"Rock of the Month # 16, posted October 2002" Chert is a very fine-grained variety of silica, composed largely of cryptocrystalline quartz, SiO₂ --- arguably the most familiar mineral in Earth's crust. Strictly speaking, chert, as described below, is a type of rock, the principal constituent of which is fine silica. This silica, especially when microscopically fibrous, is generally termed chalcedony, familiar to rockhounds in other contexts, such as the fillings of geodes, and known in banded form by yet other names such as agate or onyx. Sample 1-PC, courtesy of A.D. Marble and Company, Inc., of Conshohocken, PA, consultants in environmental planning and cultural resource studies.

The photo displays a mm-scale chalcedony veinlet in chert from Pennsylvania, eastern U.S.A. Silica fibres, with length-fast optical orientation, form radiating fans and spherulites up to 500 microns (0.5 mm) wide, nucleated with minor rhombohedral calcite on vein margins. The host rock is a mixture of fine-grained silica and granular carbonate, with traces of iron oxides and pyrite. Taken in cross-polarized transmitted light, 40x magnification, long-axis field-of-view 2.8 mm. For an alternative view of this sample, and further discussion of silica, see the page on the recently described polymorph known as moganite. Cherts found in the bedrock of Pennsylvania are of Paleozoic (Ordovician and especially Devonian) ages.

Fine-grained to cryptocrystalline silica of sedimentary origins is generally referred to as chert in north America. The term flint is perhaps best reserved for the characteristic chert nodules found in the Cretaceous chalk of Europe, where it is usually black to grey, less often brown or red, with a fine conchoidal fracture and typically a thin, porous, white or yellowish rind formed by weathering. Red, yellow or brown chert is often termed jasper. Chert may form from siliceous sea-floor ooze, incorporating the remains of millions of individual organisms such as plankton and sponges that secrete silica. More generally, it may form at a later stage in the history of the host sediments (limestone, dolomite, shale) by processes of replacement and deposition, as siliceous fluids migrate through the host sediment. This process will generally occur at ambient temperatures, so further confusion is introduced by informal use of the term cherty silica to refer to some fine-grained quartz found in hydrothermal veins, hot-spring siliceous sinters, and associated alteration systems (whence the noun jasperoid). Whatever the origin of the silica in chert, the usual connotation is of a chemical precipitate, as opposed to more abundant clastic sediments composed of fine sand particles, such as siltstone or quartzite.

However, the most familiar form of chert is that which is found as nodules and thin beds, typically conformable with the bedding of the carbonate host rock. Such chert has been prized, around the world and for millennia past, as raw material for the preparation of stone tools and weapons. Archaeologists take careful note of the occurrence of chert artefacts, and an arsenal of mineralogical, chemical and statistical techniques has been applied to assemblages of chert artefacts, in hope of tracing the origins of particular lithic finds, and so throwing light on the trading patterns of vanished communities. Such detective work is termed an archaeological provenance study.

References

GORDON,SG (1922) The Mineralogy of Pennsylvania. Academy of Natural Sciences of Philadelphia Spec.Publ. 1, reprinted by Dover Publications, Inc., New York, 1959, 255pp.

LUEDTKE,BE (1992) An Archaeologist's Guide to Chert and Flint. Archaeological Research Tools 7, Institute of Archaeology, University of California, Los Angeles, 172pp.

McBRIDE,EF (compiler) (1979) Silica in Sediments: Nodular and Bedded Chert. SEPM Reprint Series 8, 184pp.

PROTHERO,DR and LAVIN,L (1990) Chert petrography and its potential as an analytical tool in archaeology. In `Archaeological Geology of North America' (Lasca,NP and Donahue,J editors), GSA Centennial Spec.Vol. 4, 633pp., 561-564.

SHULTZ,CH (editor) (1999) The Geology of Pennsylvania. Pennsylvania Geological Survey / Pittsburgh Geological Society, Spec.Publ. 1, 888pp.