MINERALOGICAL NOTES ELECTRON-PROBEDETERMINATION of INTER-ELEMENT RELATIONSHIPSIN MANGANESENODULES Rocrn G

]AN MINERALOGIST, VOL. 51, MAY.JUNE, 1966

MINERALOGICAL NOTES ELECTRON-PROBEDETERMINATION OF INTER-ELEMENT RELATIONSHIPSIN MANGANESENODULES Rocrn G. BunNs, .tNn D. W. FunnsrnNtv, Department oJ Geology and Geophysics,Uniaersity oJ Calif ornia, Berkeley; Departmenl of Mineral Technology,(Iniaersity oJ Calif ornia, Berheley.

INrnooucrroN Manganesenodules have attracted considerableattention in recent years as a potential sourceof Mn, Fe, Co, Ni, Cu and other minor elements. These elements are enriched in manganese nodules relative to igneousrocks, sea-water,and deep-seaclays (Willis and Ahrens, 1962). The nodules consist of an initimate mixture of crystallites of several minerals, together with organic, colloidal, and detrital matter (Mero, 1962;Arrhenius, 1963).Frequently, the nodulesform about a nucleusof pumice, altered volcanic glassor rock, pebble,or skeletalremains. Pre- viously, inter-element relationships were deduced from bulk chemical analyses of manganesenodules (Goldberg, 1954; Riley and Sinhaseni, 1958; Mero, 1962; Skorniakova et al., 1962; Willis and Ahrens, 1962). In view of the complex mineralogy of the nodules, and the obscure de- finition and difficulty of separation of extraneousmaterial, element correlations derived from bulk chemical analytical data are sometimes ambiguous. With the development of the electron-prober-ray microanalyzer, it is now possible to analyzeminute amounts of material (down to one cubic micron or 1 micro-microgram) on the surface of a solid specimen, and to examinesubmicroscopic phases. In particular, examinationof selected characteristic r-radiation during electron-beam scanning of a selected area of a manganesenodule produces a picture of the relative concentration of a specific element throughout the area and allows inter-element relationships to be establisheddirectly. X-ray intensity profiling of one, two, or three elements simultaneously enables chemical variations of elementsto be comparedacross a nodule.Inter-element relationships obtained from electron-probe measurements are phase correlations and these may differ from correlations deduced from bulk chemical analyses. The present paper describesmethods by which element correlations may be determined with the electron-probe,and summarizes the results obtained by electron-beamscanning and specimentraversing.

895 MINERALOGICAL NOTES

Pnovrous Wonx oN THEMrlnneloey AND CnBursrny ol MeNc.rxpso Noourcs The manganeseoxide phaseswhich dominate the mineralogyof manganesenodules have been characterizedprincipally by Buser and co- workiers (Buseret al. 1954;Buser and Graf, 1955a,b;Buser and Griitter, 1956; Griitter and Buser, 1957;Buser, 1959).These phases consist of D-MnOzand two "manganites,"l possessinglaver structures.Amorphous hydrated IeOOH is alsopresent, frequently as a major constituent.The three manganeseoxides, which representdifferent degreesof oxidation of manganese)possess high specificadsorption properties and undergo rapid isotopic exchange.Minor elements(Mg, Ni, Co, Zn, Ca, AI and Fe) are believed to be concentratedin the ',manganite,'phases (Buser, 19s9) . Numerous trace element studies have been made on manganese nodules.On the basisof bulk chemicalanalyses. Goldberg (1954) deduced correlationsbetween Co, Ti, Zr and the iron-rich phaseand betweenNi and Cu and the manganese-richphase. Riley and Sinhaseni (1958), after re-examiningGoldberg's (1954) data, believedthat oniy Fe-Co-Ti and Mn-Ni correlationsare shown by the data. Mero (1962) showed that trace element concentrationsin the nodules are subject to geo- graphicalvariations. In the Pacific Ocean,manganese-rich nodules with or without Cu and Ni occur near the American continents,and iron-rich nodules containing Ni, Cu, and especiallyCo are found in the Central Pacif,c.Willis and Ahrens (1962)believe that the presenceof iron in sub- stantial amount is necessaryfor the enrichment of certain trace elements. An inverse correlation between iron and manganesewas ob- served, and direct correlations were establishedbetween nickel and copper, and between iron and cobalt.

