Identification, structural transformation, and geological implications of saponite from the Coast Range of Taiwan

Saponite minerals in two different modes of occurrences were identified for the first time in the basaltic rocks at the Wu-shi-pi area of the Coastal Range. In the first type, white colored saponite was found in association with phillipsite in the basalt cavities; and in the second type, green colored saponite was found as a veinlet mineral in the host basalt without any zeolite association.
Chemical composition determinations indicate that the white saponite has no detectable Fe content, and the green saponite has an average of 1.5 wt% total Fe oxide concentration. The difference in the color for two types of saponite is believed to be attributed to the variation of the total Fe content in the saponite samples.
X-ray diffraction measurements were carried out at room temperature, 100, 250, 500, 550, 600, 700, 800, 850, and 1000°C. The results indicate that the saponite structure remained stable up to about 600°C, and at this temperature the (00l) reflections began to disappear with non (00l) peaks remained unchanged. At 700°C saponite started to transform (decompose) into orthoenstatite structure and the transformation (decomposition) had completed before the temperature of 1000°C was reached. At temperature between ambient condition and 600°C, the contraction of the saponite structure was not observed with increasing temperature, in contrast to montmorillonite which displays a significant reduction in the interlayer spacing with increasing temperature due to the loss of the molecular water adsorbed between the silicate layers.
The existence of the phillipsite-saponite assemblage suggests that the last stage volcanic activity of the Wu-shi-pi area was taking place at a temperature of less than 250°C.