The effect of copper sulfate on scorodite precipitation and its mechanism of formation at 150 degrees C was investigated. Scorodite was determined to be the dominant phase formed under all conditions explored (0.61 < Fe(III)/As(V) < 1.87, 0.27-0.30 M Fe(SO4)(1.5), 0-0.3 M CuSO4, 0-0.3 M MgSO4, at 2.5 h and 150 degrees C). The produced scorodite was found to incorporate up to 5% SO4 and <= 1% Cu or Mg in its structure. The precipitation of scorodite was stoichiometric, i.e. the Fe/As molar ratio in the solids was equal to one independent of the starting Fe/As ratio in the solution. The presence of excess ferric sulfate in the initial solution (Fe/As > 1) was found to slow down the ordering of the H-bond structure in scorodite. Precipitation under equimolar concentrations (As = Fe = Cu = 0.3 M), short times and lower temperatures (30-70 min and 90-130 degrees C) revealed the formation of a Cu-Fe-AsO4-SO4-H2O short lived gelatinous intermediate that closely resembled the basic ferric arsenate sulfate (BFAS) type of phase, before ultimately converting fully to the most stable scorodite phase (96 min and 138 degrees C). This phase transition has been traced throughout the reaction via elemental (ICP-AES, XPS), structural (PXRD, TEM) and molecular (ATR-IR, Raman) analysis. ATR-IR investigation of an arsenic containing industrial residue produced during pressure leaching of a copper concentrate (1 h and 150 degrees C) found evidence of the formation of an arsenate mineral form resembling the intermediate basic ferric arsenate sulfate phase. (C) 2011 Elsevier Inc. All rights reserved.