www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 494 (2000) 87 – 95 SEM, EDX and EIS study of an electrochemically modified electrode surface of natural enargite (Cu 3 AsS 4 ) P. Vela ´squez a , D. Leinen a , J. Pascual a , J.R. Ramos-Barrado a, *, R. Cordova b , H. Go ´ mez b , R. Schrebler b a Laboratorio de Materiales y Superficie, Unidad Asociada al CSIC, Departamento de Fı ´sica Aplicada I, Departamento de Ingenierı ´a Ciil, Materiales y Fabricacio ´n, Uniersidad de Ma ´laga, E29071 Ma ´laga, Spain b Instituto Quimico, Facultad de Ciencias Ba ´sicas y Matema ´ticas, Uniersidad Cato ´lica de Valparaı ´so, Casilla 4059, Valparaı ´so, Chile Received 6 March 2000; received in revised form 12 July 2000; accepted 10 August 2000 Abstract Enargite electrodes, made from mineral originating in the ‘Salvador’ mine (Chan ˜ aral, Chile), have been studied using SEM, EDX, CV and EIS in an alkaline solution for a range of different potentials in the positive sweep direction (PSD) and negative sweep direction (NSD). The results of SEM and EDX have shown that the principal changes in morphology and chemical composition take place on the surface of the electrode for oxidation potentials close to the value that corresponds to the most intense peak in the PSD, where there is clearly formation of a layer of oxidised material with areas of morphology which are irregular and high in oxygen content. These results are in accordance with the findings from XPS studies published previously. The charge transfer resistance and the resistance of the anodic layer of the electrode were determined by EIS measurements; the values depend heavily on the applied potential in the PSD or NSD. These changes are related to the changes on the surface of the electrode studied by SEM and EDX © 2000 Elsevier Science B.V. All rights reserved. Keywords: Enargite; EIS; SEM; EDX; Mineral electrode; Copper minerals 1. Introduction Enargite (Cu 3 AsS 4 ), a metal sulphide which is fre- quently found as an impurity in copper ores, is a mineral that corresponds to a semiconductor species of type A 3 I B V C 4 II with p-type conductivity [1,2]. Its crystal structure is orthorhombic, based on wurtzite (-ZnS) [3]. The values of the crystal lattice parameters a, b and c are 6.47, 7.44 and 6.19 A  respectively [4]. Each As and Cu atom is surrounded by four S atoms. Further, each S atom is centred in a tetrahedron formed from one As atom and three Cu atoms. This results in the appear- ance of discrete AsS 4 groups in the enargite structure. Given the tetrahedral fashion in which the atoms are arranged, one can expect effects due to strong covalent bonds and highly delocalized holes. The main impuri- ties in natural enargite are iron and antimony, but lead and silver can also be present to some extent [5]. Enargite is a mineral which gives rise to significant arsenic pollution during the industrial procedure of copper mining; moreover, it is necessary to eliminate traces of arsenic from the metal after processing [6,7]. Many copper sulphide ores in Chile contain enargite and it would thus be useful to improve the separation of enargite from the other copper sulphide minerals during the initial processing steps. The behaviour of the mineral particle during the flotation process is affected by its surface properties and its interaction with the electrolyte. During flota- tion, ground mineral is placed in a basic solution containing a collector (usually a xanthate) and molecu- lar oxygen (air bubbling). In this process the surface of the mineral undergoes considerable changes in its chem- ical composition due to interaction with the electrolyte. On the mineral surface there are redox reactions, ad- sorption of molecules, and in general, diverse electro- chemical processes which modify the surface to such an extent that it can be considered as a heterogeneous film * Corresponding author. Tel.: +34-521-31922; fax: +34-952- 13200. E-mail address: barrado@uma.es (J.R. Ramos-Barrado). 0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0022-0728(00)00341-7