American Mineralogist, Volume 74, pages 656467, 1989

Corrosion of Fe-Ni alloys by Cl-containing akagan6ite (p-FeOOH):The Antarctic meteorite case

YacN F. Bucnwar-oDepartment of Metallurgy, Building 204, The Technical University, 2800 Lyngby, Denmark

Rov S. Cr-anrc, Jn.Division of Meteorites, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A.

Ansrnacr

A suite of 12 iron and 8 stony meteorites from various environments and locations inAntarctica was studied, representing a range of compositions, metallographic structures,and degrees of weathering. Polished sections of meteoritic metal with adhering corrosionproducts were examined microscopically, with the electron microprobe and seu, and wereused as samples for X-ray diffraction examination. The major corrosion products are Cl-containing akagan6ite (up to 5 wto/o Cl) and goethite, with minor amounts of lepidocrociteand maghemite. Cl is not native to the meteorites but, owing to the electrochemical natureof the corrosion of Fe-Ni alloys, is attracted from the snow and ice, or rocky soil, envi-ronment that contains low amounts of Cl. Akagan6ite precipitates near the reaction front,incorporating Cl ions into ion-exchange sites, where they may be retained, made availablefor further depassivating, corrosive action, or become flushed from the system. With timeand distance from the reaction surface, akagan6ite ages and transforms to mainly goethiteand maghemite. It is again argued that lawrencite (Feclr) is not a mineral in meteorites.In the stormy Antarctic, wind-blown terrestrial particles may become attached to theweathering products on the meteorite and in time themselves decompose, delivering ter-restrial ions to the meteorite corrosion system. The ions may move a significant distancefrom the surface, and in particular Cl may be found along almost invisible cracks in thedeep interior. Trace-element work and age determinations (36C1) should take this problemlnto account.

INrnonuctroN study of iron-meteorite corrosion in all terrestrial envi-ronments. Although a number of recent papers have con-

Meteorites are our most accessible samples of extrater- sidered various aspects of the weathering of stony mete-restrial bodies. They provide us with direct evidence of orites, little attention has been given to iron meteoritesearly planetary system processes, along with information since the works of Buddhue (1957), White et al. (1967),on many subsequent interactions throughout their com- Faust et al. (1973), and Buchwald (1977).plex histories prior to and after arrival on the Earth. They Early in our studies it was realized that Cl-containingfall randomly in terms of both time and geographic dis- akagan6ite is a mineralogical key to the understanding oftribution. Our life-supporting atmosphere laden with the corrosion of meteoritic metal and that its role is mostoxygen and moisture, however, is hostile to these intrud- clearly demonstrated by the comparatively simple asso-ers from the less reactive environment of space. Corro- ciation of corrosion products that develops on meteoritessive processes begin as a meteorite enters the atmosphere retrieved after long exposures to the cold and low-hu-and continue until nothing remains that is recognizable. midity environment of Antarctica (Buchwald and Clarke,The metallic phases that are present in all but a few me- 1988). The process that emerges and overcomes passiv-teorites are among the first to show obvious effects of ation barriers includes anodic metal going into solution,terrestrial weathering. Two Fe-Ni alloys, kamacite (low- Cl- ions that originate from the terrestrial environmentNi Fe-Ni, bcc) and taenite (high-Ni Fe-Ni, fcc), are the moving to the reaction surface to maintain charge bal-main metallic minerals present, and they are gradually ance, and Cl-containing akagan6ite precipitating at theconverted to rust. Understanding this corrosion process reaction surface. Over time, the initial corrosion productshas obvious implications for developing strategies to ter- undergo an aging process that converts them to mainlyminate, or at least minimize deterioration that continues goethite. This same process is also responsible for thewhen these scientifically important materials are incor- corrosion of the metal particles in Antarctic chondrites,porated into research and exhibit collections. For these stony meteorites containing from 6 to 15 wt0/o metal. Pre-and other reasons, we have undertaken a comprehensive liminary indications are that Cl-containing akagan6ite

0003-004x/89/0506-o656s02.00 656

BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE 657

TABLE 1, Characterization of selected Antarctic meteorites

Bulk comoosition".State ofMaSS

Name' (kS) Type wt% NiAget

(10'yr)wt% P corrosion

ALH A77283ALH 477289ALH A78252ALH A81013ALH 84165DRP A78OO9EET 83390GRO 8s201tLD 83500NEP 642614PGP 477006Y 751 05

10 .52 . 1 9279

1 7 80 . 1 0

1 38.10.o21.402.521.07

1 e o

0.02

lA, coarse octahedritelA, coarse octahedritelVA, fine octahedritellA, hexahedritelllA, medium octahedritellB, coarsest octahedrite(lA), octahedritelllA, medium octahedritelVB, ataxitelA, coarse octahedritelA, coarse octahedritellA. hexahedrite

7.336 8 49.335 5 zL I O

6.59

6,CY

1 8 97.267 2 75.26$

0.220.210.170.270.140.340.2+0.230.280.200.200.25+

1'I

02

2

0

'I

0

1 1 0 + 7 0170 + 70360 + 70

-3000

9 0 + 7 0

- Allan Hills (ALH), Derrick Peak (DRP), Elephant Moraine (EET), Grosvenor Mountains (GRO), Inland Forts (lLD), Neptune Mountains (NEP), PurgatoryPeak (PGP), Yamato Mountains (Y).

'- E. Jarosewich, analyst. Remainder is Fe with -1% total Co, S, Cr, C, etc.t Nishiizumi (1984) and Nishiizumi et at (1987).f Our estimate.$ Kracher et at. (1980).

plays an important role in the weathering of essentiallyall meteoritic metal, and we suspect that in the corrosionof other ferrous metals under similar circumstances, thismineral plays a role that has not been previously recog-nized.

