Surface Characteristics Growth Patterns in Some Manganese ...nopr.niscair.res.in/bitstream/123456789/39026/1/IJMS 10(1) 57-60.pdfSurface Characteristics & Growth Patterns in Some Manganese

Indian Journal of Marine Sciences

Vol. 10, March 1981, pp. 57-60

Surface Characteristics & Growth Patterns in Some Manganese Nodules fromthe Central Pacific

as ROONWAL

Department of Geology, University of Delhi, Delhi 110007and

G H FRIEDRICH

Institut fiir Mineralogie und Lagerstiittenlehre, R.W. Technische Hochschule, Aachen, W. Germany

Received 20 August 1980; revised received 12 December 1980

Scanning electron micrographs of nodules indicate that they are made up of small and large botryoids. When largebotryoids are present as aggregates, the nodule texture becomes rough and smaller botryoids give smooth texture.Petrographic study indicates that the growth of nodule starts with the penetration and accumulation of Mn-oxides solutionsaround a nucleus. The nucleus is generally composed of fossiliferous clayey sediments containing grains ofvo1canic material,ancient nodules, and fish tooth. With subsequent accumulation of Mn-Fe materials around the nucleus, and diageneticprocesses several features such as dentrites and cusps develop. Authigenic minerals also grow due to diagenic reconstitution.

Marine manganese material occurs commonly asnodules, slabs, crusts and cylindrical types. Physicalappearance, surface, growth rings and metal content ofnodules have been reportedl-3• Lalou and Brichet3have emphasised the role of the core material andconcluded that the oxide frame work of the nodules issupported by a 'frame' of either silicate or phosphaticmaterial supporting the layers. Nature of the corematerial in nodules from the Central Pacific is beingreported separately4. In the present communicationsurface characteristics and petrography of nodules arereported.

Materials and MetbedsThe study is based on manganese..nodules collected

from the Pacific on board R V Sonne, c,iuringcruiseSO-06/1 and SO-06/2 in Aug.-Oct. 197&. Surfacecharacteristics of the nodules were studied by scanningelectron microscopy, and petrography using thinsections. Internal structures were studied in polishedsections of nodules by reflected microscopy.

Results aDd Discussion

Most of the nodules collected were medium to largeand discoidal to ellipsoidal containing a large nucleusof fragment of clayey material with grains ofpalagonite,basalt and even older Fe-Mn material thatint1uence3the shape (Figs 1and 2).They can be classedas adult type Band AB2, or S/m (SjD)s/rgroup of Raaband Meylan5 suggesting that mostly they vary betweenlarge and medium size with spheroidal to discoidalshape showing·· microgral1ular to microbotryoidelsurface morphology.

When dissolved in 1:I hydrochloric acid (35/LRgrade) it is found that major part of many large nodulesis actually non-manganese. Although it is generallythought that morphology is probably unrelated to themode of genesis of nodules, Mayer6 noted that largediscoidal nodules are richer in Mn, Ni and Cu, andpoor in Fe. Halbach et al.2 have suggested thatalthough shapes of nodules cannot be correlated withtheir chemical compositien, certain surface texturesmay be associated with higher metal contents. Noduleswith smooth surface from the Central Pacific are richerin Mn, Ni and Cu and poorer in Fe than gritty andrough surface nodules which are rich in Co and Fe.This has been generally observed durin~ R V Sonnecruises of 1978. However, the nodules from the SWPacific are generally more rounded than those from theCentral Pacific, as has been observed earlier 7•

Surface !:haracteristics-Nodule size and shape varywithin a small area more in the Central PacifICthan inthe Aitutaki Passage (l6Qo-167°E; 17°-23°S)oftheSWPacific. It is generally observed that intheCe8tralPacific although the size and shape of nodules vary; thesurface textures appear to remain relatively uniform,and possibly independent of nodule size, morphology

.and internal structures. However, large ellipsoidal ordiscoidal nodules have a distinct 'equatorial' zonecharacterized by rough texture. This divides thenodules into top and bottom side.andin these cases thetop surfaces are relatively smooth whereas the bottomsurfaces are gritty. Scanning electron micrographs ofafew nodules from the Central Pacific as well as fromthe Aitutaki Passage are shown in Fig. 3. Thesemicrograms show that the nodule surface is made up of

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INDIAN 1. MAR. SCI., VOL. 10, MARCH 1981

2.5cmD

?cm

o

E&F

2cmB

oI

o

Eo

o

Fig. I-A to C, nodules from the Central Pacific at location 75DK20 [Area F (132°E; 7°S,South Equatorial Pacific) Nodules oblong in torm,surface smooth, single and twinned (poly) nodules]. D to F, nodules from Aitutaki Passage, SW Pacific at location 170 GD, Area K (160­

167°E; 17-23°S) [Cannon ball type nodules)

Fig. 2-Polished sections of nodules fromlocation 75 DK in the equatorial Pacific[A, single nodule with a smooth surfaceand a compact core; B, single nodule withsmooth surface texture but uneven shapedue to a large dispersed core; C, cannonball type nodule from Aitutaki Passagewith a large core and rounded concentric

growth rings]

