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Transformation of Synthetic Birnessite to Cryptomelane: An Electron Microscopic Study

Published online by Cambridge University Press:  02 April 2024

Cy-Chain Chen ,
D. C. Golden  and
J. B. Dixon
Cy-Chain Chen
Affiliation:
Department of Soil & Crop Sciences, Texas A&M University, College Station, Texas 77843
D. C. Golden
Affiliation:
Department of Soil & Crop Sciences, Texas A&M University, College Station, Texas 77843
J. B. Dixon
Affiliation:
Department of Soil & Crop Sciences, Texas A&M University, College Station, Texas 77843
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Abstract

Na-saturated bimessite was synthesized by oxidizing an alkaline MnCl2 solution with gaseous O2. Transmission electron microscopy (TEM) showed no morphological change upon K-saturation of the bimessite, but selected-area diffraction (SAD) revealed structural disorder. Stepwise heating of the K-birnessite to 800°C yielded cryptomelane, as indicated by X-ray powder diffraction (XRD). Various degrees of transformation to the final cryptomelane product were observed by TEM and SAD, but not by XRD. Unaltered bimessite formed chiefly as thin, platy crystals of apparent hexagonal outline. The cryptomelane crystals that formed by heating the K-saturated bimessite were acicular. Bimessite crystals, partially transformed to cryptomelane and displaying non-integral diffraction spots, formed plates having linear striations and rods twinned at 60° to each other. These intermediate products commonly retained some of the original hexagonal appearance of the parent bimessite. Structural disorder was detected in the partially transformed crystals by SAD. TEM revealed 6.8- and 4.8-Å spacings of the (110) and (200) planes of cryptomelane, respectively. The basal planes of bimessite layers (001) appeared to correspond topotactically to the (110) plane of cryptomelane during these transformations.

Keywords

BimessiteCryptomelaneSelected area diffractionStructural disorderSynthesisThermal treatmentTransmission electron diffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 34 , Issue 5 , October 1986 , pp. 565 - 571
Copyright
Copyright © 1986, The Clay Minerals Society

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