Hydrothermal synthesis of chlorite from saponite: Mechanisms of smectite-chlorite conversion and influence of Mg2+ and Al3+ supplies

Abstract Conversion of smectite to chlorite (S C conversion) is a common clay transformation reaction in a range of geological settings. Yet, current models for S C conversion are still under debate. This study systematically investigated the mechanisms of S C conversion and the influences of Mg2+ and Al3+ supplies. Saponite (a representative trioctahedral smectite) was reacted with Mg2+ and Al3+ under 340 °C and autogenous pressure. Reaction was found to reach steady state around 24 h, and either randomly mixed-layered chlorite/smectite (C/S) or chlorite formed from saponite. Solution chemistry and mineralogical evidence suggested that the S C conversion occurred through solid-state transformation, characterized with gradual formation of hydroxide layers from Mg Al polymers via polymerization in interlayer and concomitant substitution of Al for Si in 2:1 layers. The formation of mixed-layered C/S or chlorite depended on Mg2+ and Al3+ supplies. When Mg2+ and Al3+ were limited, mixed-layered C/S would form from saponite, and growing Mg2+ and Al3+ supplies would continuously increase the percentage of chlorite in mixed-layered C/S until chlorite formed (saponite → mixed-layered C/S → chlorite). In contrast, chlorite could form directly from saponite under sufficient Mg2+ and Al3+ supplies (saponite → chlorite). This study contributed to an improved understanding of S C conversion, and provided a general methodology and knowledge basis for future studies involving this mineral conversion.