| Clay
is not just clay, there are various different
sorts.
All types come from the transformation of
metamorphous or eruptive parents rocks such
as : granite, gneiss, trachyte.
The following
is encountered :
Transformed clay
in situ, that is to say the parent rock has
been turned into clay over thousands of years.
In this type of formation, the transformation
is rarely complete, and often the presence
of loose silicia, which can be inconvenient
due to its abrasiveness, can be noted. This
implies that if we treat by cleaning and spin
separation, pratically all Kaolins undergo
the treatment.
- Sedimentary types
of clay which are formed by the erosion
of older clay which is then washed away
by water. The lightest parts are carried
further than the heavier, and this causes
a sort of natural purification, meaning
that sedimentary clay is much less contaminated
by silicia.
Mineralogy
Clay is always a simple
or complex silicate of aluminium, magnesium
and iron.
Irrespective of the type it can be differently
coloured.
In fact the colouring is due to the presence
either of ferrous or ferric iron, or of
other contaminants, albeit in very small
quantities, which in no way affect the physico-chemical
properties of the clay.
There are four big
families :
Double layers : Kaolinites
or Halloysites.
In this class electronic
microscope show “plate stacks”
for the kaolin. In the case of the Haloycite
these same plates rolled up like a jam roll
lend a fibric appearance.
Pseudo-layers ( fibric
clay ), attapulgites, sepiolites.
Analysis by electronic
microscope shows interwining of fibres with
the following sizes : 2 to 5 µm ( µm
= one millionth of a metre ) for attapulgite
and 5 to 10 µm for sepiolites.
Triples layers, comprising
two families :
glauconites illites
( non gonflant because of the fixed inter
reticular space ) and smectites ( montmorillonites
and beidellites ) which can be gonflant
because the space between the layers is
variable.
Looking at these two
families under the electronic microscope
one sees many rough layers.
Different types of
clay cannot be identified precisely by classical
chemical analysis. In fact, two sorts of
clay can have the same type of chemical
analysis whilst being in themselves completely
different ( for example attapulgites and
montmorillonites in the same quarry at different
heights in the stratigraphy can have exactly
the same chemical analysis ).
The only precise way
of identifying them is therefore X diffraction
which in measuring the space between two
layers lets us classify the mineral with
confidence. |
