Iron-rich oolites from a Middle Ordovician (Llanvirn-Llandeilo) sequence in south-central Sweden are associated with volcanic ash beds, clear evidence that volcanic activity was coeval with a period of extensive iron ooid formation in Baltoscandia. Relict texture of the ash is preserved as illitical rims on primary glass shards that underwent rapid calcitization in a marine environment. XRD and Mossbauer spectroscopy reveal that the ooids are composed mainly of chamosite with a Fe (super 2+) /Fe (super 3+) ratio of 6.3. It is suggested that authigenic chamosite was formed from elements released into the sediment when the ash dissolved on the sea bottom. Chamosite precipitated on skeletal grains, and repeated reworking of the sediment oxidized the outer surface of the grains to goethite/hematite, thus creating the typical concentric ooid lamination. The presence of Fe (super 2+) in ooids is in contrast to the agitated environment in which they are formed. Fresh volcanic ash, however, usually contains more Fe (super 2+) than Fe (super 3+) , which can explain the high Fe (super 2+) /Fe (super 3+) ratio in the ooid chamosite, if the iron is derived from the volcanic ash.