Canadian MineralogistYol.29, pp. 149-161 (1991)

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA

SYLVIE LLORCACSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, New South Wales 2113, Australia

PIERRE MONCHOUXLaborotoire de Mindralogie et Cristallographie, Univenitd Paul Sabatier, i9 Alldes Jules Guesde, 31400 Toulouse, France

ABSTRACT

A study of cobalt mineralization present in weatheredultramafic rocks from New Caledonia revealed the follow-ing minerals: heterogenite (2H and 3R polytypes), asbo-lan (with cobalt and nickel), lithiophorite, intermediarephases between asbolan and lithiophorite, cryptomelane,ramsdellite and todorokite; only lithiophorite and cryptome-lane had previously been characterized in New Caledonia.Cryptomelane, ramsdellite and todorokite contain onlytraces of cobalt. The other minerals, the main cobaltbearers, occur not only as cryptocrystalline aggregates butalso as crystals or fibers up to several hundred pm long.They form concretions, but more often occur as pseudo-morphs after silicates and plant roots, which are habits notpreviously documented for these minerals. Microprobe ana-lyses show that (l) the heterogenite is much more nickelifer-ous (up to 7.5t/0 NiO) and the lithiophorite, more cobal-tiferous (up to I 1.5q0 Co2O3), than those found elsewhere;(2) the asbolan shows more variable compositions thanthose described elsewhere, rangrng from purely cobaltifer-ous to purely nickeliferous end-members; and (3) newphases exist with compositions between those of asbolanand lithiophorite. Thermal, chemical and crystallographicdata suggest structural models that differ from those in theliterature, and allow for the existence of continuous ser-ies, between cobaltiferous and nickeliferous end-membersof asbolan and between asbolan and lithiophorite.

Keywords: cobalt, asbolan, heterogenite, lithiophorite,weathering, ultramafic rocks, New Caledonia.

SotrtMalns

L'6tude des mindralisations d cobalt pr6sentes dans leslat6rites de roches ultrabasiques en Nouvelle,Cal6donie amis en 6vidence les mindraux suivants: h6t€rog6nite (polf'ty-pes 2.Aet 3R), asbolane (i cobalt er nickel), lithiophorite,phasc intermddiaires entre asbolane et lithiophorite, crlpto-m6lane, ramsdellite et todorokite; seuls lithiophorite etcryptom6lane avaient ddji 6t6 signalds en Nouvelle-Cal&onie. Cryptom6lane, ramsdellite et todorokite ne ren-ferment que des traces en cobalt. Les autres mindraux, prin-cipaux porteurs de la min€ralisation, se pr6sentent non seu-lement d 1'6tat cryptocristallin, mais aussi en cristaux oufibres atteignant quelques centaines de pm; ils forment desconcr6tions, mais le plus souvent remplacent les silicateset les racines, formes d'expression nouvelles pour ces min6-raux. Les analyses i la microsonde montrent que (l) leshet6rogenites sont nettement plus nickelifbres (usqu'i 7.590

NiO) et la lithiophorite, plus cobaltifire (usqu'd 11.590Co2O3) que celles 6tudi6es ailleurs; (2) Ies asbolanes pr6-sentent des compositions plus vari6es que celles d€crites ail-leurs, allant de termes purement cobaltifires i des termespurement nickelifdres; (3) de nouvelles phases existent, decompositions intermddiaires entre les asbolanes et la lithio-phorite. Les donn6es thermiques, chimiques et cristallogra-phiques suggbrent des modbles structuraux diff6rents deceux proposds dans la litt6rature et autorisant I'existencede sdries continues, d'une part au sein des asbolanes, d'autrepart entre asbolanes et lithiophorite.

Mots<lds: cobalt, asbolane, h6t6rogdnite, Iithiophorite, lat6-rite, roches ultrabasiques, Nouvelle-Cal6donie.

INTRoDUCTIoN

Ultrabasic rocks constitute more than a third ofthe surface of New Caledonia. They are essentiallycomposed of harzburgites and' dunites, serpentinizedto various degrees (Troly ef ol. 1979). These rocksare deeply weathered and covered by a thick mantlecomposed of two horizons. In the lower horizon, inwhich the structure is preserved (saprolite), the sili-cates €lre hydrolyzed; the earthy residual upperhorizon is essentially composed of iron hydroxide(Trescases 1975, Troly et al.1979). These weather-ing profiles host supergene cobalt mineralization ofeconomic interest; no detailed studies have previouslybeen made of such mineralization.

This paper reports the mineralogical results of thefirst comprehensive study of the cobalt mineraliza-tion in New Caledonia. The cobalt minerals are iden-tified, and details of their chemical composition,structure and physical properties are given. Themetallogenic results are described in detail elsewhere(Llorca 1990).

REvIEw oF PREVIoUS STUDIESON NEW CALEDONIAN ORES

Glasser (1903) proposed that the cobalt in NewCaledonia is present in noncrystalline or poorly crys-talline manganese oxides called "asbolans"; theseblack products were used as the only guide to miner-alization. However, to date, the exact nature of theseproducts and their precise location within the

t49

150

Ti6boqhi

THE CANADIAN MINERALOGIST

FIc, l. Occurrences of ultrabasic rocks in New Caledoniaand location of the two main areas that were sampledfor this study.

weathering profiles remained obscure. Some authors(Queneau 1971, Perseil 1972, Trescases 1975)attempted to determine the nature of these phases;Perseil identified lithiophorite and cryptomelane.

