Neoformation and transformation of trioctahedral clay minerals in diagenetic process

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Trioctahedral clay minerals such as saponite, chlorite-saponite mixed layer and chlorite are widely found in the Green-tuff formation composed mainly of Miocene pyroclastic rocks. Two modes of occurrence in saponite are recognized, namely cavity fillings precipitated from solution and glass replacements. The former is rich in Fe but poor in Al, while the later is rich in Al and Fe.Al-Fe saponite seems to be transformed to chlorite through interstratification of chlorite and saponite by absorption of Mg(OH)2 and release of SiO2. Trioctahedral clay minerals in the Green-tuff formation is rich in Al than that in deep sea basalt. Tetrahedral Al is likely to be controlled by formation temperature.Refractive indices of trioctahedral clay minerals are shown in Fig. 4. Birefrengence is the most diagnostic feature in the distinction between them under microscope.
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