ExpBnrnrnNr.qr,MBtnoos A radial block (0.5 cmX0.5 cmx2-4 cm), cut from a hemispherical section of each nodule, was mounted in epoxy-resinand polished.l-he polishedspecimens were coatedwith a thin layer of carbon and storedin a vacuum desiccator.The phasesin the noduleswere identified by r-ray powder photograph (FeK" r:rdiation, Mn filter; and Cux" radiation, Ni filter) of samplesdrilled from the matchins surfaceof the other half of each nodule. I The term "manganite," which is prevalent in oceanographicliterature, is a chemical name applied to manganeseoxide phasesfound in manganesenodules. The so-called"10 A manganite" and "7 A manganite" possess tbe approximate empirical formulae 3MnOr "Mn(OH)I.nHzO and 4MnO:.Mn(OH)r.nHzO, respectively. The phases should not be confused with the mineral manganite, 7-1vlnooH, *'hich has a different crystai structure. MINERALOGICAL NOTIIS 897

An ARL electron-probe microanalvzer was used to obtain information on inter-element relationships in the sectioned nodules. Three techniques rvere used:

1. Wavelength scanning at various points throughout the specimen.This identifies the elements present and yields a semiquantitative analysis at each point. 2 Specimen traversing (96p per minute) under a stationary beam. With appropriate ,calibration, this technique allows one, two, or three elements to be analyzed simut taneousll'by ratemeter and provides a graphical means of comparing variations in element concentrations across a nodule. 3. Electron-beam scanning. The electron beam, focussed to a diameter less than one micron, is srvept in a regular manner across a square area of sample while a synchrono.rs oscilloscopedisplaf indicates the two-dimensional intensity distribu- ticn of back scattered electronsor a selectedcharacteristic r-radiation. Photographs of the oscilloscopescreen provide pictures of textural features and phase relationships (back-scattered electrons) and the relative enrichment of a particular element (characteristic r-radiation) throughout the area A comparison of photographs taken during electron-beam scanning enablesinter-element relationships to be rec- ognized at a glance, provided the element is present in sufficient concentration not to be obscuredby continuous radiation

Rnsurrs Measurementshave been made on manganesenoduies from fourteen widespreadlocalities and inter-elementrelationships were found to be the same in each of the nodulesexamined. To illustrate the type of results that can be obtained by wavelength scanningand electron-beam scanning,measurements on a representativespecimen (DWHD-16) are presented. This nodule was dredgedfrom a depth of 1270meters from an area approximately 2.50miles north-east of Tahiti, which is classi- fied as zrregion in which the cobait concentrationof manganesenodules is unusuallyhigh (Mero, 1962). ',fhe principal phasesconstituting specimen DWHD-16 are D-MnOz and an iron-rich tr-ray amorphous phase believed to contain hydrated FeOOH. Traces of the "7 A manganite" are also present. Chemical analysisb-v electron-probe,using metal (Mn, Fe, Co, Ni, Cu, and Zn), oxide ('fi), clinopyroxene(Ca, Mg, and Si), and muscovite (K and Al) standards, ).ields the following average concentrations (wet weight per centof totalnodule): Mn:24.5; Fe: 11.5;Co:1.15;Ni: 0.70; Cu: 0.25; Zn:0.10;K:0.4; Ca:2.1.;Ti:1.1;Mg: 1.0;Al: 1.5;:Si:2.0. The calibration error arising from differencesin averageatomic number between specimensand the standards trimits the accuraclr of the anaiysesto the range plus 10 per cent of minus 5 per cent. A wet chemical anal)'sisgave a water loss at 110' C. of 21.2 wt. per cent. Electron-beamscanning photographs of an areaof specimenDWHD-16 are reproduced in Fig. 1. The picture at top left is a photograph of an 898 MINERALOGICAL NOTES