Gsoloclcel OCCURRENCE AND SAMPLING

Cobalt mineralization in New Caledonia resultsfrom the supergene concentration of the cobalt,which occurs as traces in silicates and oxides of thefresh ultrabasic rocks. The present studies werefocussed principally on the Poro and Ti6baghi blocks(Fig. l), which are considered to be representativeof the ultrabasic rock types, climates and types ofcobalt mineralization known throughout the island.Geological and geochemical studies of variousrepresentative profiles located principally on these

blocks (Llorca 1986a, 1990) showed that cobalt con-tent gradually increases from the fresh rock (150ppm) to a transition horizon between saprolite andthe ferruginous horizon, where it is concentratedowing to downward leaching from the ferruginoushorizon. In areas such as the Poro block, where thedepth of weathering is highly variable because theparent rock is variously serpentinized, accumulationis stronger in depressions (Llorca 1986a, 1990).Cobalt values in the transition horizon, usually0.5590, attain 1.3090 in those depressions. Cobaltmineralization occurs as bluish black-grey productsthat can exist as spots, coatings or concretions alongjoints and cracks, particularly in the saprolite, or asroot fossils and impregnations in the overlying earthymaterial. Cobalt concentration is not, however,proportional to the quantity of visible "blackproducts" (Llorca 1986a, 1990).

From the profiles examined, 40 samples having aminimum cobalt content of 0.590, and representingthe various macroscopic forms, horizons, geologi-cal and morphological settings observed (Fig. 2),were selected for mineralogical studies. For all miner-alogical techniques using powders, the blackproducts (except for a few samples with 4 to l0q0Co) were concentrated by washing off the limoniteand sieving; the 100-250 pm fraction was collected.Further concentration could in some cases beobtained by a l5-minute treatment in l2N HCl. Inorder to preserve the structure of the soft samplesfor microscopic study as polished blocks, thesesamples were collected by hammering 20-cm long5-cm diameter PVC tubes into the mine faces. Thesamples were then impregnated under vacuum andunder pressure (70 bars).

I

crust

c

sop rot i le

freshuttmbqsic rock

Ftc. 2. Ceneralized diagram showing the cobalt mineralization and sample locationsin the various types of profiles observed. l: Co mineralization as spots, coatingsor concretions along the joints and cracks. 2: Co mineralization as root fossilsand impregnations. 3: Sampling interval A: Complete profile, Ti6baghi (samplesC,F,G, in particular). B: Profile with strong Co accumulation, truncated andpedoturbed, Poro (samples A,B,E,H,I,J,K, in particular). C: Surficial cobalt miner-alization, truncated profile, Ti€baghi (samples D,L, in particular).

r ron

fe r rd g inou shor i zo n

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA 1 5 1

MINERALS IDENTIFIED

Nqture snd nomenclature

In her paper on the presence of lithiophorite andcryptomelane in a sample from New Caledonia, Per-seil (1972) concluded that the "asbolans" are mix-tures of cobaltiferous varieties of these two minerals.More recently, Chukhrov et al, (1980a.b, 1982)studied "asbolans" from the Urals; they did not findany cobaltiferous varieties of previously known man-ganese oxides, but instead a new mineral consisting ofMn layers alkrnating with Co andlor Ni layers. Theseauthors referred to this new mineral as asbolan serzsastricto. It is in this restricted sense that the term asbo-/an is used in this paper.

Mineralogical study of the 40 selected samplesshowed that the "black products" of the NewCaledonian profiles are made up, in various propor-tions, of several minerals with different cobalt con-tents. However, each of these minerals occurs underparticular Eh-pH and chemical conditions that limittheir occurrence to a particular horizon or area(Llorca 1990).

Three of these minerals, cryptomelane, ramsdel-lite and todorokite, are rare (found in three samples)and contain only traces of cobalt (0.7). The others,more widespread and in which cobalt is a major con-stituent, can be considered as responsible for thecobalt mineralization and have been examined indetail. These minerals are heterogenite (found in 8samples, and dominant in 4 of them), asbolan (17samples, 12 of which contain no other cobaltiferousphase), lithiophorite (10 samples, 6 of which con-tain no other cobaltiferous phase), and members ofa series between asbolan and lithiophorite (20 sam-ples, dominant in 10). Only lithiophorite and cryp-tomelane had previously been noted in New Caledo-nia (Perseil 1972). This is the first reporr ofheterogenite forming as the result of supergene con-centration of cobalt from trace quantities in ultra-basic rocks. Before this study, heterogenite had beenfound only in the oxidation zone ofcobaltiferous sul-fide deposits (mainly in Shaba, Zaire: Deliens 1974).This is the first evidence of a series existing betweenasbolan and lithiophorite, the members of which arereferred to as "asbolan-lithiophorite intermediates".

Textures

Microscopic examination of polished blocks of the"black products" revealed a variety of habits (Fig.3). It showed not only cryptocrystalline aggregates(optically isotropic), but also larger crystals withobservable optical properties. Cryptocrystallineaggregates, as well as microcrystalline aggregates(with crystallites showing anisotropy), were observedfor all the minerals except ramsdellite. Asbolan,

lithiophorite, "asbolan-lithiophorite intermediates"and heterogenite also were observed as flexibleacicular crystals, up to 50 pm long, occurring eitheras individuals (independent or taneleO or joined sideby side (parallel or divergent), forming tufts (Fig.3.7), spherulites, or monocrystalline areas about 100to 500 pm across (Figs. 3.3, 3.2). Heterogenite,lithiophorite and ramsdellite also form prismaticcrystals about 20 pm long (Figs. 3.1, 3.8).

These crystalline forms can produce concretions.More commonly, however, they occur as pseudo-morphs after the primary silicates or after plantroots, habits which were previously unknown forthese minerals. The pseudomorphs after the silicates(serpentine, olivine, and talc derived from thepyroxene in the early stages of weathering) occuralong joints, giving a macroscopic appearance ofblack spots or coatings.

Cobalt minerals commonly replace veinlets of ser-pentine. In the least common occurrence, bands ofsquat crystals of heterogenite crudely replace the ser-pentine veinlets (Fig. 3.1). Images of these crystalsobtained with a scanning electron microscope @ig.4) show barrel-shaped crystals terminated on the(0001) face, a previously unreported habit forheterogenite. Between these crystals, rose-petal-shaped crystals show well-developed growth striae.Crystals of both shapes have the same chemical com-position and commonly show orthogonal associa-tions, probably twins, which had not been notedbefore. In the second, more common type of replace-ment of the serpentine, single crystals are formed,accurately reproducing the serpentine veinlets (Fig.3.2); Ni-rich asbolan and Ni-rich "asbolan-lithiophorite intermediates" commonly occur in thishabit.

