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

Nikita V Chukanov,Natalia V Zubkova,Sergey N Britvin,Anatoly V Kasatkin,Dmitry A Varlamov,Igor V Pekov

doi:10.3390/min11090969

Nikita V Chukanov, Natalia V Zubkova + Show 4 more

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https://doi.org/10.3390/min11090969

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Abstract

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.

Highlights

Especially compounds with tunnel structures, are important materials used as pigments, ion exchangers for water purification, and cathode materials [1,2,3,4,5,6,7]
This paper provides new data on isomorphism in natural MnO(OH) polymorphs mainly based on the IR spectroscopic data
3300–3500 and 1080–1150 cm−1 are due to stretching vibrations of OH groups forming relatively weak hydrogen bonds and overtones and/or combination modes related to the

Summary

Introduction

Especially compounds with tunnel structures, are important materials used as pigments, ion exchangers for water purification, and cathode materials [1,2,3,4,5,6,7]. We discuss the mechanisms of isomorphic substitutions in natural Mn3+ O(OH) polymorphs and their relationships with the IR spectra. Three polymorphs of the compound MnO(OH) are known. All of them were found in nature. Α-MnO(OH), is an orthorhombic (space group Pnma) mineral with the 1 × 2 tunnel structure and unit-cell parameters a = 4.55–4.56, b = 10.47–10.70, c = 2.87 Å;. The mineral is isostructural, with diaspore AlO(OH) and goethite Fe3+ O(OH)

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