Kaolinite-to-Chlorite Conversion from Si,Al-Rich Fluid-Origin Veins/Fe-Rich Carboniferous Shale Interaction

Franck Bourdelle,Cyril Durand,Ahmed Addad,Schéhérazade Bounoua,Philippe Recourt,Sandra Ventalon,Michel Dubois,Emily Lloret

doi:10.3390/min11080804

Abstract

The kaolinite-to-chlorite conversion is one of the chloritization processes that occurs in low temperature diagenetic and hydrothermal systems. The mechanism of this mineralogical transformation is still under discussion, since direct transformation, conversion via berthierine as intermediate phase or direct formation of berthierine/chlorite mix, either by dissolution-crystallization or by solid state transformation (or a combination of both), are all hypotheses put forward. In this context, each description of a kaolinite-to-chlorite conversion occurrence becomes an opportunity to shed new light and to renew this debate. Studying Carboniferous shale–crosscut by large quartz-kaolinite veins–from the mining basin of the North of France, we report therefore an uncommon kaolinite-Fe-rich chlorite assemblage. This assemblage appears as a chlorite fringe 20 µm wide along the interfaces between the shale and the quartz-kaolinite veins. All petrographical, mineralogical and chemical data suggest that the Fe-chlorite results from the interaction between the shale, providing the Fe,Mg supply, and the Si,Al-rich veins, leading to the chloritization of the kaolinite at a small scale via at least one dissolution-recrystallisation step. High-resolution observations highlight that neoformed Fe-rich chlorite contains some 7Å isochemical layers, as relict of berthierine. Therefore, we advance that the conversion takes place either through the precipitation of berthierine following by a second step involving solid state berthierine-chlorite conversion, or through the direct precipitation of a chlorite-rich/berthierine-poor mix driven by the Fe/(Fe + Mg) ratio, at low temperature and in reducing conditions. The comparison of our data with the recent literature allows to prefer the second hypothesis.

Highlights

Introduction iationsChlorite, a 2:1:1 phyllosilicate crystallizing from 80 ◦ C [1] up to 600 ◦ C [2], is ubiquitous in diagenetic, hydrothermal and metamorphic rocks
All these mineral phases are similar to those previously identified by X-ray diffraction (XRD)
We report the formation of Fe-rich chlorite as the result of an interaction between a black shale supplying Fe, Mg and large kaolinite areas formed from a Si,Al-rich fluid, like a small size system leading to a noteworthy mineralogical reaction front

Summary

Introduction

A 2:1:1 phyllosilicate crystallizing from 80 ◦ C [1] up to 600 ◦ C [2], is ubiquitous in diagenetic, hydrothermal and metamorphic rocks. Considering that the chlorite composition depends in part on the pressure-temperature conditions [3], chlorite is widely used as an indicator mineral and provides the basis of many geothermobarometers (e.g., [4,5,6,7,8,9,10,11,12]). Chlorite formation, and more widely clay mineral transformation, may depend on kinetic factors. This point is coupled to the complexity of the conversion pathways of a precursor into chlorite, and more widely, of the chlorite formation processes

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Discussion

Conclusion