Synchronous pseudomorphs after talc crystalswere commonly observed for the prominent minerals(asbolan, Ni-rich in particular, "asbolan-lithiophorite intermediates", heterogenite,lithiophorite; Fig. 3.3). Crystallinity of the newlyformed phases is invariably good, as there is orientedgrowth of the fibers from and perpendicular to thecleavage planes and faces of the dissolving talccrystals.

Cavities between the serpentine veinlets, previouslyoccupied by olivine cores, are in some cases filledwith a disordered cryptocrystalline cobaltiferousmaterial. These pseudomorphs, composed essentiallyofheterogenite or Ni-rich asbolan, are asynchronousor subsynchronous with the dissolution of olivine(Figs. 3.1, 3.2). They are less commonly observedthan the pseudomorphs after talc or serpentine.

Fossilized roots are cornmon. Microscopic andmacroscopic examinations show that the root is firstmolded by an optically isotropic cryptocrystallinemass, which can even enter the pores and fill theinner wall while the plant is alive. The tissues them-

THE CANADIAN MINERALOGIST

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA 153

selves are then more or less completely replaced,sometimes at the level of individual cells @ig, 3.4),by an optically anisotropic material of the samenature, usually "asbolan-lithiophorite intermedi-ates", but also Co-rich asbolan or lithiophorite.

Around the roots in particular, where the porosityis high, cobalt minerals can impregnate the earthymass. Microscopic examination of such impregna-tions showed an isotropic and cryptocrystallinematerial with ineeular or diffuse contours. The sameminerals are found here as in fossilized roots.

Various forms of concretions (colloform orfibrous) were observed (Figs. 3.5, 3.6). These con-cretions consist of alternating layers of smaller orIarger crystallites, possibly reflecting seasonal vari-ations. Co-rich asbolan, Iithiophorite and "asbolan-lithiophorite intermediates" wsre found as concre-tions that had developed on the walls of open frac-tures. Some lithiophorite concretions also wereobserved as cement in surface colluvium.

Cobaltiferous fibers, consisting particularly ofasbolan, and showing a tendency to spheruliticarrangement, were found to have grown in cavitiesleft by the dissolution of rock-forming silicates(talc or olivine?) (Fig. 3.7). In this case, the pseudo-morphs are subsynchronous with the dissolution, andonly the outer shape of the mineral is preserved.

In addition, cryptocrystalline or microcrystallineaggregates or fibers can fill small open fissures. Inparticular, microkarsts filled with a corroded cryp-tocrystalline material typically composed ofcryptomelane-todorokite mixtures (cobalt-poorphases) were observed; in the small cavities causedby corrosion, squat crystals have developed, whichconsist exclusively of ramsdellite (cobalt-poor phase)(Fig. 3.8).

Opticol properties

Heterogenite, asbolan and lithiophorite show areflectance of about 200/0, a white color with apleochroism in the whites and greys, and a stronganisotropy, in agreement with previous observationsmade elsewhere. However, the anisotropy appearswith some color tints, which had not previously beenobserved in lithiophorite or asbolan. The tints arepale (lightly tinted white with pale yellow or beige)in the least cobaltiferous asbolan and lithiophorite,

and more pronounced (straw-yellow to ochre) in thecobalt-rich varieties. They are brown in the heter-ogenite. The "asbolan-lithiophorite intermediates"show the same optical properties as observed for theasbolan and lithiophorite. Between uncrossedpolarizers, the tints are straw-yellow and blue in allminerals. Occasionally, the todorokite-cryptomelanemixtures also give a strong anisotropy colored withochre, whereas ramsdellite shows a strong anisotropywith beige and brown. These optical similarities mayhave led to the misidentification of some of theseminerals in the past.

Thermol data

Thermal data collected from the heterogenite-richsamples are comparable with those obtained byDeliens (1974) for heterogenite from Shaba; anendothermic peak at about 340oC (Fig. 5) may befollowed by a less marked one at about 930"C. Theasbolan shows endothermic reactions at 130-190'C,420-4ffi'C and570-625oC (Fig. 5), similar to thosereported by Chukhrov et ol. (1980) for the asbolanfrom the Urals. The Co-rich asbolan, however,shows an additional endothermic reaction at about270"C (Fig. 5), not reported by Chukhrov et al. Thelithiophorite and the "asbolan-lithiophorite inter-mediates" both show two endothermic reactions, oneatU10450'C and a smaller oneat625-650"C, analo-gous to those of asbolan (Ftg. 5).

Chemical dato

Microprobe analyses of the heterogenite revealeven higher nickel contents (4.5 to 7.590 NiO) thanin heterogenite-2H from Shaba (1.990 NiO) (Tablel; Llorca 1986b). However, there is a notable absenceof copper, which attains several percent inheterogenite-3R from Shaba (Deliens 1974),

The lithiophorite contains very little lithium com-pared with the type mineral from Postmasburg(Table l) and in this respect is similar to most exam-ples found elsewhere in the world. However, it ismuch richer in cobalt than lithiophorite from otherlocalities, which generally contain none or very little(Table l, Fig. 6a).

