Enargite (Cu3AsS4) is a common penalty mineral in the copper mining industry. Different treatment methods have been proposed to passivate the enargite surface for effective flotation of non-arsenic copper minerals, including pulp potential control, pre-oxidation and chemical depression. Magnesium ammonium mixture (MAA) has been previously tested, showing good selectivity for arsenic rejection. It was hypothesised that MAA would make the enargite surface hydrophilic by adsorption of magnesium ammonium arsenate hexahydrate (MgNH4AsO4 center dot 6H(2)O). Here we show that MAA does not selectively adsorb onto the enargite surface as conjectured. X-ray photoelectron spectroscopy and cyclic voltammetry show that magnesium compounds do not precipitate onto the enargite surface at pH 10, whereas magnesium hydroxide on the enargite surface is formed at pH 12. MgNH4AsO4 center dot 6H(2)O was not observed at either pH 10 or pH 12. The effect of pre-oxidation to promote dissolution of (AsO4)(3-) before conditioning with MAA, which could help catalyse the formation of MgNH4AsO4 center dot 6H(2)O, was also tested. However, the results were the same as those without pre-oxidising the sample. Our investigation shows that under alkaline conditions it is not possible to passivate the enargite surface in MAA solutions for effective flotation of non-arsenic copper minerals. (C) 2014 Elsevier Ltd. All rights reserved.