Predicting iron staining of siderite-bearing microsyenites intended for dimension stone use

This paper reports on the investigation into the performance of a microsyenite rock containing siderite, as a secondary mineral phase, from the disused Mt Gibraltar quarry in the Bowral district, 100 km south-west of Sydney in New South Wales, Australia. Such a rock has been extensively used in the past as a dimension stone for a number of major building facades, curbstones and pavements in Sydney. Laboratory and field exposures of stone samples, conducted under controlled or measured environmental conditions, have shown through petrography that the siderite secondary mineral in the Mt Gibraltar stone is subject to alteration to iron oxides/hydroxides through oxidation processes. The rate of such alteration is generally slow to moderate, but is more profound in an alkaline environment. These findings are consistent with the theoretical desktop study of Eh–pH diagrams. Although the in situ oxidation of the siderite is visually observable, as pinhole-sized rust spots, the mobility of the iron and subsequent staining on the stone surfaces is negligible. Examination of existing structures in Sydney, built from siderite-bearing microsyenites, has confirmed some limited degree of in situ oxidation of the siderite to iron oxides/hydroxides. However, apart from some weathered veins and general browning of the stone with age, no visual defects were observed in structures with service lives of up to 80 years. Thus, the method developed can be used to assess similar rock sources as to the likely long-term iron-staining performance of dimension stone intended for external use.