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Geology is the Way

Chert nodule

Nodules of chert are rounded to elongated silica-rich lumps that occur within sequences of limestones (commonly chalk and calcilutites). They originate from the diagenesis of an original silica-rich carbonatic ooze, containing remains of organisms like radiolarians or diatoms, which produce shells made of silica. The silica produced by these organisms is amorphous hydrous silica, which is very unstable in oceanic waters and in the sedimentary environment. Consequently, during diagenesis, silica dissolves in the water-rich sediments and starts to migrate through the surrounding carbonates. Where the pH conditions allow it, silica replaces calcium carbonate, producing nodules of microcrystalline quartz that often mimic the orientation of bedding.

Distinguishing features
Nodules of chert in limestone are easily recognizable because of their characteristic shape, hardness (around 7, harder than metal on scratch) and conchoidal fracture. The color of chert nodules is highly variable depending on the trace elements and minerals present. Chert is more resistant to erosion than limestone. Therefore, chert nodules usually stand out compared to the surrounding carbonates.

Nodule of chert that shows concentric variation in color. The characteristic conchoidal fracture is very evident. “Indiana hornstone”, U.S.A. Photo by James St. John.

Nodules of chert in the crinoidal limestone of the Butte Lake Group, Vancouver Canada. Chert appears in relief compared to limestone, since it is more resistant to erosion. Moreover, limestone shows evident karst features that do not affect chert. Photo courtesy MarkuMark.

Detail of the nodules of chert of the previous image. Sets of fractures are visible in the cherts. Photo courtesy MarkuMark.

Nodules of chert in chalk. Chert shows the typical conchoidal fractures, while the surrounding chalk show stylolites, which are common diagenetic structures of carbonatic rocks. White Park Bay, Northern Ireland. Photo by Siim Sepp.

References
Abdel‐Wahab, A., & El‐Younsy, A. R. M. (1999). Origin of spheroidal chert nodules, Drunka Formation (lower Eocene), Egypt. Sedimentology46(4), 733-755.
Gao, G., & Land, L. S. (1991). Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined field, petrographic and isotopic study. Sedimentology38(5), 857-870.
Knauth, L. P. (1979). A model for the origin of chert in limestone. Geology7(6), 274-277.
Maliva, R. G., & Siever, R. (1989). Nodular chert formation in carbonate rocks. The Journal of Geology97(4), 421-433.

        

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