Asbolan has a more variable composition than thatfrom the Urals (Table l, Fig. 6b). Three varieties of

Ftc. 3. Principal habits shown by the cobaltiferous minerals from New Caledonia. l. Heterogenite and asbolan occur-ring as crude pseudomorphs after serpentine veinlets and olivine (sample A). 2. Perfect pseudomorphic replacementof serpentine veinlets (sample G). 3. Pseudomorphic replacement of a talc crystal (sample I). 4. Pseudomorphicreplacement of a root (represented here: part of the outer wall) (sample J). 5. Colloform concretions (sample B).6. Fibrous concretions (sample D). 7, Automorphous fibers, here showing spherulitic tendency (sample C). 8. Cryp-tocrystalline mass filling a dissolution cavity (from sample K). Symbols: H, heterogenite; A, Co-rich asbolan; A',A",Nirich asbolans; AL, "asbolan-lithiophorite intermediate"; L, lithiophorite; TC, todorokite-cryptomelane mix-ture; R, ramsdellite; l, "limonite"; q, quartz.

r54 THE CANADIAN MINERALOCIST

FIc. 4. Scanning electron micrographs ofheterogenite showing barrel and rose-petal shapes, with common 90o associ-ations (sample A).

asbolan were described in the Urals, referred to as"Co-Ni asbolan", "Ni-asbolan" and "Co-asbolan"(Chukhrov el ol. 1980a,b,1982). Samples from NewCaledonia contain asbolan varieties similar in com-position to those from the Urals, as well as varietiesshowing intermediate compositions, which werereferred to as "Co > Ni asbolan" and "Ni > Co asbo-lan" (Llorca 1988). However, none of the aboveterms was submitted to the IMA Commission onNew Minerals and Mineral Names, as the use ofprefixed chemical symbols is not accepted by theCommission. For these reasons and to avoidunnecessary confusion, the expressions "Ni-richasbolan" and "Co-rich asbolan" are preferred in thispaper, covering respectively the Ni-dominant andCo-dominant varieties.

About half of the cobaltiferous grains analyzedusing the electron microprobe show a compositionbetween those of heterogenite, lithiophorite and thevarious types of asbolan, but mostly betr:veenlithiophorite and asbolan (Table l, Fig. 6c). Twocases can be distinguished:l. Grains that look like imperfect single crysralsunder the microscope. They occur in association withother cobaltiferous grains of various compositions.Slight compositional variations can be observedwithin them under the microprobe. A scanning elec-tron microscope coupled with energy dispersion(EDAX) reveals a mosaic of domains with variouscompositions but identical orientation. Grains with

a composition intermediate between that of heter-ogenite and the other minerals are made of these sub-tle mixtures.2. Euhedral crystals or grains that look like perfectsingle crystals under the microscope. They occur inassociation with other cobaltiferous grains of iden-tical composition. No compositional variations wereobserved within them under the microprobe. Trans-mission electron microscopy coupled with energy dis-persion (EDAX) of two finely cmshed and dispersedsamples confirmed their chemical homogeneity.These pure phases, with compositions between thoseof lithiophorite and the asbolan varieties, have beentermed "asbolan-lithiophorite intermediates".

Crystallographic dstq

In most cases, X-ray diffractometry of concen-trates prepared from samples containinglithiophorite, asbolan or "asbolan-lithiophoriteintermediates'l gave only the n1ain diffraction linesatabout4.75 A (002) and 9.50 A (@l) that are com-mon to these minerals; mixtures gave broader andmultiple lines. Concentrates from samples contain-ing heterogenite usually showed only one line, at 4.45A. However, more complete patterns were obtainedfor the various minerals from the purest samples(Table 2).

A heterogenite-rich sample gave a patterncomparable to that from crystalline heterogenite

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA

i6 io il -T'c

HETERO6ENITE ---------:.

co-RICH ASBoLAN

Co -R l tH ASB0LAN

Ni- RICH ASBOLAN'

Ni -RITH ASBOTAN

155

LITHIOPHORTTE

ASBOLAN -

LITHIOPHORITE

INTERMfiNATE

FIG. 5. Results of differential thermal analyses of the cobaltiferous minerals fromNew Caledonia (from samples A,B,D,E,F,G,H). G, goethite; Q, quartz; g, gar-nierite; C, cryptomelane; R, ramsdellite.

from Shaba (Deliens 1974), which is interpreted asa mixture of both the 2H and 3R polytypes (Table2). The (002) and (003) reflections ofthe respectivepolytypes are supe^rimposed, giving an intense lineat about 4.45 A. The identification of theheterogenite-2f/ polytype makes New Caledonia onlythe second known locality for this polytype. Twosamples of Co-rich asbolan gave X-ray-diffraction

patterns and electron-microdiffraction pattems fromwhich numerous lines and spots could be indexedClable 2). Some of these reflections were not reportedby Chukhrov et al, (1980a,b) for the asbolan fromthe Urals, as the samples from New Caledonia arepurer and better crystallized. A lithiophiorite-richsample gave a very complete X-ray-diffractionpattern (Table 2), similar to those in the literature

156 THE CANADIAN MINERALOGIST

TABLE 1. CH&IICAL COMPOSITION OF COBALT.BEARINC MINERALSFROM NEW CATEDONIA, SHABA, THE URATS AND POSTMASBURC DIScUSSIoN

Review of previous concepts of crystal structure

Two polytypes of heterogenite were defined: the3R polytype, CoOOH, is rhombohedral, with siq-gle layers stacked along the Z axis with a 13.157 Aperiodicity (I!ey 1962); the 2H polytype is hexagonal,has a 8.805 A periodicity along Z, and contains upto 2t/o NiO, resulting in an idealized formula(Co,Ni)OOH (Deliens & Goethals 1973).

The asbolan from the Urals was described (Chukh-rov et ql. 1980a,b, 1982) as hexagonal, with layersof octahedra Mn4* O2-IOH), reguiarly alternatingwith discontinuous layers of (Co,Ni,Ca)2* u(OH)zr*, or (Ni,Ca)2+/(OH)zy** or Co3*/Caz*ro6yn1*z+u/2\, depending on the variety,stacked along the Z axjs (x, y and z < 1).According to the authors, these three varieties havewell-differentiated structures. In the Co-Ni variety,Co2+ and Ni2+ ions would indiscriminately occupythe same octahedral sites; two superimposed sublat-tices are observed, wiqh the sarne Z axis, identicalc parameters (9.34 A) apd slightly different aparameters (2.83 and 3.04 A), which would be due,respectively, to Mn layers and Co-Ni layers. In theNi variety, there are also two sublattices, one ofwhich has rm a parameter almost double the other;this led the authors to consider a dioctahedral Ni layer(one empfy site out of tlree). In the Co variety, cobaltwould be trivalent and in tetrahedral coordination.

