American Mineralogist, Volume 71, pages 805-814, 1986 Topotactic relations among pyrolusite, manganite,and MnrOr: A high-resolution transmissionelectron microscopyinvestigation J*ms H. Rlsx Department of Geology, Arizona State University, Tempe, Araona 85287 Pnrnn R. Busncx Departments of Geology and Chemistry, Aizora State University, Tempe, Arizona 85287 Ansrnrcr Pyrolusite (MnOr, tetragonal) is commonly created in the natural environment by the oxidation of manganite (MnOOH, monoclinic). Such secondarypyrolusite typically dis- plays anomalous, nontetragonalcharacteristics. In this high-resolution transmission elec- tron microscopy (Hnrrvr) study, MnrO, (monoclinic) was found intergrown with pyrolusite that had formed from manganite.This occurrenceof MnrO, is ofinterest for severalreasons: (l) MnrO, has been identified in a natural manganite-pyrolusitemixture. Assuming that it is not an artifact of nnreu observation, this is the first report of naturally occurring MnrOr. (2) Aligned monoclinic MnrO, intergrown with pyrolusite may be partly responsible for the nontetragonalcharacter of secondarypyrolusite. (3) MnrOr-pyrolusite intergrowths occur adjacentto manganite.This occurrencesuggests that the oxidation of manganitemay form both pyrolusite and MnrOr. Another new Mn oxide has been identified through selected-areaelectron difraction. We hypothesizethat this phaseis a structural modification of pyrolusite. Ordering of OH groups and Mn atoms of different oxidation statesis a possibleexplanation for this mod- ification. fNrnooucrroN translations, a, and ar. Either of these should have an equal probability of becoming the manganite a (or b) Pyrolusite (MnOr), a tetragonal mineral with the rutile translation when pyrolusite is reduced to manganite. structure, is the most stable form of manganeseoxide in However, that is not what is observed. In the sequence manyterrestrialenvironments.Distinctionshavelongbeen manganite (primary) * pyrolusite (secondary) + man- recognized between the relatively rare primary form of ganite (secondary), primary and secondary manganite in- pyrolusite and the much more common secondaryform variably have the same orientations. A memory of the that occursas pseudomorphicreplacements of other man- original manganite orientation is conveyed by the pyro- ganeseoxide minerals, particularly manganite (MnOOH, lusite intermediate, but there is a question as to how this monoclinic). Primary pyrolusite has a hardness of 7, memory is transmitted. whereas secondary pyrolusite displays variable, much The nontetragonal characteristicsof secondary pyro- lower hardness values. Secondary pyrolusite also pos- lusite have been attributed to microstructures formed in sessesseveral characteristics suggestiveof a symmetry pyrolusiteuponitscreationfrommang;anite(Strunz, 1943; lower than tetragonal. For example, reflectedJight mi- Champness,l97l).Pyrolusiteandmanganitehave similar croscopy shows only one cleavagedirection as well as structures.The manganitea and c translationsare halved optical anisotropy in the (001) plane of secondarypyro- to form the a and c pyrolusite unit-cell translations,while lusite. Until Strunz (1943) showedthrough single-crystal D of manganite contracts from 5.28 Arc q.q0 A to form X-raymeasurementsthatthetwoformsofpyrolusitehave the other a translation of pyrolusite (Fig. l). This 150/o identicalcrystalstructures,primarypyrolusitewastermed contraction along b presents the possibility that micro- polianite and considereda distinct mineral. Later studies scopic cracksparalleling the manganite (010) planes sep- (de Wolfl 1959; Potter and Rossman,1979) have found arate newly made crystallites of pyrolusite. Images ob- that some secondarypyrolusites are actually orthorhom- tained by transmission electron microscopy confirm the bic. existenceof lamellar micropores in secondarypyrolusite Another anomalous property of secondarypyrolusite, (Champness, l97l). Such microcracks may explain the termed the memory effect,was noted by Dent Glasserand aberrant optical properties, the decreasedhardness, and Smith ( 1968).Ideally, pyrolusite hastwo equivalent lattice the great chemical activity and adsorptivity of secondary 0003{04xl86/0506-0805$02.00 805 806 RASK AND BUSECK:HRTEM STUDY OF Mn OXIDES Table l. Unit-cell dimensions of pyrolusite, manganite,and MnrO, Pyrol usi te Mangani te l'4n-0^ D6 a = 4.3999A a=8.98A a = 10.347A c = 2.8140A b=5.28A b= 5.72A c = 5.71A c = 4.852A I = i09'25' = (#136) = (#i4) = (#12) SG P4^./mnnz sG B2rld SG czlm Source : Source l Source : Gl,*illr% Bauer,1976 Buerger, 1936 oswald and Hanpetich, 1967 for 20 h at 5 kV andfor 2 h at 1.5kV. A reor--rernr200CX 200- kV anda Philips400T 120-kVmicroscope were the instruments usedin this study. Fig. l. A drawingto illustratethe