The Tetravalent Manganese Oxides
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American Mineralogist, Volume 64, pages I199-1218, 1979 The tetravalent manganeseoxides: identification, hydration, and structural relationships by infrared spectroscopy RussBrr M. Porrnn'AND GpoRcn R. RossueN Division of Geologicaland Planetary Sciencef Califurnia Institute of Technology Pasadena,Califurnia 9I 125 Abstract A compilation of the infrared powder absorptionspectra of most naturally occurring tet- ravalent and trivalent manganeseoxides is presentedwhich is intended to serveas a basisfor the spectroscopicidentiflcation of theseminerals in both orderedand disorderedvarieties, in- cluding those too disorderedfor X-ray diffraction studies.A variety of synthetic manganese oxidesare also included for comparisonto the natural phases.The samplesinclude: aurorite, birnessite,braunite, buserite, chalcophanite,coronadite, cryptomelane,groutite, hausman- nite, hollandite, lithiophorite, maoganite, manganese(Ill) manganate(IV), manganosite, manjiroite, marokite, nsutite, partridgeite, pyrolusite, quenselite,rancieite, ramsdellite,ro- manechite, sodium manganese(Il,flI) manganate(Iv), todorokite, and woodruffite. The spectraindicate that well-orderedwater occurs in ramsdellite,chalcophanite, and most ro- manechites.Disordered water is observedin the spectraof nsutite,hollandites, birnessite, to- dorokite, buserite,and rancieite.The infrared spectraof well-orderedtodorokite, birnessite and rancieite differ which indicatesthat they possessdiflerent structuresand should be re- garded as distinct mineral species.Much variation is observedin the spectraof pyrolusites, nsutites,birnessites, and todorokiteswhich is interpretedas arising from structural disorder. Spectraltrends suggest that todorokite,birnessite and rancieitehave layered structures. Introduction oxide crystal structures and structural relationships. The finely-particulate and disordered nature of X-ray powder diffraction patternsare the only struc- manganeseoxides in many of their concentrationsin tural data available on severalof the oxides impor- the weatheringenvironment has made identification tant in the weatheringenvironment, and thesegener- of their mineralogy by means of X-ray diffraction ally show lines which are few in number, broad, and difficult and sometimesimpossible. Characteristic X- sometimesvariable in position. This has causedun- ray powder diffraction lines are frequently broad, in- certainty in the structural relationships among syn- distinct, or absent altogether and do not serve as a thetic and natural samplesand a consequentcon- satisfactorybasis for identification. The frequent oc- fusion in their classification. currence of silicate phasesadmixed with the low- This work will show that infrared (IR) spectros- temperaturemanganese oxides is an additional com- copy is often a necessaryalternative and generally a plication becausetheir X-ray lines can sometimesbe useful supplement to X-ray diffraction for mineral- confusedwith those of the manganeseoxides. ogical analysisof the manganeseoxides. Because it is Problems of particle size and disorder are a char- sensitiveto amorphous componentsand those with acteristic of pure manganeseoxides and have been short-rangeorder as well as to material with long- responsiblefor our poor understandingof manganese range order, IR spectroscopyyields a more complete and reliable descriptionof materialssuch as the man- I Presentaddress: Owens-Corning Fiberglas Corporation, Gran- ganeseoxides, where crystalline disorder may be ex- ville, Ohio 43023. pected.In addition, it is sensitiveto the structural en- 2 Contribution N o. 3126. vironment of the hydrous components, which is 0np3-{0/'X/7 9/ I I I 2- I 199$02.00 POTTER AND ROSSMAN: TETRAVALENT MANGANESE OXIDES frequently diagnosticof the manganeseoxide mrner- Appendix figures are indicated by an "A" or "B" fol- alogy. lowing the figure number.' The intensities of IR BecauseIR spectroscopyis not a primary struc- spectramay vary by a factor of 2 or 3 due to differ- tural technique like X-ray diffraction, it is necessary encesin sample particle size and dispersion in the to "calibrate" it against well-crystallized materials pellet. For this reason we have presentedthe IR whose mineralogy has been previously determined spectra at concentrationssuch that all spectrahave by X-ray diffraction. Once a mineral's characteristic the samemaximum intensity in the 1400cm-' to 200 IR spectrum has been determined, it can then be cm-' region. Each spectrumin the 4000cm-'to 1400 usedto identify whether that phaseis presentin more cm-' region is presentedat 4 times the concentration disorderedsamples. of its lower energy spectrum. The presentationin- More than 20 predominantly tetravalent manga- tensitieslisted in the figure