The finer subfractions contain only the 1Md illite.After leaching with ammonium acetate, Rb–Sr ‘leachochrons’ for the untreated subfraction, leachate and residue were derived for all subfractions.
However, barring an extraordinary coincidence, the result of such events will be that when we draw the isochron diagram, the minerals will no longer lie on a straight line.
The glaucony series (of which glauconitic mica is the K-rich end member) is one of the few widely occurring minerals in the sedimentary record.
So their isotopic ages determined mainly by the K–Ar method represent a major database for the geologic time scale.
As the clay particle size decreases from 1–2 to 0.1–0.2 μm: (1) the proportion of chlorite, where present, is reduced; (2) the Crystallinity Index Standard (CIS) increases; (3) the Sr coordinates is observed for the data points of leachates.
This suggests that mixtures of at least two, non-cogenetic, illite generations are present in all the shales, and that these illites crystallized in environments with dissimilar .