!-rc. 1. Electron beam-scanningphotographs of back-scattered electrons (B.S.E.) and selected characteristic Ka radiations from a 360X360 micron section of a manganese nodule (specimenDWHD-16). Accelerating potentiai 25 kv. Average concentrations in the bright areas and banded region, respectively, were determined by making traverses along MINERALOGICAL NOTES

the verticalsection XZ of photographB.S.E. (wt. /6 of total nodule)to be: Mn: 23, 31; Fe: 12,2;Co: 1.2,0.2; Ni: 0.6,3.0; Cu: 0.2, 0.5; Mg: 0.7,2.0;Al: 1.1,3.0; K: 0.3,1.2; Ca: 2.2,0.8; Ti: 1.2,0.2; Si: 2.5,0.5; Zn (notshown) : 0.1, 0.3. qOO MINERALOGICALNOT'ES oscilloscopescreen displaying back-scattered electrons (B.S.E.) and illustrertesthe char:rcteristictextural features in manganesenodules of concentricand fluted banding, and accumulationsof micro-nodules.The bands,which are 10-150pwide and rather sporadicin specimenDHWD- 16, are narrower and more continuousin some of the other nodulesexamined.The brighter areasin photograph B.S.E. (Fig. 1) are regionsof higher mean atomic number, a contrast control having been adjusted to accentuatethe differences.The bright areasconsist of an intimate mixture of hydrated d-MnOz (averageatomic number ( 29) and hydrated amorphous FeOOH (average atomic number 122), the cracked appear- anceresulting in part from dehydrationof the surfaceby heating under the eiectron beam. The darker, banded areas are composed mainll' sf hydrated "7 A manganite" (averageatomic number of 4MnOz.Mn(OH), .2HzOis 20). The remaining pictures in Fig. 1 are photographsof the oscilloscope screen displaying Ka radiation of va.riouselements. Bright areas are regionsof higher elementconcentrations relative to dark areas.Exposure times for the minor elements(Co, Ni, Cu, K, Ca, Ti, Mg, AI and Si) are variable and are longer than those for manganeseand iron with the re- sult that continuous radiation contributes significantly to the signai and tends to obscuredetails of minor elementenrichment (for exampie Cu). Nevertheless,minor element correlationis verl' evident from inspection of Fig. 1, which showsthat Ni, Cu, Mg, Al and K occur with Mn in the banded areas,and Co, Ti, Ca and Si occul with Fe in the regionsof higher averageettomic number. Wavelength scanningat various points in the bright and banded areas(photograph B.S.E.) indicate that Zn is enriched with Ni and Cu, Ba with K, and Mo, Pb, Ce and Zrare enriched with Ti in the bright, iron-rich areas containing D-MnO2. Analyses taken during the specimen traverses along the vertical section XZ in photograph B.S.E. of Fig. 1 are illustrated in Fig. 2. The inter-elementrelationships and phaseenrichments shown in Fig. 2 com- piement those portrayed in the beam-scanningphotographs in Fig. 1, even to minor fluctuations. Element correlations similar to those describedfor specimenDWHD- 16 existin eachof the other nodulesexamined. Differences are of a minor characterand are related to the nature of the manganeseoxide phases. Many of the nodulescontain the "10 A manganite" insteadof the "7 A manganite" and d-MnOr, and have correspondinglya lower cobalt concentration. This observation is in accord with thermodl'namic calcula- tions (Burns, 1965)which indicate that cobalt is enrichedin manganese nodulescontaining higher oxidationstate manganeseoxide phases.In the f,[INERAI,OGICAL NOTES 901

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U I

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I 0 '

rrrlllll o 50 loo 150 200 230 300 150 OISTANCE TRAVERSED ACROSS THE NODULE' MICRONS

