The illite-crystallinity (IC) method was developed during the early
1960s, first in Texas and later in France. It was applied in petroleum
exploration to detect diagenetic stages, but essentially to characterize
the ultimate evolution stages before metamorphism. This empirical
approach had no precise theoretical support for clay mineralogists
or crystallographic scientists. With the development of computerised
XRD treatments, the application field of the IC method has essentially
been for the anchizonal and its immediate limits, for which the method
is most accurate. It corresponds to an indirect measurement of the
mean consecutive illite layers contained in coherent scattering domains
of mixed-layer illite-smectite. Its application field went beyond
petroleum exploration to be applied to regional geology in studies
of low-metamorphic orogenic belts.