Lithiophorite is hexagonal @auling & Kamb 1982)and would consist of Al,nLiu(OH)a2 and Mn2+3Mna* 16042 layers regularly alternating alortg the Zaxis, six layers leing necessary for thp unit cell havingan a of 13.37 A and a c of 28.20 A. fne presenceof Mn2* cations in this formula is due to a decreasein the number of positive charges of the Mn layerin order to compensate for the introduction oflithium in the normally vacant sites of the hydrar-gillite layer. Hydrogen bonds permit the transfer ofthe charges.

New concepts derivedfrom the New Coledoniqn data

Crystallochemical data recorded for the heter-ogenite confirm that cobalt is in the trivalent stateand occupies octahedral sites, and thus support thestructural hypotheses of Hey (1962) and Deliens &Goethals (1973). According to Deliens (1974), theendothermic reactions recorded for the heterogeniteat 340"C and 930'C are due, respectively, todehydroxylation (transformation of CoOOH intoCorOn spinel) and reduction of the spinel to Comonoxide. In the New Caledonian samples, however,the second reaction is without doubt due to thepresence of talc, which is invariably associated withthe heterogenite.

1 2 3uzo - 0.3'Mgolrzog - 8.9sroe 0.3 0.3 4.0CaOM€z O.2 !6.0Mr|o - 0.4Fe2O3 0.7 0.01 0.2co/\ 76.7 u.6 7.ecooNlo 4.7 1.9 1.5CUO - 0.1BaOH/J 17.4+ 127 14.f

4 5 e 7 74.2

228 0.6 0.1

1 .0 - 1 .6 1 .917.O Q.3 68.7 46.9 56.68.0

8 8 ' , 90.01'

o.2 0,20.8 11.7

80.0 5.00.3 1.8 0.510.0 55.1 45.4

73.4

'to.7

0.9 15.3r " 0.4 1.31.0 13.9- 6.9 8.3 0.1 0.4

17.9 11.4 13.8 3.9 21.6 2.7

n 2

14. i3.{ tz*17.sG) 19.a 3.s(-) l.r rg.g*

TOTAL(wt .$ ) 16 lm 1m 1m 100 lm 1m tm 99.8 lm 100

Compoaitions wsre derived frcB el€ctmn-Elcrcprcbe data unlesa statedothers'ise. l. Hetercgenile frcm New Csledonia (wpl€ A). 2. Heterc-genite-2u frco Shaba (D€Uens l9?4). 3. Lltbiophorlte frcm New Cals-donl,B (snple D) . 4. Uthlophorits frcB postmsbu.g (frcm the Btruc-tursl fomula glven by Pauung & Kamb 1982). 5. A! example of Co-ltchasbolan frcm New Caledonia (wple B). 6. An e@pl€ of Nl-rlch asbo-lan frcm New Csledolia (smple C). ?. nco-Ni asbolan" frcn the Umls,amlyzed by wet-cheni€l m@ns (Chukhrcv et al. 1980a), in column ?,realculated for the pure phase. 8. nNi-asbolann frcn the UBls, ana-lyzed by wet-chesd@l @qns (Chukhrcv et al. 1980b), ln @lumn 8're-@lculalsd for the pure phase. 9. An example of nasbolan-lllhio-

phodte intemediBtet (wple E). r: obtalned by atonlc absorption onths whole smple, then re@lculiBted. +: obtalned by dlfference frcm100$. -: oblalned frcm welght lossea lncurred dudng ther@l amlyBis.

[that of type-locality lithiophorite from Postmasburgfor example: Wadsley (1950) quoted in Mitchell &Meintzer 1967)1.

Crystallochemical dqta

Cobalt, nickel and manganese X-ray-absorptionspectra (EXAFS and K-thresholds) were collectedfrom samples rich in heterogenite (sample A), Co-rich asbolan (sample B), lithiophorite (samples D andL) and "asbolan-lithiophorite intermediate,' (sam-ple I). It is the first time thar this very high resolu-tion technique of investigation has been used on suchminerals. Details of these analyses and their interpre-tation are given in Manceau et ol. (1987).In short,the results show that cobalt is invariably in the triva-lent state. It is never simply adsorbed on the layersbut always integrated into the structure, surroundedby ions with an atomic mass comparable to its own.It occupies octahedral sites, comparable in size withthose of manganese and clearly smaller than thoseof nickel. It is probably low-spin, the ionic radiusof Co3= low spin being 0.53 A, and thus verysimilar to Mna" (0.54 A) and clearly different fromNi2* (0.69 A;. tn rhe lithiophorite-rich samples,nickel is surrounded by light atoms, and the highstructural order around Co contrasts with the struc-tural disorder observed around Mn.

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA 157

At M n

Ftc. 6. (a) Cationic composition in terms of Mn-Al-Co of five samples of lithiophorite from New Caledonia, ana-Iyzed several times by electron microprobe (filled circles). Comparison with the lithiophorite from Lecht (Wilsonet al. 1970), Mikhalkovo (Int. Mineral. Association 1982), Charlottesville (Mitchell & Meintzer 1967), Ottr6 (Fran-solet 1979), Postmasburg (in Pauling & Kamb 1982) and Groote Eylandt (Ostwald 1984) (open circles). (b) Cationiccomposition in terms of Mn-Co-Ni of ten samples of asbolan from New Caledonia, analyzed several times by elec-tron microprobe (filled circles). Comparison with asbolan from the Urals (Chukhrov et al. 1980 a,b, 1982) (opensquares). (c) Cationic composition of 158 areas analyzed by electron microprobe in 36 cobaltiferous samples fromNew Caledonia (Mn is indicated in brackets). A,B,C,D,E analyses given in Table l. Symbols: r pure phase, o possi-ble mixture.