MrrnonrrB SpECIMENSAfter preliminary examination of corrosion products

on 40 iron meteorites from a variety of environments,we focused our attention on a suite of l2 Antarctic irons.Collected between 1964 and 1986, they represent a va-riety of compositions and metallographic structures, dif-ferent environments within Antarctica, and separationsby large distances on the continent (Table l). The AllanHills, Elephant Moraine, Grosvenor Mountains, and ya-mato Mountains meteorites were found on either bare iceor in an ice and rock moraine, generally at altitudes of1600-2000 m (Yoshida et at., t97t yanai, 1976; Cas-sidy, 1980; Clarke et al., l98l;Clarke, 1982a). The Der-rick Peak meteorites were found on a ridge and on a sideof Derrick Peak, a mountain protruding above Heather-ton Glacier in the Britannia Range (Kamp and Lowe,1982; Clarke, 1982b, 1984). The Inland Forts specimenwas found partially within soil at Inland Forts on thesouth slope of the Asgard Range above Taylor Glacier(Clarke, 1989). The Neptune Mountains meteorite wasfound on glacial debris about 30 m above the rce on anunatak in the central Neptune Range (Buchwald, 1975).The Purgatory Peak meteorite was recovered from thebase of the mountain in the Victoria Dry Valley (King etal., 1980; Clarke, 1982a). Common to these specimens isthat they were found completely or largely exposed onthe ice, on rock surfaces, or on soil. Great care was takento prevent contamination or deterioration of specimensduring collection and transport to curatorial facilities, andthey have received special attention within these facili-ties. Background on the field and curatorial programs that

provided the specimens is given by Marvin and Mason(1980, 1982, 1984) and Marvin and MacPherson (1989).

Table I provides total specimen weights, chemical andstructural classification, and bulk Ni and P values. Theseverity ofcorrosion is also evaluated on a scale from 0(no corrosion) to 4 (destructive corrosion), a scale similarto but different in detail from the scale used for Antarcticstony meteorites (Lipschutz, 1982). The final column re-ports terrestrial ages (Nishiizumi, 1984; Nishiizumi et al.,I 987) .

ExpsnrN{nNTAL METHoDS

Regions with interfaces between metals and oxides oroxide fragments were selected for examination that in-cluded microscopic identification on polished sections,electron microprobe and scanning electron microscope(srv-eo,lx) analysis on these same sections, and X-raypowder-diffraction studies of oxide isolated from selectedsections. Polished sections were made in the standardway: cutting with tap-water-cooled carborundum blades,followed by wet-grinding with garnet grit, and wet pol-ishing with diamond and finally AlrO, paste. In order toestablish that Cl was not introduced by the use of tapwater, control sections were prepared without its use fromcorrosion products that had not been previously exposedto tap water. Dry cutting and coarse grinding were fol-lowed by petroleumJubricated fine grinding and polish-ing with diamond paste. Akagan6ite in these sections con-tained comparable levels of Cl and convinced us that Clwas not introduced or moved during our sample prepa-ration. However, it would be reasonable to expect that inprocedures such as abrasive-wire sawing, where long cut-ting times and large amounts of tap water are involved,a slight modification of the mobile Cl concentration inakagan6ite is possible.

Chemical data were obtained with an anr- seuq elec-tron microprobe with six fixed spectrometers for Si, Al,

6 5 8 BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE

TABLE 2. Oxide weathering products of Antarctic iron meteorites

FormulaName Structure General formula wt% Fe

Observed range (wt%)

Akagan6iteGoethiteLepidocrociteMaghemite

hollanditediasporeboehmitecubic spinel

B-FeOOHa-FeOOH7-FeOOH7-FerO"

39-6041 -6145-6058-63

0.5-191.0-8.00.5-110.4-7.0

0.3-5.4<0.2<0.2<0.1

62.962.962.969.9

Fe, Mg, Ca, and K, and three adjustable spectrometersset for Ni, P, and Cl, or for Co, Na, and S, operated at15 kV and 0.025-mA sample current on brass. Magnetiteand Kakanui hornblende were used for Fe standards (Ja-rosewich et al., 1980), and synthetic Ni-Fe alloys wereused for the Ni standards. Values for Ni in the oxideswere corrected by using the uecrc-rv program.

Photographs and additional chemical data by energy-dispersive X-ray analysis were obtained using a PhilipssEM-EDAx operated at 20-kV accelerating voltage.

Samples for X-ray diffraction analyses were dispersedwith acetone onto a low-background quartz slide, anddata were collected with a Scintag automated diffractom-eter system, operated under the following conditions: 0.03"step size, 2- ard 4-mm divergent slits, 0.3- and 0.5-mmreceiving slits, Ge detector, CuKa radiation, \ : 1.54091A, +S tv, 40 mA, and a 1.0"/min scan rate.

ConnosroN ASSEMBT.AGES

Corrosion assemblages on iron meteorites that have sofar been retrieved from Antarctica are much less exten-sive than those that have been observed on many speci-mens found in temperate and tropical environments.Corrosion on the exterior surfaces of those selected forstudy is typical and normally consists of a relatively thinlayer of oxides, less than I mm thick. More severe cor-rosion develops on under-surfaces, in cleavages, or infissures within the meteorite.