captionsallow the origi- nese oxide phasesare recognizedas valid mineral nal intensity of the spectrato be calculated.The pre- species.X-ray criteria for the identification of the sentation intensity is 100 times the intensity in the well-crystallizedoxides are well established(Cole et figure divided by the intensity measuredusing the al., 1947;Burns and Burns, 1977a).This paper pre- standardpreparation techniques (Section 2). sentsthe basisfor determinationof their mineralogy Experimentaldetails by IR spectroscopy.The IR spectraof many of the manganeseoxides have been published (Gattow and Purity and mineralogy were determinedby X-ray Glerrser, l96la, b; Glemser et al., 196l; Moenke, powder diffraction using CrKa radiation and a 1962;Yalarelhet a1.,1968;Agiorgitis, 1969; Kolta el Debye-Scherrercamera. The IR spectrum of some al., l91l; van der Marel and Beutelspacher,1976), minerals is so distinctive that after an initial correla- but the quality of the data are generally too poor to tion was made betweenthe X-ray diffraction pattern show clear differencesamong the oxides. Using im- and the IR spectrum, further X-ray work was not proved instrumentation and sample preparation needed.When necessary,qualitative chemicalanaly- techniques,we have found that the differencesin the sis was usedin addition to X-ray diffraction to deter- IR spectra of the tetravalent manganeseoxides are mine mineralogy. sufficiently diagnostic to permit their identification in IR spectra were obtained with a Perkin-Elmer manganeseoxide concentrationsof the natural envi- model 180spectrophotometer on 2.0 mg of powdered ronment evenwhen the oxidesare highly disordered. sampledispersed in TlBr pelletsfor the 4(XX)cm-' to We have usedthese spectra as the basisfor the deter- 1400cm-' region and on 0.5 mg in TlBr and KBr pel- mination of manganesemineralogy in a variety of lets for the 1400cm-' to 200 cm-' region. Pelletsof occurrences including desert varnish, manganese 13 mm diameter were pressedfor I minute at 19,000 dendrites, stream deposits, and concretions (Potter psi under vacuum. Pellets were prepared without and Rossman,1979a,b, c). evacuationto verify that dehydration of the manga- This paper also contains the results of research nese oxide did not occur under these conditions. into somefundamental problems of tetravalentman- SinceKBr is hygroscopic,it was not usedin the 4000 ganeseoxide mineralogy.We have usedour spectro- cm-' to 1400cm-' region,where water and hydroxide scopic results in conjunction with a variety of other absorptionoccurs. TlBr is preferableto KBr because techniquesto investigatestructural variations within it is non-hygroscopic.Also, becauseits refractive in- the manganeseoxides, to addressquestions about the dex better matchesmost manganeseoxides, it gives doubtful structures,to proposestructural models for spectra of better quality. The figures presentedare structureswhich are as yet unknown, and to test the from TlBr pellets. Where the corresponding spec- validity of synthetic materials as analogsof natural trum in KBr differs significantly,it is included in Ap- samples. pendix B. A vacuum dewar with KBr windows was The presentationof theseresults follows the classi- used to obtain IR spectraat liquid nitrogen temper- ficationscheme of Burns and Burns (1975,l977a,b), ature in the 4000 cm-' to 1400cm-' region. Qualita- which is based on the nature of the polymerization of tive chemical analyseswere done with an SEM MnOu units, in which six oxygenssurround a central 3 manganesecation in approximatelyoctahedral coor- To receivea copy of Appendic.esA and B and a table of IR band positions, order Document AM-79-ll7 from the Business dination. For each structure representativespectra Office, Mineralogical Society of America, 2000 Florida Avenue, are included with the text. Spectraof other samples NW, Washington,D. C. 20009.Please remit $1.00in advancefor are containedin Appendix B as indicated in Table l. the microfiche. POTTER AND ROSSMAN: TETRAVALENT MANGANESE OXIDES l20l equipped for energydispersive X-ray analysis.Man- the resolution of bands 3,4, and 5 improves,and the ganeseoxidation statewas determinedby room tem- intensity of band 3 growswith respectto band 4. perature dissolution in excess0.05M Fe2* in 0.5M All the pyrolusite powder diffraction patternscon- HrSO4followed by back titration of excessFe2* with tain linesat 3.40,2.63,2.32,1.78, and l.7lA. These 0.002M KMnOo and spectrophotometricdetermina- cannot be indexed on the tetragonal pyrolusite cell tion of total Mn as MnO;. This procedureis a modi- nor on a superlatticeof it. They can all be attributed fication of that of Moore et al. (195O).Far-infrared to manganiteand accountfor its four strongestlines. spectrain the region 200 cm-' to 35 cm-' were ob- The +3.99 manganeseoxidation state of