It has been shown previously that amorphous FeO-SiO2 slags are not amenable to high pressure oxidative acid leaching - unlike their crystalline counterparts. Independent studies of glass and silicate mineral dissolution at ambient conditions suggest that acid attack can be hindered by the formation of a passive silica layer. The current work extends this finding to the case of high temperature dissolution of amorphous FeO-SiO2 slags by providing evidence for the formation of a passive silica layer within slag particles under high pressure oxidative acid leaching conditions (250 degrees C, 70 g/L initial H2SO4, 0.62 MPa [90 psi] O-2). Based on the percolation model of glass dissolution, a mechanism of amorphous slag leaching is proposed. (C) 2016 Elsevier B.V. All rights reserved.