Akagan6ite, goethite, lepidocrocite, and maghemite arethe four minerals that constitute the bulk of the corrosionproducts observed. They are listed in Table 2 by name,structure type, and general formula. Also given is the for-mula weight percent of Fe and the observed ranges inweight percent for Fe, Ni, and Cl as determined by elec-tron microprobe. Low Fe totals were observed on all fourof the oxides and are due mainly to the presence of waterof hydration and to irregularities of surfaces caused bythe presence ofvoids in the oxide. For this reason, ana-lytical data are presented to no more than two significantfigures. The oxides also contain Ni in varying amountsas a consequence of derivation from an Fe-Ni alloy. Aka-gan6ite contains significant amounts of Cl, and smallamounts of Cl were occasionally observed in goethite andlepidocrocite.

Observations on specific meteorites are summarized inTable 3. The major and accessory weathering productsare given for each meteorite, along with the range in Ni,Cl, and S for the major product. Also noted are other

elements that were observed in the oxides and foreignmineral grains that were Present.

Akagan6ite is rust colored, and its powder is orange tobrownish. It is grayish blue in polished section, is softerthan maghemite and magnetite, and is often rich inshrinkage cracks, like dried mud. It may contain a sig-nificant content ofadsorbed water, and this is particularly

evident when the electron-microprobe beam is kept on aspot. Within 2-10 min, 5-10 wto/o HrO may be lost, as isindicated by an increase with time in Fe counts. In po-

Iarized light, akagan6ite is microcrystalline, with orangeto reddish internal reflections that are intermediate be-tween those ofgoethite and lepidocrocite, appearing sub-dued like a coke fire in a fireplace.

Akagan6ite is observed in various relationships in Ant-arctic iron meteorites. It fills corrosion pits 0.5-2 mmacross on the surface, and in the interior it loosely ce-ments cracks as a disordered and microcracked oxide'Akagan6ite is usually in contact with as yet uncorrodedmetal, whereas goethite, maghemite, and lepidocrocite areoften found as oxide intergrowths or as coatings on orbetween other oxides. Our data show (Tables 2 and 3)that natural akagan6ite is rich in Cl. In a given meteoriteoxide, the Cl content may vary from the limit of detection(0.1 wto/o) to several weight percent, as analyzed by theelectron microprobe. There is both a large-scale variationamong different parts of the meteorite and a small-scalevariation, for example, within the akagan6ite-goethite fill-ing of an extended crack, due to varying proportions ofthese two minerals. Most of the Cl is located in the aka-gan6ite, whereas a few tenths ofa percent may be presentin goethite and lepidocrocite. Cl is apparently absent inmaghemite (Table 2).

The Cl content of akagan6ite is highest near its inter-face with metal or in the deep interior when it is a crackfilling. In a typical crack filled with akagan6ite, 100 pmwide at the surface and narrowing to 5 pm at a depth ofl5 mm, Cl ranges from 0. I wto/o at the surface to 4.9 wto/onear the bottom ofthe crack. The Cl concentrates wherecorrosion is currently active, and levels of 4-5 wto/o aretypical at these locations.

Lepidocrocite is sometimes observed coating exteriorsurfaces and in internal cracks. It has clearly been trans-ported in solution and deposited as botryoidal layers, orit forms laminated bands 5-50 pm thick, sometimes en-veloping foreign particles such as quartz and feldspargrams.

Although our efforts focus on iron meteorites, we have

BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE

Taele 3. Corrosion products and nonmeteoritic minerals on Antarctic iron meteorites

6s9

Weathering productsMajor oxide composition

Range in wt%

Maior AccessoryElements introduced

into oxides Foreign minerals

ALH A77283ALH A77289ALH 478252ALH A81013ALH 84165DRP A78OO9EET 83390

GRO 85201

rLD 83500NEP 642614PGP A77006

Y 751 05

akagan6iteakagan6iteakagan6iteakagan6iteakagan6iteakagan6iteakagan6ite

akaganeite

akagan6iteakaganeitegoethite

akagan6ite

goethitegoethitegoethitegoethitelepidocrocitegoethite, lepidocrocitegoethite, maghemite,

lepidocrocitegoethite, maghemite,

lepidocrocite

goethite, maghemiteakagan6ite, maghemite

lepidocrocitegoethite

0.5-5.5 0 7-2.5 nd1.8-6 0 0.3-3.4 nd1 5-8.5 0.3-5.0 nd0.9-4.5 0.7-1.6 0.2-0.71.8-12 0.4-1.5 0.1-1.91.0-5.6 0.8-4.5 0.2-0.51 .0-13 0.3-2.5 0-2.0

0.4-12 0.6-2.4 nd

1.2-16 0.4-3.0 0-0 4 Mg, Ca, Al, Na, (S)22-19 0.4-3.6 nd Ca, Mg, Al1.0-5.0 <0.2 <O.2 Si , Al , Mg, Ca, K, (S)

1 5-4.7 0.4-5.4 nd Mg, K, Al

quartz, olivine, zeolite

calcite,- quartz

feldspar

calcite,* quartz

orthoclase, plagioclase, calcite,-olivine, quartz, amphibole

Ca, KMg, Ca, AlMg, SiAl, Mg, Ca, (S)Ar, K, (S)Si, Ca, Mg, (S)Mg, Al, K, (S)

Ar, K, Si

ivote; nd : not determined.' ldentified in vugs and veins

also examined the corrosion of metal particles in potishedthin sections of a suite of 8 chondritic meteorites fromAntarctica. Both H- and L-group meteorites of petro-graphic grades 4-6 (Van Schmus and Wood, 1967), andofweathering classes B and C (Lipschutz, 1982), are in-cluded. Specimens selected are from the Allan Hills, theElephant Moraine, and the Taylor Glacier, and they werecollected during the 1976 through 1982 field seasons.Their names, total weights, classifications, their range inakagan6ite wto/o Cl, Ni, and Na, and their terrestrial agesare given in Table 4. In general the same relationshipsbetween oxides and metals developed for iron meteoritesare observed in association with the metal particles inthese stony meteorites. Metal particles are in the milli-meter and smaller size range and constitute varyingamounts of the specimen. Average metal content for ob-served-fall H-group chondrites is 18 wt0/0, and for L-groupchondrites 7.4 wto/o (Mason, 1965). A by-product of thesechondrite studies was black and white photographs illus-trating important relationships; photographic equivalentsof iron meteorites are difrcult to obtain without resortinsto color photography.

