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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The following K-Ar ages have been obtained on mineral separates from some metamorphic rocks in the Oban massif: amphibolites (592-930 Ma), banded gneisses (492-538 Ma), granodioritic gneiss (502 Ma), schists (514-519 Ma) and charnockite (481 Ma).
     Comparison of these ages with previous dates obtained by the Rb-Sr whole rock method and the Pb-Pb evaporation technique on single zircon throws light on the cooling history and orogenic events in the Oban massif. The banded gneiss in the Oban massif yields a zircon age of 1932±5 Ma, indicating that it was emplaced during the early Proterozoic (Eburnean orogeny). The K-Ar age of 492-538 Ma suggests that these rocks were metamorphosed during the Pan-African orogeny. This age is similar to the Rb-Sr isochron age of 510±10 Ma earlier suggested to be the age of migmatization in the Oban massif.
     The amphibolite yielded Rb-Sr isochron ages of 784±31 Ma and 1313±37 Ma. The age of 1313±37 Ma fits into the Kibaran event. The K-Ar hornblende age of 930±37 Ma on the same amphibolite strongly suggests that this amphibolite belongs to an event older than Pan-African (600±150 Ma) and has not been reset perfectly by the Pan-African event.
     The migmatitic schists in the Oban massif yielded a Rb-Sr isochron age of 527±16 Ma which has been interpreted as the age of its migmatization. One sample of this schist, however, gave a model age of 676±24 Ma. The K-Ar ages for the Oban massif schists (514-519 Ma) are close to the Rb-Sr isochron age of ca. 527 Ma and confirm that a metamorphic event occurred in the area at this time. This was followed by final cooling at 515±10 Ma.
     Charnockite in the Oban area has yielded a zircon age of 584±20 Ma, which is Pan-African. The age possibly dates the time of formation of the charnockites. The K-Ar age (481 Ma) obtained in this study is approximately 100 Ma younger than the zircon age and possibly dates the subsequent cooling of the charnockite through hornblende blocking temperature.

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