Sideronatrite [Na 2 Fe(SO 4 ) 2 (OH).3H 2 O] occurs as yellow botryoidal encrustations on low cliffs of weathered pyrite-bearing mudstones at Barton-on-Sea, Hampshire, England. Extensive areas of the cliffs, up to approximately 100 m 2 , are coated with sideronatrite and its low solubility in cold water secures its longevity. Dry samples of sideronatrite convert readily to metasideronatrite [Na 2 Fe(SO 4 ) 2 (OH).H 2 O], the reaction to sideronatrite being reversible. Sideronatrite, it is suggested, forms as a result of weathering of pyrite that is present in the argillaceous sediments and reaction with Na from the sea-salt spray.

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