58

·----- ---~~.- ---- -

ROONWAL & FRIEDRICH: MANGANESE NODULES FROM CENTRAL PACIFIC

Fig. 3-Scanning electron micrographs showing nodule surfaces [A, botryoidal growth features x 80; B, close up of a botryoid as shown inA x 800; C, surface botryoidal features as seen in nodules from Aitutaki Passage at location 170GB x 500; D. cut nodule from the Central

Pacific at location 175 DK showing flakes of birensite x 1200]

large and small botryoids. Surface texture is. determined by the number of botryoids. The rough

and more gritty surfaces in nodules from Aitutaki areaare due to many small botryoids(Fig. 3C).The externalrough surface appears to be due to the presence ofcomparatively large botryoids. In some nodules (Fig.1C) individual botryoids are composed of aggregatesof larger botryoids produced by different sizes ofbotryoids (Fig. 3). The diameter of individualbotryoids ranges greatly and the large ones are of thesize of> I mm. Further magnification of individualbotryoi~s sho~s that the botryoids are made up oflayers of Mil and Fe material around a tiny nucleus of

either sediment or even earlier Mn-Fe material (earliermicron size nodules). Such tiny nodules ap~ar to havebeen formed within the sediment before being joinedinto the nodule growths.

Nature of nucleus-Nodules comprise large coresand of various shapes (Fig. 2). The nucleus is generallyfossiliferous unsorted clayey sediment containinggrains of volcanic glass, phillipsite, Mn-oxide cuspsand dendrites. In nodules from Aitutaki area, in somecases the large core consisted of lithified sedimentshowing sharply defined to irregular form. Thelithified sediment shows colour variation .l?etweenlight

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INDIAN 1. MAR. SCI., VOL. 10, MARCH 1981

and dark brown to yellowish brown with cusps offerromanganese oxide.

Under polarizing microscope thin sections ofnodules show that the first fine layer surrounding the

clayey core material consists mainly of Mn-oxides

(black). This is followed by a distinct Fe-oxide (red)

layer, and again a Mn-rich layer showing colloform tocolumnar growth patterns. This rhythemic patterncontinues. Petrographic evidences prompt us toconclude that during the growth process, Mn is first toaccrete around the nucleus or seed.

The core is largely composed of unsorted clayey­fossiliferous material. Sediment grain size is large and

more elongated in the core material with no basaltic

grains in nodules from Aitutaki Passage than in thecores of nodules from the Central Pacific. Thinsections of nodules from different locations haveshown these features.

Authigenic minerals are commonly seen in clayeypart, and includes alkali-feldspars, calcite, apatite andphillipsite.

Growth patterns in nodules-Variations in the

surface texture in nodules suggest that slight variation

in the environment may produce different growth

characters. In fact, the individual botryoids are

themselves internally micro laminated and appear asmicronodules.

Petrography of the cores of nodules andsurrounding layer of oxides shows clearly the growthof Mn-Fe oxides around nucleus. Around the core ornucleus, the first substance to accrete is the Mn-oxide(hydrous dioxides) which grows as columnar andcolloform rings in quite rhythemic patterns assuggested also by Fewkes8, filled by clayey materialsoften -containing phillipsite needles, and palagonitegrains. Several other features such as dendriteformation are produced due to penetration of Mn-

60

oxides into core. This indicates that the growth ofnodules started under fl~id condition. The first Mn­

oxide layer, however, is not uniform, and leaves several

uneven projections and grains. The metal accumu­

lation in the form of Mn-Fe oxide results in growth of

tiny botryoids in this way. Such precipitationphenomena suggest that generally all nodules shouldbe of hydrogenous origin. Earlier influence ofvolcanism has to be considered to explain the supply of

metals because there are always grains of palagoniteand similar volcanic material found in the nodules.

The later growth hngs and subsequent compactionseem to be related to diagenetic processes and

colloform layerings have been formed due to

accretionary process.

Acknowledgement

Thanks are due to colleagues on board R V Sonne

for useful discussions, the German Research Society,Bonn for financing the cruises and the Indian Instituteof Technology, New Delhi, where the Scanning

Electron Micrography was done.

References

1 Raab W J, Ferromanganese deposits on the oceanj1oor, edited by

D.R. Horn (N.S.F., Washington, D.q 1972, 31.2 Halbach P, Ozkora M & HenseJ, Mineral Deposita, 10(1975) 397.3 Lalou C & Brichet E, Mineral Deposita, II (1976) 267.4 RMnwal G S, Friedrich G H & Rajamanikam G Y, Unpublished

data.

5 Raab W J & Meylon M A, Marine manganese deposits, edited byG.P. Glasby{Elsevier, Amstradam) 1972, 109.

6 Mayer K, Papers on the origin and distribution of manganesenodules in the Pacific and pro:.pectfor exploration, edited byM. Morgenstein(Hawaii lnst!. Geophysics, Honolulu) 1973,125.

7 Backer H, Glasby G P & Meylan M A, New ZealandOceanographic Institute, Field Report, (1976),88 pp.

8 Fewkes R H, Hawaii Inst Geophysics, (1973), 21.

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