dimensionsof the pyro- Cnysrl.r-r-ocRApHrc RELATToNSAMoNG lusite,manganite, and MnrO, unit cellsand their relativeori- MANGANTTE,PYROLUSTTE, AND MnrOt entationsin the topotacticreactions. The unit cells of pyrolusite, manga.nite,and MnrO, are closelyrelated (Baur, 1976;Buerger, 1936; Oswald et al., 1967), and in their topotactic transformations,their crys- pyrolusite, but probably cannot explain observed slight tallographic axes remain in nearly the same relative ori- deviations of such pyrolusite from tetragonal symmetry. entations. The a, b, and c axes ofmanganite correspond The memory efect may also be a result of these aligned directly to the a axes and the c axis ofpyrolusite. The a lamellar microcracks. axis of MnrO, is at an angle of 19'from one a axis of Here we report the frndings of high-resolution trans- pyrolusite and a of manganite; b of MnrO, corresponds mission electronmicroscopy (nnrnrvr) examinations of un- to the c translation of pyrolusite and manganite;and c of heated natural mixtures of manganite and pyrolusite and MnrO, has the same orientation as b of manganite, and similar examinationsof portions of thesemixtures heated a of pyrolusite (Dent Glasserand Smith, 1968).Pertinent in air to 300.C.We have observedmicropores, as reported crystallographic data are given in Table 1, and the ori- by Champness(1971), but we have also determined that entation relations among the unit cells of these minerals an intermediate phase, MnrO, (Mn;+Mn1+Or), plays an are illustrated in Figure L important role in the anomalous behavior of some sec- Sincemanganite has perfect (010) cleavage,crushed grain ondary pyrolusite. Our discovery of MnrO, in unheated, mounts commonly contain manganite particles oriented ion-milled, natural samples is of additional interest be- with the zone axis nearly parallel to the electron causeMnrO, in a natural occurrencehas not previously [010] beam. The ion-thinned sample also was examined only been reported. Furthermore, since MnrO, is an interme- in this orientation. The preponderanceofthis orientation diate phase in oxidation and reduction reactions of Mn has advantagesand disadvantages.Although the changes oxides,it should be given considerationin studiesof Mn- from manganite to pyrolusite and MnrOr are clearly ev- oxide phaseequilibria. ident in the selected-areaelectron diffraction (seeo) pat- ExpnnrvrnNTAl DETArrs terns taken from this orientation, micropores that parallel the manganite (010) planes are not observable. The specimensstudied are from the StanfordUniversity min- patterns in eral collection.They are labeledas manganiteoriginating from sAED of these minerals, all the orientation the Iake Superiorregion (sample no. 5l ll0) and from Ilfeld, correspondingto [0 l0] of parent manganite,are illustrated Harz,Germany (sample no. 7l 52).Powder X-ray diftaction (xno) in Figure 2. The reflections along the a* direction are revealedthese specimens to be mixturesof pyrolusiteand man- virtually identical in the manganite [010] and the pyro- ganite.To continuethe reaction ofmanganite to plnolusite,por- lusite [010] diffraction patterns. However, the manganite tionsofthe specimenswere powdered and heatedin air for 1.5 (101) spacingis double that ofpyrolusite, so the 101 and to 3 h. An xno powderpattern ofa samplethat hadbeen heated 101 manganite reflections appear halfuay between the at 300"Cfor 3 h showedno manganitereflections. This pattern origin and the more intense spots that correspond to the pyrolusitepeaks, contained,along with strong lessintense re- pyrolusite 101 and l0I reflections (in the manganitepat- flectionsattributable to MnrOr. Bothheated and unheated sam- terns theseare the 2OZana 202 reflections).The l0l, 10I, pleswere examined by nnrerraas grains mounted on holey-carbon in manganite film. To obtainbetter observations of the relationshipsamong 200, and 200 reflections do not appear the intergrownphases, an ion-thinnedsample ofthe LakeSuperior pattern becauseof the ll + l: 4n requirement for allowed specimenwas preparedand observedbefore and after heating. ftOl reflectionsimposed by the diamond glide in the man- In themilling process, ionized argon bombards the samplein an ganite structure. evacuatedchamber. The lake Superiormanganite was ion-milled The [001] MnrO, pattern was commonly observed to RASK AND BUSECK: HRTEM STUDY OF MN OXIDES 807 MnO o o202 58 o o 101 o oo 000 o o 101 o o MANGANTTE [o1o] oo Mnn. ooMn'. OO o Fig. 3. A schematic illustration of a MnrOr-pyrolusite inter- o o growth as seenlooking down the pyrolusite [001] axis (MnrOt r01 [010]). Unit-cell boundariesare shown by dotted lines. The rep- resentationsof pyrolusite and MnrO, are adapted from Dent Glasserand Smith (1967). In the illustration ofpyrolusite, filled circles represent oxygen