Abstract
The clay mineralogy of bentonite in Balamban, Cebu Province, Philippines was studied. Specimens were randomly collected from lenticular seams of bentonite concordantly intercalated with sequences of calcareous and tuffaceous shale and sandstone of the Middle Miocene Toledo Formation. Multiple analytical techniques of the <2μm size fraction reveal that smectite is the dominant clay constitution of the bentonite. Minor association of mixed-layer kaolinite/smectite, kaolin, and mica is indicative. Diagenetic facies of calcium-rich bentonite and cristobalite-rich bentonite are diagnostic features. Non-phyllosilicate components are mainly quartz, feldspar, cristobalite, calcite, and zeolite. Volcanic glass shards, considered as the precursor material, occur sporadically in the clay-dominated matrix. Calcareous microfossils or nanofossils also exist as subordinate impurities.
Rehydration capacity of the calcium-rich bentonite was observed to be generally greater than the cristobalite-rich bentonite.
Bentonite formation is ascribed to be generated by halmyrolysis of the volcanic glass fragments contained in the vitric tuff exhalative unit of the Toledo Formation. The devitrification and transformation of the glass into nearly pure smectitic clay suite apparently ensued in a marine environment. Diagenetic formation of the calcium-rich facies appears syngenetic with the accumulation of the volcanic ash in saline waters while the cristobalite-rich facies appears authigenic, constituting the silica-rich layers.
