The Transformation of Groutite (α-MnOOH) into Pyrolusite (MnO2)

J. Lima-De-Faria and A. Lopes-Vieira
Laboratório de T.F.Q.M.P., Junta de Investigações do Ultramar, Lisbon, Portugal

Summary: The transformation of groutite (α-MnOOH) by heating has been studied at 300° C in air, by single-crystal and powder X-ray methods. At this temperature groutite transforms topotactically into pyrolusite (MnO2), the a, b, and c axes of groutite becoming respectively the a, b, and c axes of pyrolusite (in pyrolusite b = a). At various stages of the transformation other weak and diffuse spots were observed on X-ray oscillation photographs, which could not be ascribed to pyrolusite. Some of these extra spots fit well to an α-Mn2O3 structure (isostructural with hematite), with c 14·3 and a 4·9 Å; the other few spots could not be identified.

The transformation of α-MnOOH into MnO2 is explained by a homogeneous mechanism, with migration of protons and electrons to the crystal surface. A detailed interpretation of this mechanism is presented on the basis of the close-packing characteristics of these two structures.

Mineralogical Magazine; September 1964 v. 33; no. 266; p. 1024-1031; DOI: 10.1180/minmag.1964.033.266.10
© 1964, The Mineralogical Society
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