MECHANISM OF ILLITIZATION OF BENTONITES IN THE GEOTHERMAL-FIELD OF MILOS ISLAND GREECE - EVIDENCE BASED ON MINERALOGY, CHEMISTRY, PARTICLE THICKNESS AND MORPHOLOGY

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.