Frc. 2. Analyses made cluring specimen traverses across section XZ (photograph R'S'E' of Fig. 1) of the manganesenodule DWHD-16. Calibrations were made against pure metal standards.

manganesenodules containing the "10 A manganite,"Ni, Cu, Zn,K and Mg ure enriched with manganesein the "10 A manganite" layers, and Co, Ca and Ti are enriched in regions high in iron concentration'

CoNcrusroNs The results obtained in the presentelectron-probe investigation of manganesenodules may be summarized as follows: l.The iron concentration fluctuates whereas the manganese concentration is relatively uniform throughout a manganese nodule, being slightly higher where iron is low. 2. There is a distinct correlation between Fe, Co, Ti, and Ca' 3. There is a pronounced element coherence between Ni, Cu, Zn, and Mg, and between K and Ba. These elements are enriched with Mn and Al in regions of a nodule where the iron concentration is low. 902 MINERAI,OGICAL NOTES

4. The pronounced inter-element relationships suggest that isomorphic substitution is an important factor controlling minor element distribution in manganese no{ules. The ions Ni2+, Cu2+,2n2+,Mgr*, K*, and Ba2+substitute for Mn2+ in the ..manganite,,phases, (II! co occurs with Fe (rII) in hydrated iron oxide phases (Burns, 1965) and ri (IV) in 6-MnO:.

AcrNowr,BocEMENTS

The authors wish to thank Dr. B. w. Evans for his instruction in the use of electron-probe,and for his assistanceand interest in the project. The authors are also indebted to professorA. pabst for his assistanceand helpful suggestions.This rvork was supported by a National science Foundation grant, which the authors gratefully wish to acknowledge.

RnlrnBNcrs

Annnnmrus, G. (1963) Pelagic sediments. The Sea.,M. N. Hill, ed.,3, 655_722. BurNs, R' G. (1965) Formation of cobalt (III) in the amorphous FeooH.nH:o phase of manganese nodules. N ature 2OS, 999. Busen, w. (1959) The nature of the iron and manganesecompounds in manganesenodules. Inlern. Oceon Congr., Preprints, M. Sears,ed. AAAS publ., Washington, D.C.,962_ 963. -- AND P' Gnnp (1955a) Radiochemische Untersuchungen an Festkorpern rrr. ronen- und rsotopenaustauschreaktionen an Mangandioxyden und Manganiten. IIeh:. Chim. Acta 38,810-829. -- AND P. Gn.cn (1955b) Ditrerenzierung von Mangan(Il)_manganit und 6-MnOs durch Oberfliichenmessung nach Brunauer-Emmet-Teller. Hetzt Chim. Acta 3g. g3O_ 834. P. Gnen lNo W. Irrrrxwrcrr (1954) Beitrag zur Kenntnis der Mangan (II)_ manganite und des d-MnOe. Hela. Chim. Aeta, 37. 2322-2333. A. GnUrrnn (1956) Uber die Natur der Manganknollen. Schwei.z Min. Petrogr- M iu. 36, 49-62. Goldberg, E. D. (1954) Marine geochemistry. 1. chemical scavangers of the sea. Jour. Ge0L62,249-265. Gniittrn, A. lxn w. Busrn (1957) untersuchungen an Mangansedimenten. chimea ll. 132-133 Mrno, J. (1962) Ocean-floormanganese nodules. Flon. Geol,.57,747-767. Rrr,nv, J. P. aNn P. Srwrusoxr (1958) chemical composition of three manganese nodules from the Pacific Ocean. four. Marine Research 17r 466-482. SxonNrn'xovr', N. s., P. F. ANonusncnENKo AND L. S. Fourw,l. (1962) chemicai composi- pacific tion of iron-manganese nodules in the ()cean. Ahad. rr'ozra, s.s.,s.R. okeano- Iogiia 2, 264-277. wrr,r.rs, J. P. axr L. H. furnnNs (1962) Some investigations on the composition of manganese nodules, with particular reference to certain trace elements. Geochim. et Cosrnoch.im.Acta 26. 751-764.