Isolation and Characterization of an Iron‐rich Chlorite‐like Mineral from Soil Clays

This study reports on the isolation and characterization of a naturally occurring Fe‐rich, 1.4‐nm nonexpanding chlorite‐like interphase from soil clays. The mineral was isolated from the coarse clay (2‐1 µm) fraction of a Boralf (Gray Luvisol) from Saskatchewan, by using a high‐gradient magnetic separation (HGMS) at 0.20‐T level of magnetic flux density (MFD). While most chlorites contain Fe 2+ , analyses showed that all the Fe in this mineral was in the Fe 3+ form. High‐resolution transmission electron microscopic (HRTEM) examination confirmed a 1.4‐nm spacing throughout isolated particles. Iron was extracted from the interlayer of the mineral by repeated dithionite‐citrate‐bicarbonate (DCB) plus citrate‐dithionite (Tamura‐CD) extractions. These treatments resulted in the collapse of the mineral from 1.4 nm to 1.0 nm following K‐saturation and 105 or 300 °C heating. Extractions of several Fe‐rich chlorites with the same DCB and Tamura‐CD treatments showed that, although some Fe was extracted from them, they retained their characteristic 1.4‐nm spacing after the K‐saturation and heat treatments. The DCB and Tamura‐CD extractions resulted in a marked increase in cation‐exchange capacity (from 27 to 150 cmol c kg −1 ), and a layer charge of 0.75 per half unit cell. The high initial cation‐exchange capacity (CEC) values and the rapid acquisition of a micaceous (1.0‐nm) characteristic are atypical of the normal chlorites also included in this study. It is suggested that the mineral isolated is an Fe‐hydroxy interlayered vermiculite rather than a true chlorite. Regarding the genesis of the isolated mineral, two possibilities exist: the Fe lost from the octahedral layer of an original mica could have been responsible for the formation of an Fe‐hydroxy interlayer, thus producing a chlorite‐like mineral, or the interphase may have resulted from weathering of an original chlorite since chlorites were detected in the soil parent material.