clay minerals are a special class of silicate minerals commonly with
disordered structures. The crystal structures of many clay minerals (Space
group, x,y,z coordinated of atoms), are not well defined due to
disorder and flexibility of the structures. X-ray powder diffraction
pattern of most of clay minerals are site-specific. However, there are
well investigated and documented in scientific papers and books describing
patterns of clay minerals, which could be used for identification
purposes. Separation of clays from soils and other minerals, as well
as preparation of clay minerals for diffraction studies are requiring
Minerals Society Links Page
A very extensive set of links which is slanted towards the analysis of
clay minerals but also includes lots of more general crystallography,
crystal chemistry, X-ray diffraction links and geological links. There are
links to some great Earth Science sites here (and Dilbert too). Maintained
by Steve Chipera of Los Alamos National Lab.
Clay is a generic term for an aggregate of hydrous
silicate earth particles less than 4 micrometers in diameter. They are
generally formed by the chemical weathering of silicate-bearing rocks by
carbonic acid, but some are formed by hydrothermal activity. Clays are
distinguished from other small particles present in soils such as silt by
their oblong shape, affinity for water and high plasticity index.
There are three main groups of clays: Kaolinite-Serpentine, Illite, and
Smectite. Altogether, there are about thirty different types of "pure"
clays in these categories, but most natural clays are mixtures of these
different types, as well as other weathered minerals.
Description of clay minerals
Sample preparation techniques of clay minerals well discussed in
Flow diagram for clay mineral identification
Other Application Notes are also available: