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.