Cob: clay

[Matthew HALL(SED)]

Anoraks at the ready...

Clay minerals are the most important constituents of soils and are classified as particles equal to
or less than 2microns (0.002mm) in diameter. However, clay differs from other soil particles such
as silt (0.002 - 0.063), and sand (0.063 - 2.00mm) in that it is chemically active whereas as sand,
silt and gravel are all inert aggregate. Sand, silt and clay are all formed by the process of
physical weathering (eg rain, freeze/thaw, abrasion etc) and simply represent small particles of
the parent rock from which they have been eroded. Typically these particles are 90-95% quartz in
origin although they can be anything from feldspar, haemetite and so on. Clay is formed by chemical
weathering and is of different composition to the parent rock from which it was formed. Clay
minerals consist of very small platy-structured mineral fragments and so are characterised by their
layered crystalline structure. There are three main members of this group of minerals; kaolinite
(or kaolin - china clay), montmorillonite and h
ydrous mica. Many other transitional forms occur between each type however (there is actually a
clay called nontonite!). All clay minerals are fundamentally built up of layers of silica (oxidised
silicon) and alumina (aluminium oxide) collectively known as sesquioxides. The layers are held
together with their attendant oxygen atoms. Although clay particles are very small their specific
surface area is very large indeed. Montmorillonite for example has a surface area of 800m2; even
though the particle is only 0.002mm in diameter if you could flatten it out such that it had
neglidgable thickness it would make a sheet 20m by 40m! As a result montmorillonite is particularly
known for swelling when in contact with water. Kaolinite is much more stable in contact with water
and does not sweel significantly. The type of clay mineral you have can only correctly be
identified by a technique such as x-ray diffraction which can identify the mineral composition of
the clay although various empirical techniques such as th
e use of certain acids & bases can give a good indication. The presence of silt and clay in a soil
(particles less than 63microns) can be determined by wet sieve analysis but for measuring the
particle size distribution below this size you must use a technique such as sedimentation where you
extract samples of suspended particles from different depths in the vessel and weigh the amount of
soil present after a certain time. 

Glad to discuss any of the above and more besides in much more detail with any who are interested...
Regards
Matthew
____________________________________________________________________
Matthew Hall BSc (Hons) GradBEng
PhD Research Student
Centre for the Built Environment
Unit 9 Science Park
Sheffield Hallam University
Pond Street
Sheffield S1 1WB
England

Tel: +44 (0) 114 225 3200
Fax: +44 (0) 114 225 3206
E-mail: M.Hall at shu.ac.uk