ba

I oC o

NiAI

Crystallographic data show that both 2,I{and 3Rpolytypes of heterogenite are present in New Cale-donia. An examination with the microprobe suggeststhat heterogenite-3R can, like the 2H polytype,

contain some nickel. This systematic presence andhigh values of nickel and the absence of copper inthe heterogenite from New Caledonia, comparedwith that from Shaba, obviously reflect the ultrabasic

8.,*:N,'..8 q

o t . l o

3*-a ' .

oO O

158 THE CANADIAN MINERALOGIST

TABLE 2 DTFFMCTION I.INES OF THE CO&qLT€EAFING MINEMLI! FROM NEW CATEDONIA SHABA THE URAIS AND PGTMASzuRC

Caledonbt h€isogenite (4

zf|1AR

d l

46 r&

2.q ^N2.€ .A

2J9 r02,32 *2.20 22.17 Al.s 5r '8l 5

tsv a

l l 0io l

r o l1 Q

@I Qt0 l

l.4lt 10

t.169 5t.3il m

I t 0

l @1 1 3

Od€debi Gbdsl

Hrh (F) Nlddr (C) s.6ld€ l 8t/blald€Co-tldr (B)

d l d d l d l d l

{M$co) {Nl)

BI &I

@ t m ]w w

t &

@3 @i[ 0

l @l 0 l

9.S r4Al 3

1.23 r

9,55 3 9.5 m4.8 s

d6

3.18 ?2-7 llsJ

4 n 3

2.& nv 2.ta 44 el8ala r 2.ta s

Z l 7 * l QrJ3 s lot

l .8 l et.791.78

t .vt .a

.s

.t6 t.4 m 1.f,6

I q

il0

N

2 t o3Sm

mt1 A210

J&

l r o[0

4t0t q

l.t0l.to

l .gt . t9

o.s0.?90.8

CabdqdanEbpttrrlb (D)

0 I MI

9.$ S Sr47J r00 @2

3.16 5 @J

2 6 2

z t T r tr .s t0

|.54 2t.@ 5l.q' 2I.J7 J

20lbz

2oJ204

' i l5

M,W3r r,ou26

'A8bol6-

lttbptbtlb

(E)

9.55 m477 I

if,.15 w

o2.6

Hetogsfi€ frqn Shaba

2H+gF aio 3F

d I d c l u l d c l M l

4854.& 1@ 4.43 s @ 4J77 s 6

Agbolan tqn thg UEb

@!! subH€l e6H€ll ti! $bld@l8!6ld€ll(Mn) (cet{) (Mn) 0{)

o t a i c M d c u r 2

d c [ ] E t r l

tgn w \Uw r03

1.& 3 t6s l q

2.365 10r ?.4 102

216 t@

1.923 101 Zot r03

1.69 1g 1.74 r&

1.518 I 10

a6 l.q m3asz6 26 16

2.t6 237 101

zt7 2"16 t02 219 2t0ZQ N2

1.89 r.9t 1051.82 %3

1.67 l .s t&

t-Erhphdlb trDmPodna$ug

d I d c M l

3.12 mw 3.1: @l

252 2i\2.51 I l0L 4 ' NZ d i l l

2.39 nv 2:9'n2:7 111235 M230 | t2.ml2.15 b3zta 112L6 1t3,m

l.S m 1.89 204r.s m,tl.s lt41.79 At1.6 26t .g [4

t .s l { 1 .$ l l5t.57 tu

1.4 * r .6 3t l

2.310 w 102w @ 2 . r s w 6s l @

zq x 2.416

z s l

2.fi 5 LW

232 I2,m 7 Lb12.t6 4 2.1*

r.s 8l,& 15 r.891l.s5 5t.@ lo 1.6

9.6 m 9J9 @147O 3 47O 62

q t 8 @ l459 &2451 l@416 tol3.2 ter.& @32 6 1 1 r 0

9. la 9.t8 @t459 459 024$

q t u w4.82 I

9.3 ql4 @r 9J4 6l1.67 467 @ \67 @2

l.@ 3.t r @3 t ll &l2-63 a6 r@2.6 z6 r@ Z5! tol

l .Q

l .4 t 1 .415 |01 . 2 t % M

1.112 l l0rm 26

r,st t21.67 51.49 23t.t4 6l.s9 6

2.6 mv

2.6 n iol

2.17

t,9l

r . 4 3 t r i 0 7 L t . 5 l

1.40 m* l r0 r .44 m3 r r0 | 1 .419 r t .4 lr . 3 6 w r m l t , 2

1 . 3 & n v l 1 2 l . g m l l l

t.s 5@

1.6 w t.4 261,4 312,115

l.€ w Jlo,Ar

d, d {3unces l ln l ) obb lned by t - r ry d l f f . l c@try , r l th Fe L , o r C i l - + boc l @ncn ' .@tor ; d r , by e l f t t rcn d lc rcd l f f rac t lon i G, r l th @1az l@th; dc , ca lcu labd d lsunces ; - , a - .x ls o f f te tu sub la f t l ces l rd super l@sod i4 d l f fe ren t fM eoch o the . by 30 ' ; I , ln te t rs l ty i i ,stnng; r!. tet? 9tmng; vr3, tet? rery s!rcng; o. @dl@: @, @dl@ real; r, @l; fr, very sal; wi veta rera real; hll, crftlllographlclndlces. (r), obtalned by r@vlng the 3R-polytype llnes (repo&d by Hey, 1962) fM th€ oltture.

nature of the parent rock in New Caledonia. Becauseof the nickel content, it is not possible to leave thiselement out of the structural diagram, as did Deliens& Goethals (1973). It is unlikely that nickel replacescobalt randomly, because of the difference in theirionic radii. It is more likely that the nickel formsseparate domains, such as those in the asbolan @ig.7).