Conversion in place of chondritic kamacite to akaga-n6ite is illustrated by seu back-scattered electron photo-graphs (Fig. l) and semiquantitative chemical data. Inspecimen EET 82602, a kamacite particle containing ap-proximately 6 wto/o Ni is partly converted to akagan6ite(Fig. la). The enclosing silicates appear not to have beendisturbed, indicating that akagan6ite has directly replacedkamacite without significant volume change. Assumingthat akagan6ite consists only of Fe, Ni, and Cl, the ana-lyzedarea (Fig. la) contains 5.5 wto/o Ni and 3.1 wto/o Cl.A more advanced state of weathering, also without dis-tortion in the silicate relationships, has converted a net-work of kamacite to essentially akagan6ite in EET 82604(Fig. lb). 420 x 25 pm area of akagan6ite in this asso-ciation contained 5.0 wo/o Ni, 4.4 wto/o Cl, 0.5 wto/o S,assuming that it consisted only of these elements and Fe.These associations suggest that HrO, Cl , and O, movedthrough earlier-formed corrosion oxides to react withkamacite and that Fe and some of the Ni were removedso that the less dense akagan6ite could replace kamaciteon an equal volume basis.

An akagan6ite-goethite vein penetrating a chondrule in

TABLE 4. Akagan6ite in association with metal grains in Antarctic chondrites

Akagan6ite compositionsRange in wt%

clMass(kS) Type Weathering

Age.'(103 yr)

ALH A76009ALH A77OO4ALH 477285ALH A79OO2EET 82602EET 82604EET 82606TYR 82700

170 + 70220 + 70

B

cBBtcBB

LO

H4H6H O

H4H5LO

L4

4072.20.30.21 . 8t . o1 . 00.9

0.3-23< 0 2

0.2-1.80.3-3.90.2-3.30.5-3.2o.4-3.4o642

2.1-7.41.7-4.62.7-6.7194 .72.0-5.31.7-5.91 .8-192.5-5.5

<0.1<0 .1<0 .1<0 .1nondndno

. Allan Hills (ALH), Elephant Moraine (EET), Taytor Gtacier (TyR)."" Nishiizumi (1984).

660 BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE

Fig. 1. Backscattered-electron images of akagan6ite replacing metal in polished thin sections of Antarctic chondrites. (a) EET82602 (H4), akagan6ite (gray) replaces kamacite (white). Various silicates surround the akagan6ite, and a thin akagan6ite-goethite-filled horizontal crack is at the upper right. The lighter gray rectangle at the upper right of the akagan6ite field is an area analyzedby selr-roex. The network of cracks in the akagan6ite is typical. Scale bar: 0.05 mm. (b) EET 82604 (H5), a network of kamacite(white) largely replaced by akagan6ite (gray). Darker minerals are silicates. Scale bar : 0.1 mm.

EET 82602 demonstrates deposition of these weatheringproducts by solutions moving into a crack (Fig. 2). Asemiquantitative analysis of a l0 x l0 pm area, againignoring HrO and Or, found mainly Fe, 5.5 wto/o Ni, 0.9wto/o Cl, 0.4 wto/o S, 1.1 wto/o Al, 3.1 wto/o Mg, 2.6 wto/o Si,and 0.6 wto/o Ca, suggesting that parts of the adjacenttroilite and silicates were dissolved and incorporated inthe akagan6ite-goethite filling.

Troilite and silicate associations may also be invadedby akagan6ite veins (Fig. 3a). Analysis indicates that Ni,Cl, HrO, and possibly Fe were moved along cracks toreact with the troilite and silicates to produce akagan6ite.A semiquantitative analysis of a small area, neglectingagain the light elements, indicated major Fe, 2.9 wto/o Ni,4.1 wto/o Cl, and 3.0 wto/o S. Growth contours that arereadily recognized under the microscope in these veinsare suggestive ofgrowth by solution transport.

An interface between metal and chlorapatite in EET82606 (Fig. 3b) reveals that a thin strip of the metal hasbeen replaced by akagan6ite, but there appears to be no

indication of the still distinct boundary of the chlorapa-tite having been affected. Clearly demonstrated in thisassociation is the dependence ofakagan6ite compositionon the composition of the metal from which it formed.The somewhat thicker akagan6ite in the bottom half ofFigure 3b formed by replacement of part of the adjacentkamacite field and retains the approximately l8:l Fe:Niratio of the kamacite. Analyses in this akagan6ite areawere all close to 5 wto/o Ni, 90 wto/o Fe, and 4 wto/o Cl,ignoring HrO and Or. The akagan6ite in the top of thepicture formed from taenite with an entirely different Fe:Ni ratio of about 4:1. Consequently, this akagan6ite isvery rich in Ni, averaging 19 wto/o Ni, 76 wto/o Fe, and 4wto/o Cl, by the same analytical procedure.

