Mechanism of Illitization of Bentonites in the Geothermal Field of Milos Island Greece: Evidence Based on Mineralogy, Chemistry, Particle Thickness and Morphology

George E. Christidis
Technical University of Crete, Department of Mineral Resources Engineering, 73133 Chania, Crete, Greece

Abstract: Hydrothermal alteration has caused illitization along a 40m vertical profile in the Tsantilis bentonite deposit, Eastern Milos, Greece which consists principally of a Wyoming-type montmorillonite and authigenic K-feldspar. The product K-bentonite which contains illite/smectite, kaolinite, K-feldspar, quartz, sulphates and sulphides exhibits an unusual tendency for increase of expandability with depth.

Mineralogy and I/S textures were determined with X-ray diffraction and SEM and TEM methods respectively and chemistry using X-ray fluorescence. Illitization is characterized by a 5- to 6-fold increase of K and release of Si, Fe, Mg Na, and Ca from the parent rock, indicating a K-influx (K-metasomatism) in the system.

The I/S particle morphology is characterized by both flaky and lath-like particles, the former dominating in the range 100-50% expandable layers (R0 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, indicating that the latter might affect illitization. The I/S particles are classified in populations with thickness multiples of 10 A, their thickness being probably smaller than the coherent XRD domain. As the reaction proceeds, particles grow thicker and more equant. The distribution of I/S particle dimensions forms steady state profiles showing log-normal distribution; however, sensu stricto Ostwald ripening is unlikely. It seems that the reaction proceeds toward minimization of the surface free energy of I/S, being affected principally by temperature and K-availability. The spatial distribution of expandability implies that the heating source was probably a mineralized vein with T < 200°C, directed away from the bentonite, suggesting that illitization might be used as an exploration guide for mineral deposits.

Key words: Bentonite • Hydrothermal alteration • Illitization • Illite/smectite • Kaolinite • Neoformation • Ostwald ripening • Solid state transformation

Clays and Clay Minerals; October 1995 v. 43; no. 5; p. 569-585; DOI: 10.1346/CCMN.1995.0430507
© 1995, The Clay Minerals Society
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