The word "clay" commonly denotes two separate but related things in geology.
First, clay is used to describe sedimentary particles which are less than 1/256 mm
in diameter (sand, by contrast, is defined as being between 1/16 mm and 2 mm).
Most sedimentary particles that are this small (you can't really study them without
resorting to electron microscopy) are the clay minerals. Clay denotes a whole
group of minerals and books have been written about all the different clay minerals
around (check out any introductory sedimentary geology textbook for an introduction
and references).
Clay minerals (mostly aluminum, iron, magnesium silicates) can form in many different
ways. The most common way is by chemical weathering of preexisting minerals. For example,
many igneous rocks, like granite, contain a lot of potassium feldspar (it's the mineral
that colors granite various shades of white and pink). When subjected to slightly acidic
rainwater or groundwater (and all rainwater and groundwater is slightly acidic), this
mineral chemically changes into a type of clay mineral called kaolinite. Other minerals
and such things as volcanic ash will also chemically alter through time into clay minerals.
Clay minerals are the product of weathering.
I'm not familiar with any clays having an organic origin but I'm by no means an expert
on clays and look at them mostly from a geologic perspective. Some people, working on the
problem of the origin of life on Earth, have speculated that early organic molecules may
have polymerized (linked together) on the surfaces of clay minerals because clay minerals
often have an electrostatic charge and a lot of surface area. Some have also speculated
that the alternation of sheet-like clay minerals (mixed-layer clays) acted as a template
for coding genetic information forming primitive DNA-type molecules. Maybe others can
provide you with references, I'm not real familiar with this field.
- Steve.
-- Steven H. Schimmrich, Assistant Professor of Geology Department of Geology, Geography, and Environmental Studies Calvin College, 3201 Burton Street SE, Grand Rapids, Michigan 49546 sschimmr@calvin.edu (office), schimmri@earthlink.net (home) 616-957-7053 (voice mail), 616-957-6501 (fax) http://home.earthlink.net/~schimmrich/