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Mechanism of illitization of bentonites in the geothermal field of Milos islandGreece: evidence based on mineralogy, chemistry, particle thickness andmorphology

Christidis Georgios

Πλήρης Εγγραφή

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Περίληψη

Hydrothermal alteration has caused illitization along a 40m vertical profile in the Tsantilisbentonite deposit, Eastern Milos, Greece which consists principally of a Wyoming-type montmorilloniteand 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 methodsrespectively and chemistry using X-ray fluorescence. Illitization is characterized by a 5- to 6-fold increaseof 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 dominatingin the range 100-50% expandable layers (R0 ordering) and the latter in the range 50-10% expandablelayers (R1 and R > 1 ordering). Flaky particles are also abundant in samples with R1 ordering andabundant kaolinite, indicating that the latter might affect illitization. The I/S particles are classified inpopulations with thickness multiples of 10 A, their thickness being probably smaller than the coherentXRD domain. As the reaction proceeds, particles grow thicker and more equant. The distribution of I/Sparticle dimensions forms steady state profiles showing log-normal distribution; however, sensu strictoOstwald ripening is unlikely. It seems that the reaction proceeds toward minimization of the surface freeenergy of I/S, being affected principally by temperature and K-availability. The spatial distribution ofexpandability implies that the heating source was probably a mineralized vein with T < 200~ directedaway from the bentonite, suggesting that illitization might be used as an exploration guide for mineraldeposits.

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