METACHERT

 

Metachert is metamorphosed chert.  The physical properties of chert and metachert are quite similar (in some cases, not distinctive at all) - hard, dense, relatively smooth surfaces, sharp broken edges, and conchoidal fracture.  The metamorphism involves recrystallization of the chert's microscopic quartz grains.

 

Metachert (novaculite; microporous metachert) (3.8 cm across) - in Arkansas, metachert is called novaculite.  Information given about Arkansas novaculite invariably mentions its use as a whetstone, which is a sharpening stone for knives.  But, this is not a geologic definition, and so, prima facie, “novaculite” shouldn't be a geologic term.

It turns out that metachert in Arkansas is distinctly, but variably, microporous.  This is what makes novaculite different from ordinary chert.  The texture, look, and feel of most novaculite is indeed different from chert.  Some Arkansas novaculite is not microporous, and is exactly like non-metamorphosed chert.  Most novaculites, however, are somewhat rough, even on cut surfaces.  The roughness is a consequence of the microporosity.

All that said, I’d still prefer to ditch the term “novaculite” and use “metachert” instead.  A rose by any other name is still a rose.

The novaculite shown here is composed of quartz grains ranging in size from about 5 to 20 microns.  It comes from the southern facies of the Arkansas Novaculite (Middle Devonian to upper Lower Mississippian), which is exposed in the Ouachita Mountains of west-central Arkansas, USA.

 


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