DrscussroN

Cl and Ni in akagan6ite

Akagan6ite is geologically a rather rare mineral thatwas first described as a weathering product from the Jap-

BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE

Fig.2. Backscattered-electron images of a chondrule in EET 82602 (H4). (a) A section through a silicate chondrule (dark)partially surrounded by kamacite (white), with slightly weathered kamacite also in the silicate matrix. A vein of akagan6ite-goethitepenetrates the chondrule from its lower right to the upper left. Thin veins of akagan6ite-goethite also border the chondrule. Scalebar : 200 pm. (b) Enlargement oflower right area of akagan6ite-goethite vein (white to light gray) penetrating the chondrule silicates(dark). Scale bar : 10 pm.

66r

anese limonite mine Akagan6 (Mackay, 1962). It was not-ed as a corrosion product on a few iron meteorites byMarvin (1963), who identified it in X-ray powder pho-tographs. Akagan6ite (Table 2) has the idealized formulap-FeOOH and has long been known from synthetic work(Mackay, 1960).

An important point for corrosion of ferrous metal isakagan6ite's ability to host Cl in its structure, syntheticmaterial having been reported to contain from 1.3 to 6.4wto/o Cl (Keller, 1970). Many authors have noted that iteasily adsorbs water, 2-20 wto/o, particularly when crys-tals are fine or the crystallinity is poor. Synthetic akaga-n6ite has an exchange capacity, Cl- and OH being rathereasily exchanged. By repeat washing with water, syntheticakagan6ite with 5.4-6.3 wto/o Cl was shown to lose 2l-36wt0/o of its Cl (Keller, 1970). The unusual properties ofthis iron oxide are best accounted for by the formulaproposed by Keller, Fer(O,OH)l,u(Cl,OH)=r, where 8FeOOH formula units are arranged in a monoclinic cellas in hollandite (J. E. Post and V. F. Buchwald, manu-script in preparation) and where Cl occupies some but

not all of the tunnel sites, making it rather easy for Cl toenter or leave the structure.

Akagan6ite may also accommodate Ni in its structure.In the present study, up to 19 wt0/o Ni was observed,although 3-5 wto/o Ni was the normal amount becausemost of the akagan6ite has formed from kamacite. ItsX-ray diffraction pattern typically gives broad peaks, un-doubtedly as a result of very small crystal size. This isconsistent with earlier observations (Mackay, 1962;KeI-ler, 1970) on synthetic akagan6ite by reu (transmissionelectron microscopy) of crystals averaging 2500 x 250 Awith a maximum length of 5000 A. It has, therefore, notbeen possible to identify changes in lattice parametersdue to Fe:Ni ratio variations. The roles of both Cl andNi in the akagan6ite structure will be discussed in detailelsewhere (J. E. Post and V. F. Buchwald, manuscript inpreparation).

Corrosion mechanism

The corrosion mechanism of meteoritic metal in theAntarctic environment is in large part an electrochemical

662 BUCHWALD AND CLARKI: CORROSION OF Fe-Ni BY AKAGANEITE

reaction that produces akagan6ite as the initial product.Sources ofthe required potential difference to drive cor-rosion cells are the normal components of meteoritic metaland structural cracks. The major phase is generally kam-acite (bcc Fe containing 4-7 wto/o Ni), with minoramounts of the accessory minerals taenite (fcc Fe with2540 wto/o Ni), schreibersite [(Fe,Ni)3P], and troilite (FeS)providing interfaces of differing chemical potential. Theseinterfaces, as well as cracks, are sites where O, and elec-trolytes invade the metal. The result is that anodic kam-acite goes into solution, electrons pass through metal tothe cathode where O, (and sometimes H*) is reduced,and akagan6ite precipitates. It appears that in the Ant-arctic deep freeze, such reactions may take place withoutliquid water, evidently requiring a much extended timeowing to low coefficients of diffusion in ice and in oxides.

In simplest form and using conventional half-reactions,the process may be expressed as follows. Metal (Feo) isoxidized at the anode

F e o - F e 2 + i 2 e

and O, is reduced at the cathode

02 + 2H2O -t 4e - 4OH-,

or, alternatively, H is reduced at the cathode

2H* + 2e.---."..-.-Hz.

Combining the first two equations

2Feo * 02 + 2H'O ;;;;

2Fe(OH). + 2OH-.

This is followed under oxidizing conditions by the for-mation of akagan6ite

2Fe(OH)* + 2OH-1- VzO,;;;;2B-Fej.OOH + H,O.

In order to represent the degradation of meteoritic metalmore realistically, we have adopted a variant of Keller'sformula (Keller, 1970) for akagan6ite, [Fe,rNi][O,r-(OH)rolClr(OH). It satisfies structural constraints, is incharge balance (Fe is assumed to be Fe3+, and Ni is Nir*),and contains 55.9 wto/o Fe. 3.9 wto/o Ni. and 4.7 wto/o Cl-amounts that have been commonly observed in this study.The reaction may then be expressed as

30Feo + 2Nio + 47O + l9H,O + 4H* + 4Cl-

kamacite

-.------: .- 2[Fe''Ni][O,,(OH),0]CI,(OH).

ineversible aka€n6i1e

This reaction results in the formation of akagan6ite asa replacement for kamacite. Little Ni or Fe is leachedfrom the corrosion product, and Cl is derived and accu-mulated, even from environments where it is present invery low concentrations. Three factors appear to be im-portant in explaining this dramatic accumulation of Cl:

the electrochemical nature ofthe corrosion reaction, thehigh ionic mobility of Cl-, and the size of the Cl- ion thatallows it to fit into the tunnel sites of akagan6ite andstabilize its structure.

