Pyritic cone-in-cone concretions
By: Bertram G. Woodland --.
Series: Fieldiana -- v. 33, no. 7; Field Museum of Natural History. Publication -- 1200; Publication (Field Museum of Natural History) -- 1200.
Publisher: Field Museum of Natural History
Published: 1975
Place of publication: Chicago
Subjects: Concretions -- Middle West.; Pyrites -- Middle West.; Geology, Stratigraphic -- Pennsylvanian.
Other titles: Cone-in-cone concretions
Description: A number of pyritic cone-in-cone concretions from Middle Pennsylvanian black shales of Indiana and Illinois are described. These are the only occurrences of typical cone-in-cone structure composed of pyrite that are known to the author, other than those described from Cambrian and Ordovician shales of the Oslo region, Norway. The Indiana concretions have a core of earthy, phosphatic material, probably coprolitic; the Illinois specimen is an isolated cone. Both occurrences have the typical fabric and structure of the usual calcitic (dolomitic) cone-in-cone. The latter is diagenetic in origin and has formed as a direct consequence of fibrous growth in semi-consolidated sediments. Because of this, the striking similarity of structure between the pyritic and carbonate types, and the contrasts between the crystal habits of calcite and pyrite, the pyritic cone-in-cone is interpreted as a pyrite replacement of original carbonate cone-in-cone concretions.