Beidelltie clay from Chang-yuan, Taiwan; geology and mineralogy

Beidellite clays near Chang-yuan, 80 km south of Hwalien, form vein-like bodies 5-10 m wide and 20-45 m deep. Andesitic rocks have been fractured by faulting permitting hydrothermal solutions to percolate and produce an altered mineral assemblage in which beidellite is associated with a mixed-layer illite-smectite, kaolinite, dusty pyrite, microquartz, cristobalite, calcite and dolomite, plagioclase, and gypsum. Ca-clay is common on outcrops and subsurface, where the clay is leached, Na-clay is found; some outcrops are partly kaolinized. Chemical analyses of the beidellite show that the structure is charged predominantly in the tetrahedral sheets. Exchange capacities are mainly in the range 120-140 mEq/100 g clay (110°C); exchangeable cations are Na ≫ Ca ≃ Mg > K. After K saturation and heating, ten samples still expand to near 17 A with ethylene glycol, but the two samples with the highest tetrahedral charge expand only to 14·6-15·0 Å although their total charge is similar to the other samples.