Asbolan from New Caledonia shows a whole rangeof compositions between the Co and Ni varieties,which seems to indicate a continuous series. Crys-tallochemical data indicate that cobalt is in the triva-lent state and occupies octahedral sites that are differ-ent from those occupied by Ni2*. These data areinconsistent with the models suggested by Chukhrovet al. (1980a,1982) for the Co and Co-Ni varieties

from the Urals, and indicate the presence of separatedomains for Co and Ni within the discontinuouslayer of the asbolan. As previously observed for theasbolan from the Urals, our crystallographic datashow two hexagonal sublattices with the Z axis incommon. One of these sublattices can be attributedto the Mn layers and Co domains, and the other todioctahedral Ni domains, such as those suggested byChukhrov et al. (1980b) for the Ni variety from theUrals.

According to Chukhrov et al. (1980), endothermicreactions at I 30- I 90 " C, 420-460o C and 570-625 " Creflect l) the loss of molecular water present betweenthe layers and between the domains of the discon-tinous layer, 2) the loss of the hydroxyl groups of

r59

this layer (Co-Ni or Ni), 3) the los$ of the hydroxylgroups of the fault-free Mn-layer. With the NewCaledonian samples, however, an additionalendothermic reaction at270oC was observed for themost cobaltiferous specimens. This reaction can beattributed to the hydroxyl groups attached to thecobalt, and the o\e at 42O460oC to the hydroxylgroups attached to the nickel only (Llorca 1988).Weight losses measured for these two reactions relateto the decomposition of heterogenite-type CoOOHdomains into Co3Oa and the decomposition ofIN(OH)31- dioctahedral domains into NiO, respec-tively (Llorca 1988).

All data recorded for the asbolan from NewCaledonia are consistent. They indicate a continu-

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA

W zW7l7/17711HETEROGENITE-2H ( AJ

o! vmmmmmm)zT O - R I C H A S B O L A N ( B )

r-scz- t _Vmn;- 3.Esouo

3-5 SOLUTION

4-2-1&'-_fm7t'_

5

rS

INTERSTRAnFIED

2WmTVmn: :. C d a H 2 0 tAsBoLAn - L trHIoPHoRITE tNtrnuronrr' lr t

Flc. 7, Structural models suggested for the cobalt-bearing minerals from New Caledonia. The various layers consistof a single layer of octahedra. Cobalt is trivalent, nickel divalent, manganese tetravalent, aluminum, trival-ent. 1.CoOOH; all sites are occupied. 2. tNi(OH):l-; 2 sites out of 3 are occup^i^ed. 3. [MnO2-lOH)r]"-, with x =

Ni cations/Mn cations; all sites are occupied.4. [A10.7Ni0.04Li6.92(OH)2lu''*; one site out of 5 is empty.5.[Mne.qsCos.e2O t10.2-; all sites are occupied. The projection is perpendicular to [001]. A,B,C,E samples; M average;*calculated.

o

Ni-RI[H ASBOLAN

17v7777n1

LI 'THIOPHORITE (M)

160 THE CANADIAN MINERALOGIST

ous series between Co and Ni end-members, with astructure comparable to that suggested by Chukhrovet al. (1980b) for the Ni-variety from the Urals, withthe addition, for the cobaltiferous varieties, ofheterogenite-type CoOOH domains ne1t to thelNi(OH)31- domains (Fig. 7). Because the Nidomains have to be small (Chukhrov et al. 1980b),samples of asbolan close to the Ni end-member cancontain more water. They are also less stable, asshown by their easy dissolution in HCl, and can evenbe leached with water (Llorca 1986a).

Chemical data show that the lithiophorite fromNew Caledonia is enriched in cobalt. It has a highAl content, which precludes the possibility both ofinclusions of heterogenite or asbolan (both Al-free),as well as diadochic or topotactic replacement of Alby Co. Crystallochemical data show that cobalt isnot adsorbed on the layers but integrated into thestructure. It is surrounded by ions with an atomicmass comparable to its own, that is to say it isintegrated in the Mn layer. It occupies octahedralsites comparable in size with those of manganese.Cobalt sites, however, show a higher structural orderthan those of manganese, suggesting that Mn andCo are not distributed at random in the Mn layer.Small amounts of nickel also are present; crystal-lochemical studies show that nickel is surrounded bylight atoms, which means it is distributed in thehydrargillite layer.

In the lithiophorite from New Caledonia (Fig. 7),the nickel present in the hydrargillite layer would,together with the excess Al3* cations, replace someof the missing lithium. The presence of Li+, Ni2*and excess Al3+ cations in some of the normallyempty sites of this layer creates an excess of positivecharges. The cobalt is located in the MnO2 layer,where its presence creates a lack of positive chargesin the right proportion to compensate for the chargeof the other layer (Llorca 1987). Hydrogen bondswould allow the transfer of charges. It is thus notnecessary here to suggest the presence ofMn2* ionsin the MnO2 layer to compensate the charge of theother layer, as suggested for the Postmasburg-tlpelithiophorite by Pauling & Kamb (1982).

A whole range of new phases with compositionsbetween those of lithiophorite and the members ofthe asbolan series were found, termed "asbolan-lithiophorite intermediates". There is a markedsimilarity among the structures of heterogenite,asbolan and lithiophorite (Fig. 7): they all consist ofsingle layers of octahedra stacked along the Z axis,with comparable periodicities. In both asbolan andlithiophorite, every second layer is a Mn layer, whichip separated from the next Mn layer by 9.34 and 9.39A, respectively. It thus is not surprising that asbo-lan and lithiophorite form a continuous series (solidsolutions or interstratifications) (Fig. 7).

Heterogenite, asbolan, l i thiophorite and

"asbolan-lithiophorite intermediates" form ahomogeneous group with similar structures and crys-tal chemistry: only four cations are present in thesephases, Co3+, Mn4*, 1q12+, 413+, all forming layersof octahedra, CoOOH, MnO2, tNi(OH):l- andAl(OFI)r, in different combinations to form the var-ious species. This structural mimicry influences thephysical properties of these minerals, making theiridentification difficult, particularly for asbolan,fthiophorite and the "intermediates". Only chemicalanalysis (microprobe) can lead to unambiguous iden-tification of these minerals and discrimination amongtheir varieties.

