Crystallization of the iron phosphate mineral vivianite, Fe₃(PO₄)₂,8H₂O, has been studied by simple precipitation from solutions of Mohr’s salt, Fe(NH₄)₂(SO₄)₂,6H₂O, and a mixture of ammonium dihydrogen and monohydrogen phosphate, both 0.05 M. The vials were closed to the air, so that only initially dissolved oxygen was present. Initial pH of the solutions ranged from 4.5 to 7. Crystals of vivianite, which belong to the monoclinic prismatic class 2/m, were tabular with {010} as the dominating form and normally showed overgrowth of smaller crystals. The largest dimension was about 0.1 mm. At 25°C aggregates were common, but at higher temperatures single crystals were more frequent. In spite of high affinity towards oxidation, vivianite crystals were dominating in most precipitates, presumably because dissolved oxygen is consumed in the initial stage of the process as a result of fast precipitation of much less soluble, nanocrystalline strengite, FePO₄,2H₂O, following oxidation of iron(II) to iron(III). Crystal growth kinetics was determined from pH recording on solutions during 12 hours, 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 λ deduced from analyses of the latter mechanism did not show any significant dependence on temperature; most often they agreed with λ = 27.4±0.5 or 36.0±0.4 pJ/m. Two experiments at 25ºC yielded values from the initial stage of the process as high as λ = 80.4±4.5 pJ/m. An abrupt decrease of slope of the graphs of growth rate versus supersaturation to a value in the usual range followed this stage. 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 strengite.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29768 must be provided.

1. Redox process catalysed by growing crystal – strengite, FePO₄,2H₂O, crystallizing from solution with iron(II) and hydroxylamine