The electrochemical requirement is that anions moveto the anode, the surface where Feo goes into solution, tomaintain charge balance and that electrons flow throughmetal to the cathode where anions are produced. Chlo-ride ions are apparently attracted to the vicinity of thecorrosion front preferentially, their high ionic mobilityundoubtedly being an important contributing factor. Atthe corrosion front, Cl- decomposes any passivating ironoxide film on the metal surface and often gives rise topitting attack. Under these conditions, a Cl-containingakagan6ite forms and encloses any taenite, troilite, orschreibersite located in the corrosion zone as passiveminerals. Only much later in corosion development, astage rarely observed in Antarctic meteoritic metal, dothese minerals disintegrate, usually in the order given,schreibersite, being able to survive in the corrosion prod-ucts for a surprisingly long time.

Slowly in the Antarctic environment, but more rapidlyin temperate climates, Cl in akagan6ite is exchanged withOH-, and Cl is released into solution to move again tothe corrosion front or to be flushed from the system. Theakagan6ite left behind will approach the composition[Fe,,Nd[O,r(OH)r0](OH)3 and, having lost its Cl, be-comes unstable. Under conditions of modest heating and/or seasonal changes in temperature and humidity, it de-composes according to a reaction such as

2[Fe,,Ni] [O',(OH),.](OH)3

ineversible77-FerO3 + NiO

maghemite'

+ l6a-FeooH + Nio + 15H,O,

g@thite

a reaction that may be termed the aging of akagan6ite.The products, typically maghemite and goethite, may foras yet unknown reasons occur in widely varying propor-tions. They are intimately intergrown on the micrometerscale, but may be distinguished in polished section (mag-hemite is brownish and harder and therefore stands outin relief). They contain most or all of the Ni and Fe thatwere previously in the akagan6ite. Very little loss byleaching is observed in these early corrosion stages. Thegreat terrestrial age of some of the meteorites studied (Ta-bles I and 4) combined with the limited development ofaging products when compared with temperate and trop-

tIn this work we have observed a cubic spinel that may beeither maghemite (7-FerO3) or magnetite (FerOo). It contains Niand has an X-ray difraction pattern that may be interpreted asthat of either Ni-substituted maghemite or Ni-substituted mag-netite. Data are insufrcient to distinguish between the two pos-sibilities, so we have labeled all of the observed spinel materialas maghemite.

BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE

Fig. 3. Backscattered-elecrron images of akagan6ite associations in EET 82606 (L6). (a) Akagan6ite-goethite veins (dark gray)

through troilite (light gray) and silicates (dar$. Growth contours in akagan6ite are typical of transport reactions. Scale bar : 25

pm. (b) Taenite (T) and kamacite (K) in contact with a thin band of akagan6ite (A) invading metal at a previous interface between

metal and chlorapatite (C). Scale as in (a). See text for further discussron.

663

b

ical occurrences, suggests that aging is much slower in theAntarctic environment.

Source of Cl

What is the source of the Cl that plays this importantrole in the formation of the corrosion product akagan6itein Antarctic meteorites? Chlorapatite is the only well-characterized Cl-containing mineral that occurs in ironmeteorites (Buchwald, 197 5, 1984). Although chlorapatitemay contain as much as 5 wto/o Cl, it is sporadically dis-persed in comparatively rare silicate-containing inclu-sions that could not possibly supply the amount of Cl thatis observed. Three other possibilities remain: the metalitself, the mineralogical chimera lawrencite, or introduc-tion from the terrestrial environment by the mechanismoutlined above.

Although it is not the interpretation given by Berkeyand Fisher (1967), they nevertheless have persuasively

shown that the metal of iron meteorites contains little Cl.Their neutron-activation analyses show very low levelsof Cl in unweathered metal and suggest an upper limit of10 ppm Cl (probably very high). As akagan6ite replaceskamacite in approximately equal volumes, Cl must beintroduced from some other source to provide the highlevels that are observed

The mineral lawrencite (FeCl') has a long and contro-versial history that will be treated in detail elsewhere. Thename has been used commonly in the meteoritic litera-ture, particularly in reference to Cl-containing, semiliquideffiorescences associated with the corrosion of meteoriticmetal. Jackson (1838) first reported Cl in meteorites inthe severely corroded Lime Creek iron meteorite find. Hisclaim that the Cl was indigenous to the meteorite wascountered promptly in an insightful paper by Shepard(1842). Jackson and Hayes (1844) answered with whatappears to have been an overpowering, even ifby modern

664

eyes largely unresponsive, rebuttal. Later J. LawrenceSmith (1855, 1874, 1877) reported observing the "pro-tochloride of iron" in the solid state in the Tazewell andSmith's Mountain iron meteorites. Insufficient chemicaland physical data were given to characterize the materialwith any certainty, but the idea of an iron chloride mineralbecame accepted. Daubr6e (1877) introduced the namelawrencite to honor the discoverer, and later Smith (1883)used the term himself. The name has been accepted bymodern authorities, sometimes with equivocation, andcontinues to be used despite the lack of even a minimaldescription ofnatural material in the literature (Buchwald,1977). In the course of this work we have examined anumber of dried residues from typical "lawrencite" oc-currences, and in every case an akagan6ite X-ray diffrac-tion pattem has been obtained (Buchwald and Clarke,I 988) .

The presence of Cl in iron meteorite finds is real, ofcourse, the older literature having been reviewed by Co-hen (1903). Recently it has been shown (Buchwald,,1975,1977, 1986\ for iron meteorites found embedded in tem-perate-zone soils that the Cl is introduced from thegroundwater to which these meteorites have been longexposed. During exposure, Cl-along with O, and HrO-decomposes the Fe alloy. The corrosion mechanism out-lined above expands on this earlier work and explains theaccumulation of Cl in terms of akagan6ite. Cl accumu-lation apparently begins as the meteorite lands, and ak-agan6ite continues to play a role in corrosion as long asmetal remains.

