The crystal structure of barstowite (3PbCl 2 .PbCO 3 .H 2 O, P21/m, a = 4.2023(5), b = 9.2020(7), c = 16.6608(6) (Aa), beta = 91.829(9) degrees ) from Bounds Cliff, St Endellion, Cornwall, has been determined to an R = 0.036. The structure can be described in terms of three layers stacked along [001] in the sequence ...CACBCACBC... to give a 16.7 Aa repeat. Layer A consists of face-sharing PbCl 8 distorted square antiprisms. Layer B is composed of Pb coordinated to 5 Cl and 2 O each with two H to form H 2 O groups. Layer C is formed by Pb coordinated to 5 Cl and 3 O which are from CO 3 groups. The H positions are deduced and are H-bonded to O from carbonates. The structural details were in agreement with previous deductions based on infrared spectra.

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