Abstract
Intersratified minerals are significant component of alumina-rich clays in the alteration halo formed by hydrothermal argillization of andesitic rocks at Solo, Mabini, Philippines. The observed argillic alteration lies above the zone of chloride reservoir in steam-heated acid environment within a low-sulphidation hydrothermal system. The assemblage of interstratified minerals includes abundant illite/smectite (I/S) associated with fine-grained crystals of kaolinite and jarosite, and minor amount of dioctahedral chlorite/smectite (C/S). These minerals occupy a specific zone that is transitional between the kaolinite-smectite inner zone and the chlorite-illite fringe zone in the alteration halo. Aluminous illite and smectite interstratifications show a wide range of ratio of component layers and display some distinct variations in chemical composition, structure, and morphology. Such variations can be ascribed to the conversion of early-formed smectite to illite through interstratified illite/smectites. The reaction sequence in the conversion series takes place discontinuously in terms of the degree of ordering of interstratifications from Reichweite R=0 (>55% smectite) to R=1 (50-20% smectite) and finally to R=2 (<20% smectite). This decreasing trend in the proportion of smectite layers in I/S occurs primarily with increasing distance from the kaolinite-smectite inner zone and is accompanied by progressive increase in the K2O content of the mineral towards the chlorite-illite fringe zone. The corresponding crystal morphology changes from poorly developed honeycomb texture of smectite to dominantly pseudohexagonal flakes of illite. Aluminous chlorite/smectite minerals occur sporadically near the chlorite-illite zone in the alteration halo and usually show regular to partially regular type interstratifications. They contain about 40-50% smectite and exhibit structural similarities with the completely ordered interstratification of tosudite. The mechanism of smectite to illite conversion series identified in this study is related to the effect of cation composition of hydrothermal solution and the K+ fixation in interlayers. The illite/smectite and associated aluminian chlorite/smectite is interpreted to have been formed in response to substantial influx of Al and K from leaching the andesite rock or the precursor smectite.
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