Coupled Substitutions in Natural MnO(OH) Polymorphs: Infrared Spectroscopic Investigation

• Published in: Minerals (Basel) Vol. 11; no. 9; p. 969
• Main Authors: Nikita V. Chukanov, Dmitry A. Varlamov, Igor V. Pekov, Natalia V. Zubkova, Anatoly V. Kasatkin, Sergey N. Britvin
• Format: Journal Article
• Language: English
• Published: MDPI AG 01-09-2021
• Subjects: feitknechtite; groutite; isomorphism; manganite; MnO(OH) polymorphs; solid solution
• ISSN: 2075-163X
• DOI: 10.3390/min11090969

Solid solutions involving natural Mn3+O(OH) polymorphs, groutite, manganite, and feitknechtite are characterized and discussed based on original and literature data on the chemical composition, powder and single-crystal X-ray diffraction, and middle-range IR absorption spectra of these minerals. It is shown that manganite forms two kinds of solid-solution series, in which intermediate members have the general formulae (i) (Mn4+, Mn3+)O(OH,O), with pyrolusite as the Mn4+O2 end-member, and (ii) (Mn3+, M2+)O(OH, H2O), where M = Mn or Zn. In Zn-substituted manganite from Kapova Cave, South Urals, Russia, the Zn2+:Mn3+ ratio reaches 1:1 (the substitution of Mn3+ with Zn2+ is accompanied by the coupled substitution of OH− with H2O). Groutite forms solid-solution series with ramsdellite Mn4+O2. In addition, the incorporation of OH− anions in the 1 × 2 tunnels of ramsdellite is possible. Feitknechtite is considered to be isostructural with (or structurally related to) the compounds (M2+, Mn3+)(OH, O)2 (M = Mn, Zn) with a pyrochroite-related layered structure.