MECHANISM OF ILLITIZATION OF BENTONITES IN THE GEOTHERMAL-FIELD OF MILOS ISLAND GREECE - EVIDENCE BASED ON MINERALOGY, CHEMISTRY, PARTICLE THICKNESS AND MORPHOLOGY
Ge. Christidis, MECHANISM OF ILLITIZATION OF BENTONITES IN THE GEOTHERMAL-FIELD OF MILOS ISLAND GREECE - EVIDENCE BASED ON MINERALOGY, CHEMISTRY, PARTICLE THICKNESS AND MORPHOLOGY, Clays and clay minerals, 43(5), 1995, pp. 569-585
Hydrothermal alteration has caused illitization along a 40m vertical p
rofile in the Tsantilis bentonite deposit, Eastern Miles, Greece which
consists principally of a Wyoming-type montmorillonite and authigenic
K-feldspar. The product K-bentonite which contains illite/smectite, k
aolinite, K-feldspar,quartz, sulphates and sulphides exhibits an unusu
al tendency for increase of expandability with depth. Mineralogy and I
/S textures were determined with X-ray diffraction and SEM and TEM met
hods respectively and chemistry using X-ray fluorescence. Illitization
is characterized by a 5- to 6-fold increase of K and release of Si, F
e, Mg Na, and Ca from the parent rock, indicating a K-influx (K-metaso
matism) in the system. The I/S particle morphology is characterized by
both flaky and lath-like particles, the former dominating in the rang
e 100-50% expandable layers (RO ordering) and the latter in the range
50-10% expandable layers (R1 and R > 1 ordering). flaky particles are
also abundant in samples with R1 ordering and abundant kaolinite, indi
cating that the latter might affect illitization. The I/S particles ar
e classified in populations with thickness multiples of 10 A, their th
ickness being probably smaller than the coherent XRD domain. As the re
action proceeds, particles grow thicker and more equant. The distribut
ion of YS particle dimensions forms steady state profiles showing log-
normal distribution; however, sensu stricto Ostwald ripening is unlike
ly. It seems that the reaction proceeds toward minimization of the sur
face free energy of I/S, being affected principally by temperature and
K-availability. The spatial distribution of expandability implies tha
t the heating source was probably a mineralized vein with T < 200 degr
ees C, directed away from the bentonite, suggesting that illitization
might be used as an exploration guide for mineral deposits.