Abstract

A simple procedure is described for the separation of siderite, from mixtures of siderite and calcite, for carbon and oxygen isotope analysis based on the selective decomposition of calcite by reaction with stoichiometric excess ethanoic acid. Artificial mixtures, prepared from two naturally occurring materials (concretionary siderite containing approximately equal quantities of low-Mg and high-Mg siderite, and low-Mg calcite), are used to test the selectivity of calcite removal, investigate the potential for differential acid decomposition of siderites of differing geochemical composition, and assess the effect of separation procedures on the isotopic composition of recovered carbonate. Acidification results in complete removal of calcite and only minor (< 2 wt %) siderite decomposition. Isotopic effects are limited with minimal difference between untreated (experimental control) and acid-treated materials. Discrepancies are attributed to preferential decomposition (greater reactivity) of (low δ¹³C, δ¹⁸O) high-Mg over (high δ¹³C, δ¹⁸O) low-Mg siderite in moderately acidic solvents. Observed departures from true isotopic compositions are insignificant for most geochemical applications when compared with other sources of uncertainty associated with the interpretation of siderite stable-isotope data sets in natural systems.