Lattice parameters have been determined for two coexisting siderite phases in concretions of Westphalian age. Chemical evidence indicates that, for one concretion at least, the phase of smaller cell dimensions is a magnesian siderite with around 25 mole % MgCO₃. Both abundances and compositions of the two phases vary between concretions whilst a single phase only is present in disseminated siderite. It is thought unlikely that the two phases represent an equilibrium assemblage and a mechanism is suggested by which magnesian siderite could have formed later.