MadSci Network: Earth Sciences


Date: Mon Dec 17 16:25:08 2001
Posted By: Alex Barron, Graduate Student, Ecology(Biogeochemistry)
Area of science: Earth Sciences
ID: 1008141026.Es

   You are correct: clay is often found in streambanks but it is not 
neccesarily generated there.  Clay can either mean any soil particle which 
is smaller than 2 micrometers (a micrometer is 1/1000000th of a meter) or a 
clay mineral - which is what most people mean when they talk about clay.  
Clays are called secondary minerals because they are formed from the 
weathering of minerals from rock (primary minerals).  They are composed of 
sheets of aluminum and silicon oxides.  You can think of them as tiny 
little hexagonal plates stacked on top of each other.
   Basically, crystals of minerals in rock (this includes anything from the 
size of boulders down to individual crystals smaller than sand grains) are 
weathered (broken down and transformed chemicaly) by reacting with acidity, 
water and oxidants in soil solution.  Rock minerals like feldspar slowly 
break down and are transformed and/or recyrstalize into sheets of mineral 
(clays) because it is a more stable form for the elements involved.  With 
even more extensive weathering, even clays can break down to form more 
weathered clays (like kaolinite) or even amorphous minerals like gibbsite 
(which is just a jumbled mess of Al and O).
   This process happens in virtually all soils because virtually every soil 
has some minerals which can weather over time.  As a result, almost all 
soils contain clay (with the exception of the very top of the soil which is 
usually almost entirely organic matter) but most people only notice it when 
it makes up a large percentage of the soil.
   I hope that clears things up!


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