Illite, chlorite, montmorillonite and kaolinite, as well as
natural marine sediments and mixtures of the standards were subjected
to density separation by centrifugation in a linear heavy-liquid
gradient.
The density layers yielded by centrifugation were recovered
and analyzed by X-ray diffraction. Separates were not monomineralic
but rather were polymineralic usually with minerals of invarient
density, such as quartz, in more than one layer. This incomplete
separation is attributed to insufficient centrifugation time. The
X-ray character of minerals of variable density changes in such a
way as to suggest increased crystallinity with depth in the density
gradient.
Although several samples were known to contain montmorillonite,
none of the density layers showed X-ray evidence of this material. The anomalous behavior of montmorillonite is attributed
to its imbibing of the polar organic chemicals used as surfactants
into its expandable crystal structure to produce an extremely large
basal spacing. This problem can be overcome in some cases by
heating the clay to 110°C for 8 hours, but in other cases this had
little or no effect in collapsing the expanded structure.
Even though the project was not a total success, the method
holds promise, providing the duration and intensity of the centrifugation
is increased and complete purging of polar organic molecules
from expandable layered clays can be accomplished
