Kinetics of crystal growth of vivianite, Fe3(PO4)2•8H2O, from solution at 25, 35 and 45 °C

Crystallization of the iron phosphate vivianife, Fe-3(PO4)(2)center dot 8H(2)O, has been studied by simple precipitation from solutions of Mohr's salt, Fe(NH4)(2)(SO4)(2)center dot 6H(2)O, and ammonium dihydrogen and monohydrogen phosphate. The vials were closed to the air, so that only initially dissolved oxygen was present. Crystals were tabular with {010} as the dominating form and normally showed overgrowth of smaller crystals. At 25 degrees C aggregates were common, but at higher temperatures single crystals were more frequent. Crystal growth kinetics was determined from pH recording on solutions during 12 h, and precipitate was examined in the microscope the following day. Results could be interpreted as spiral growth at low supersaturation and a combination of this mechanism with surface-nucleation growth at higher supersaturation. Edge free energies A deduced from analyses of the latter mechanism did not show any significant dependence on temperature; most often they agreed with lambda=27.4 +/- 0.5 or 37.2 +/- 0.5 pJ/m. A few experiments showed one or two pronounced minima in the graphs of growth rate versus supersaturation. This may be interpreted in terms of the Cabrera-Vermilyea theory of crystal growth inhibition, the inhibitor being most likely a minute amount of iron(III) from redissolved iron(III) phosphate. (C) 2013 Elsevier B.V. All rights reserved.