THE TRANSFORMATION OF ORDERED (S-GREATER- THAN-OR-EQUAL-TO-1) MIXED LAYERS ILLITE-SMECTITE TO ILLITE IN A DIAGENETIC SERIES, STATE-OF-THE-ART AND PROSPECTS
B. Lanson et A. Meunier, THE TRANSFORMATION OF ORDERED (S-GREATER- THAN-OR-EQUAL-TO-1) MIXED LAYERS ILLITE-SMECTITE TO ILLITE IN A DIAGENETIC SERIES, STATE-OF-THE-ART AND PROSPECTS, Bulletin des centres de recherches exploration-production Elf-Aquitaine, 19(1), 1995, pp. 149-165
Over the last twenty years, illite-smectite mixed layers (I-S), especi
ally ordered ones, have been studied extensively in diagenetic series.
Basically, I-S ate considered to represent stacks of smectite, presum
ably low-charge montmorillonite, and illite layers of fixed compositio
n (about K-0,K-90 (Si-3,Si-30 Al-0,Al-70) (Al-1,Al-80 Fe-0,Fe-05 Mg-0,
Mg-15) O-10(OH)(2)). With the increasing depth in burial, the amount o
f I-S crystals decreases for a given volume of sediments and the compo
sition of I-S crystals becomes more illitic. Illite and I-S of varied
compositions coexist in the same sample. With increasing burial depth,
the dimensions of I-S and illite crystals increase both in the plane
of the crystallographic a and b directions and along the c axis (cohe
rent scattering domains). Several models for I-S crystal structure hav
e been proposed, deduced mainly from X-ray diffraction data and from t
he observations in transmission electron microscopy. These models are
not consistent with what is known about possible growth mechanisms for
I-S and illite. The model proposed allows us to describe the ordered
I-S stackings and is consistent with these growth mechanisms. As I-S c
rystallites are composed of montmorillonite and of fixed composition i
llite, this model supposes the montmorillonite layers to be included w
ithin the I-S crystals, whereas external layers act as high-charge bei
dellite.