ACKNOWLEDGEMENTS

The work presented here forms part of a Ph.D.thesis defended in 1986 at the Laboratoire de min6ra-logie et cristallographie de l'Universit6 Paul Sabatierin Toulouse, France. We are very grateful to all whocontributed to this work, and in particular to A.Colleau and J.P. Paris (BRGM, France), G. Troly(IMETAL, France), M. Esterle, B. Pelletier and J.P.Perrier (S.L.N., France and New Caledonia), R.D.Lille (COFREMMI, New Caledonia), B. Escande(MINEMET Recherche, France), G. Calas and A.Manceau (Universit6 de Paris VII), A. Meunier andD. Beaufort (Universit6 de Poitiers, France), J.P.Fortun6 and J.J. Couderc (Universitd Paul Sabatier),and G.F. Taylor (CSIRO, Australia). We also thankM. Deliens (Institut Royal des Sciences Naturelles,Bruxelles), J. Ostwald @roken Hill Proprietary Co.,Shortland, Australia), E.H. Nickel and J. Thompson(CSIRO, Australia) and R.F. Martin for usefulcomments on the manuscript.

REFERENCES

Srvrsov, V.V. & BanEsovsKAva, V.V.(1980a): Crystallochemical nature of Co-Ni asbo-lan. Izv. Akad. Nauk SSSR, Ser. Geol, No,6,73-81.

-, -, -&- (1980b): Crystal-of nickel-asbolite. Izv. Akad.lochemical nature

NaakSSSR, Ser. Geol. No.9. 108-120.

DrLrrus, M. (1974): Les oxydes hydrat€s de cobalt duShaba m6ridional. Ann, Mu* roy. AJr. Centr. Ter-vuren Belg, S€r. in-8" Sci. Gdol. No. 76.

& Gorrnem, H. (1973): Polytypism of hetero-genite, Mineral. Mag. 39, 152-157.

FRANsoLrr, A.M. (1979): Occurrences de lithiophorite,nsutite et cryptom6lane dans le massif de Stavelot,Belgique. Ann. Soc. Gdol. Belg. 102,303-312,

SUPERGENE COBALT MINERALS FROM NEW CALEDONIA t 6 l

GLessER, M.E. (1903): Les minerais de cobalt. Lesrichesses min€rales de la Nouvelle-Cal6donie. Rapp.au Ministre des Colonies, 29-69.

Hrv, M.H. (1962): Cobaltic hydroxide in nature.Mineral. Mag. 33, 253-259.

IrrunNarroNeL MrmeneLocrcar AssocnrroN (1982):Lithiophorite in Mikhalkovo - fluorite deposits,Eastern Srednogorie zone. Int. Mineral. Assoc., 13thGen, Meet. (Varno), Guide Book 1.3, 56.

Lronca, S. (1986a): Les concentrations cobaltifTressuperglnes en Nouvelle-Calddonie: gdologie, mindr-alogie.Thbse de doctorat, Universit6 Paul Sabatier,Toulouse, France.

- (1986b): Nouvelles donn6es sur les h6t6rog6nites(Nouvelle-Cal€donie). C.R. Acad. Sci. Paris 303,sdr, II, no. 20, 1797-1802.

(1987): Nouvelles donndes sur la compositionet la structure des lithiophorites, d'aprds des 6chan-tillons de Nouvelle-Cal6donie. C.R. Acad. Sci. Poris304 s€r. II, no. l, 15-18.

- (1988): Nouvelles donndes sur la compositionet la structure des asbolanes (Nouvelle-Cal6donie).C.R. Acad. Sci. Paris 307, s6r. II, 155-161.

- (1990): Metallogeny of supergene cobalt miner-alization, New Caledonia. Aust. J. Earth Sci. (sub-mitted).

MaNcEeu, A., Lronca, S. & Caras, G. (1987): Crys-tal chemistry of cobalt and nickel in lithiophoriteand asbolane from New Caledonia. Ceochim. Cos-mochim. Acta 51. 105-113.

MrrcurlL, R.S. & Mrrnrzrn, R.E. (1967):

Lithiophorite from Charlottesville, Virginia. Am.Mineral. 52, 1545-1549.

OsrwAlD, J. (1984): Two varieties of lithiophorite insome Australian deposits. Mineral. Mog. 48,383-388.

PnuLrNc, L. & Keun, B. (1982): The crystal structureof lithiophorite. Am. Minerql. 67,817-821.

Prnsnrr, E.-A. (1972): Quelques prdcisions sur la"lithiophorite" et "l'asbolite". C.R. Acad. Sci.Paris 275, S€r. D,275,1019-1021.

QurNnau, P.E. (1971): Cobalt and the NickeliferousLimonites. Queneau, Delft, Holland.

Tnrscesrs, J.J. (1975): L'dvolution g€ochimique super-gdne des roches ultrabasiques en zone tropicale. For-mation des gisements nickelifOres de Nouvelle-Calddonie. ORSTOM (Paris), Mdm. 78,

Tnolv, G., Euenlr, M., PeLI-Ertrn, B. & RrtsnI-' W.(1979): Nickel deposits in New Caledonia. Some fac-tors influencing their formation. 1r InternationalLaterite Symposium (D.J.I. Evans, R.S. Shoemakerand H. Veltman eds.). Society of Mining Engineersof the American Institute of Mining, Metallurgicaland Petroleum Engineers, Part I, Chap. 5, 85-119.

Waosrpv, A.D. (1950): Synthesis of some hydratedmanganese minerals. Am. Mineral. 35,485-499.

WrLSoN, M.J., BERRow, M.L. & MCHARDY, W.J.(1970): Lithiophorite from the Lecht mines, Tomin-toul, Banffshire. Mineral. Mag. 37, 618-623.

Received September 29, 1988, revised manuscriptaccepted September 6, 1990.