm after further annealing at 300 ºC for 144h and remains constant upon prolonged annealing for up to 789h. The crystal structure of the hydrothermal synthetic violarite was refined using the Rietveld method.