Antarctic meteorites add a new dimension to this pic-ture, as many of them have never been buried in soil. Forexample, the Allan Hills meteorites, found on the wind-swept bare ice fields at an altitude of about 2000 m, con-tain akagan6ite with up to 5.0 wto/o Cl. This Cl was ap-parently introduced from the snow and ice ofthe Antarcticenvironment. Unfortunately, Cl analyses of the ice in con-tact with the meteorites are unavailable, but concentra-tions may well be in the low ppm range as was found byHeumann et al. (1987) and Mulvaney et al. (1988) forsea-shelflocations. The annual average temperature at theAllan Hills is low (-30 oC), and precipitation is low anddifficult to measure. The meteorites have presumably spentmost of their residence time on Earth within the ice, beingeventually exposed by the strong katabatic winds out ofthe South that remove surface snow and evaporate theupper part of the snow and ice fields. The ultimate sourceof the low levels of Cl in the snow and ice is probablylargely sea spray from the oceans, with an increment ofhydrogen chloride exhalations from volcanic action. TheCl is carried as either an aerosol or a gas over the snowand ice f,elds, where a portion is incorporated in newlyformed snow and ice deposits (Duce et al., 1973; Heu-mann et al., 1987; Shaw, 1988). Cl may also be concen-trated in the residual ice surface, being preferentially leftbehind as ice sublimes.

Concern over the ozone layer, among other reasons, hasmade the halogen geochemistry of Antarctica a topic of

BUCHWALD AND CLARK-E: CORROSION OF Fe-Ni BY AKAGANEITE

much current interest. Irregularities in halogen abun-dances have been noted in both Antarctic meteorites androcks (Dreibus and Wiinke, 1983; Dreibus et al., 1985;Heumann et al., 1987). Although our findings do not speakdirectly to these anomalies, the weathering mechanismproposed is an obvious source of Cl contamination formetal-containing meteorites. Weathering-product akaga-n6ite accumulates Cl, perhaps in part from decomposingchlorapatite in some meteorites, but certainly in the mainfrom the local environment. The process apparently hasa high Cl- ion specificity. A highly mobile anion is re-quired to provide charge balance at the metal interface,bringing large numbers of Cl- ions to the region of aka-gan6ite formation. The ionic radius of the Cl- ion fits therequirements for tunnel-site occupancy in the akagan6itestructure and is incorporated in it and becomes localized.This ionic selectivity provides a rationale for unexpectedhalogen-abundance relationships. Under these circum-stances, one would not expect to see a relationship be-tween the sea spray-derived component of the Cl presentin akagan6ite and Na and K. Interpretations of isotopicdata, such as 36Cl age determinations, could also be af-fected.

Antarctic environmental factors

The areas where Antarctic meteorites have been foundare notoriously stormy. Wind velocities are frequentlymore than 20 m/s, and under these circumstances, blow-ing mineral grains and ice crystals become mild abra-sives. This is probably why we have not yet seen an Ant-arctic iron meteorite with a heavy corrosion scale.Meteorites from temperate latitudes often display coat-ings several millimeters to centimeters thick, whereasAntarctic iron meteorite coatings rarely exceed I mm andare usually only 0.1 mm thick. The severe winds com-bined with corrosive processes in Antarctica produce dis-tinctive specimen-surface morphologies. Corrosion pref-erentially attacks kamacite, and since abrasive particlesin due time remove most of the akagan6ite, iron meteor-ites often show a pronounced surface relief. On the sur-face of Allan Hills 477283, for example, the more resis-tant taenite lamellae stand in relief, displaying a delicateWidmanstiitten pattern and revealing the internal octa-hedral structure of the meteorite (Clarke et al., l98l). OnDerrick Peak 478009 and other specimens of that fall,the effect is even more dramatic: the large, hard schrei-bersite crystals protrude as irregular, bronze-colored knobs0.5-5 mm above the rust-colored surface, an effect thatis unknown from any other iron meteorite (Clarke, 1982b).The reason is, no doubt, that the rate ofabrasion due towind-carried particles is significantly greater than the rateof corrosion-product accumulation.

The absence of liquid water because of the low Ant-arctic temperatures is an obvious limitation to corrosiondevelopment. The meteorites, however, are dark colored,and they do absorb heat during limited periods of expo-sure to the Sun's radiation. Specimens found on the iceon sunny days have, on occasion, been observed to be in

contact with tiny pools ofwater. Schultz 0986) measuredthe internal temperature of a stony meteorite lying on theblue ice surface of the Allan Hills and found that on 2austral summer days of the 2l the experiment ran, tem-peratures were just above the freezing point of water. Ourobservation, mentioned above, of botryoidal lepidocro-cite in some corrosion assemblages strongly suggests thatliquid water is sometimes present. Marvin (1980) iden-tified hydrated magnesium carbonates and sulfates inwhite surficial deposits on Antarctic stony meteorites. Thedeposits were explained as formed from salts leached fromthe meteorites by liquid water produced from snow onsun-warmed meteorite surfaces.

Introduced elements and minerals

As may be seen from the last two columns of Table 3,foreign elements and minerals have been incorporatedinto the akagan6ite corrosion assemblages. Even in theinterior of the Antarctic ice cap, wind-transported min-eral grains from eroded nunataks are apparently ubiqui-tous and may adhere to the corroding meteorite, ulti-mately to be cemented to it by growing iron oxides. Mostparticles are qvartz, in the l0 to 100-pm range, but oliv-ine, feldspar, amphibole, and zeolite grains have alsobeen observed.

