Sand-size concretions composed of aggregates of pyrite spherules are associated with organic matter in a postglacial lacustrine clay from South Bay, Lake Huron. These concretions resemble pyrite described in the literature, however, the origin of the characteristic framboidal form remains uncertain. Although it is a rare mineral in Great Lakes sediments, pyrite may have formed in this clay because of the presence of sufficient sulphur, possibly derived from sulphate-bearing ground water.

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