Chlorite and chloritization processes through mixed-layer mineral series in low temperature geological systems a - review

This present study provides an overview of the clay-mineral reactions involved in the chloritization process in a mixed-layer mineral series, and focuses on the properties of the resulting low temperature chlorites (formed at <220 degrees C) in diagenetic and hydrothermal systems. According to the literature, most chlorite species occurring in low-temperature geological systems are derived from specific clay precursors except for direct precipitates from solution in veins. in addition, three main types of clay mineral series have been associated with these chloritization processes: saponite-to-chlorite, berthierineto-chlorite and kaolinite-to-sudoite reactions. The conversion of saponite to chlorite results in the most common sequence of trioctahedral clay minerals related to the occurrence of Mg-Fe trioctahedral chlorite in a wide variety of hydrothermal and diagenetic to very low-ade metamorphism environments. Two models were proposed in the literature to describe the saponite-to-chlorite conversion through con-ensile. The first model is a continuous transition model based on solid-state transformation (SST) mechanisms and is valid in rock-dominated systems (closed micro -systems with very low fluid-rock ratios). The second model is a stepwise transition model based on dissolution-crystallization mechanisms (DC) and is efficient in fluid-dominated systems (open systems with high fluid-rock ratio). The berthierine to Fe chlorite transition results in a sequence of trioctahedral phases which are related to chloritization processes in iron-rich and reducing environments. This transformation is a cell-preserved phase transition dominated by a SST mechanism that operates simultaneously in different domains of the parental mineral and may be considered as a polymorphic mineral reaction. Finally, the conversion of kaolinite to sudoite (Al-Mg ditrioctahedral chlorite) has not been documented like the other two aforementioned conversion series. Despite the scarcity of detailed investigations, the conversion of kaolinite to sudoite through tosudite is considered a stepwise mineral reaction that is dominated by a DC mechanism. From a compilation of literature data, it appears that several parameters of hydrothermal and diagenetic chlorites differ, including the minimal temperature, the textural and structural characteristics and the extents of compositional fields. In hydrothermal systems, discrete chlorite occurs at a minimal temperature near 200 degrees C, regardless of its chemical composition. In diagenetic systems, discrete chlorite occurs at minimal temperatures that vary according to its crystal chemistry- (10120 degrees C for Mg -chlorite as opposed to 40-120 degrees C for Fe chlorite). The strung discrepancy between the lowest temperature at which Mg- and Fe-chlorite form in buried sediments and in geothermal systems should result from drastically different heating rates, heat-flow