Summary: The carbonate phase of impure sideritic concretions from a sequence of argillaceous sediments from the Westphalian of Yorkshire has been analysed using an acid-dissolution technique and corrections allowing for the decomposition of non-carbonate minerals applied to obtain accurate compositional formulae.

The siderites show considerable compositional variation, some containing Ca in excess of anticipated substitutional limits. Whilst these may be metastable it is suggested that coupled Mg-Ca substitution could provide an explanation.