Weathered chert, or leached chert, is a white to gray, or yellowish,
porous, light-weight, harsh to feel, chalky-appearing rock which occurs
over much of the southern half of Missouri. It does not effervesce in
acid. Usually it occurs as a zone from a fraction of, to more than an
inch in thickness, about a denser core of hard, compact chert (flint),
or makes up an entire small rock fragment or gravel.
[Illustration: Chert hand specimen showing quartz-lined fossil
cavity in center, compact fresh chert in interior, and
chalky-appearing weathered outside margins. From near Columbia.]
It develops as a relatively insoluble residue left when the more soluble
rock material in association has been leached away during the weathering
process. Its composition approaches pure silica. It has no established
use and no commercial value.
“Kaoleen”
“Kaoleen” is a term used locally in part of south-central Missouri to
refer to a chalky, white to tan or buff, porous weathered chert, but the
name should be dropped because it is unnecessary (use weathered chert),
confusing, and not recognized elsewhere. Most probably the term arose in
corruption of the word _kaolin_, which is the name for a true,
high-quality clay, to which the leached and weathered chert bears a
slight resemblance. Kaolin has the chemical composition of clay (hydrous
aluminum silicate), whereas “kaoleen” is impure silica. See the
discussion on Weathered Chert.