The iron(II) phosphate vivianite, Fe₃(PO₄)₂·8H₂O, has been precipitated at 37 °C from solution of ammonium iron(II) sulfate (Mohr's salt) and ammonium phosphate, to which a small amount of calcium nitrate or aluminum potassium sulfate (alum) was added. The relatively high temperature was chosen to minimize the formation of crystal aggregates, which would otherwise complicate study of morphology. Crystallization kinetics was determined by pH recording, and the resulting crystals were studied by polarization microscopy. The main effect of Al observed was formation of small, irregular crystals. In contrast, Ca had little or no effect on morphology, but in three cases at intermediate additive concentrations kinetics changed from the usual combination of spiral growth and surface nucleation to pure spiral growth. It is proposed that the screw dislocations leading to growth spirals originate either from calcium phosphate crystallites acting as heteronuclei or from nuclei containing both Fe and Ca and with structure deviating from that of pure vivianite.