Dear Rob,
You should obtain more than 95% yield of CO2 from siderite in a few 
hours in 150 degC (or  in a few days in 100 degC). See e.g. Rosenbaum 
and Sheppard, GCA (1986) 50, 1147.
Best regards,
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Tomasz Durakiewicz
Mass Spectrometry Lab, Institute of Physics
Maria Curie-Sklodowska University, Lublin, POLAND
tel. (0-81) 37 61 78, fax (0-81) 33 669
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