Important fact about CLAYS
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.
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/bentonites 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 or bentonites 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.