On decomposing, the mineral grains release smallamounts of A1, Si, Ca, and K, which may become incor-porated into the growing akagan6ite or into later-forminglepidocrocite. The amounts of these elements are gener-ally low, on the order of a few tenths of 1 wt0/0, but oc-casionally, as in Purgatory Peak A77006, the concentra-tions in akagan6ite reach l.l wto/o Mg and0.2 wto/o Al. S,probably as sulfide, is also present in akagan6ite, usuallyin the range of 0. l-1.0 wto/0. S has been put in parenthesesin Table 3 because it probably is not a foreign ion butmay originate from decomposition of troilite in the me-teorite. P is sometimes present in akagan6ite, usually inthe range of 0.1-1.0 wt0/0, but occasionally reaching the 5wto/o level. This is to be expected, however, as most ironmeteorites contain appreciable amounts of schreibersite.On penetrating the atmosphere, sulfides and phosphideslocated at the surface melt preferentially and solidify tovery fine grained multicomponent eutectics in the fusioncrust. These components are the first to be attacked bycorrosion and are responsible for the occasional high pand S concentrations ofakagan6ite in the initial stages ofcorrosion. Later, most of the P and S are leached out ifliquid water is available.

Terrestrial age and location

In a general way, one would expect that the degree ofweathering reflects the terrestrial age of the meteorite.

"That this is not the case is demonstrated by the age de-terminations of Nishiizumi (1984) and Nishiizumi er al.(1987), given in Tables I and 4. We agree with Nishi-izumi's conclusion that the terrestrial age in the Antarcticis a composite of two unknowns, the time buried in theice and the time exposed on the surface. Corrosion rates

665

for meteorites either buried or exposed are unknown. Itis reasonable to assume, however, that rates at the surfaceare higher and that this is where most of the corrosiontakes place.

We observed no correlation between the location andthe severity of corrosion except for the exposed moun-tain-side location for the Derrick Peak specimens men-tioned above. We did note, however, that foreign sub-stances were more pronounced on meteorites foundamong rock debris in the mountain ranges than on thosefrom the blue ice fields of the Allan Hills. Some speci-mens were visibly affected on the surface, having yellow-ish or whitish incrustations. On such specimens, such asDerrick Peak, Inland Forts, and Purgatory Peak, calcitewas identified in veinlets and vugs, deposited togetherwith lepidocrocite or introduced into cracks of akaga-n6ite. The presence of calcite as crystalline veinlets is fur-ther documentation of the sporadic presence of liquidwater.

CoNcr,usroNs

Akagan6ite, previously regarded as a rare mineral, is inreality a significant corrosion product of the Fe-alloyphases in Antarctic meteorites. Work in progress suggeststhat it is equally important in the corrosion process ofmetal-bearing meteorites in all environments. Akaga-n6ite's particular importance derives from its crystalstructure that readily incorporates Cl ions attracted tothe corrosion front and from its small crystal size thatgreatly enhances its water adsorption capacity. Cl- ionspositioned in the tunnel sites of akagan6ite are readilyreleased under moist conditions to move again to theakagan6ite-Feointerface region, where they serve to de-passivate the Feo and reinitiate corrosion. Invading aka-gan6ite may again incorporate the Cl ions, providing areservoir ofcorrosive agent followingjust behind the re-action front.

Purer forms of akagan6ite typically contain 4-5 wto/oCl, but, as it is often intergrown with Cl-free goethite ona microscopic scale, microprobe analyses often yietdsomewhat lower Cl values (0.34 wto/o). Akagan6ite formsin situ by replacement of metal and contains Ni whenformed from meteorites. The Ni level reflects the com-position of the phase from which it formed, having Fe/Ni ratios of about 16 when formed from kamacite andabout 4 when formed from taenite. On aging, akagan6iteloses Cl and decomposes in situ to intimate intergrowthsof goethite and maghemite that retain the Ni of the aka-gan6ite. On a given meteorite, therefore, akagan6ite willbe located at the actively corroding interface, while goe-thite and maghemite will be found nearer the surface as"dead" oxides. Lepidocrocite is occasionally found ascrusts and fillings, deposited on or in the other oxidesafter deposition from solution.

Cl apparently starts accumulation on meteoritic metalsurfaces upon landing. Over time, very small amounts ofCl are sufficient to destroy large iron bodies, because ofits quasi-catalytic role in the corrosion reaction. Our ob-

BUCHWALD AND CLARKE: CORROSION OF Fe-Ni By AKAGANEITE

666

servations demonstrate that the Antarctic environment issufrciently rich in Cl, be it ultimately from air-borne seaspray or from volcanic action (Antarctica has one activevolcano, Mount Erebus, on Ross Island), to corrode me-teoritic metal. Although the Antarctic ice may well con-tain Na as well as Cl, it is the anion that is both selectivelyattracted to the metal surface and incorporated into theprecipitating akagan6ite. There is no site for the large Nacation in the corrosion products. Precipitating akagan6iteand lepidocrocite may entrap windblown mineral parti-cles from eroding mountain ranges. These particles slow-ly decompose and provide foreign ions that become in-corporated in the various corrosion products.

Considering only bulk composition, all iron meteoriteswould be expected to be equally susceptible to corrosion.Variations in structure and their associated mineralogicalrelationships are, however, responsible for important dif-ferences. The electrochemical potentials established be-tween kamacite, taenite, schreibersite, and troilite ac-count for differing levels of corrosive activity. A meteoriterich in large-angle grain boundaries (IIB, IIIA) or in in-clusions (IA) is more exposed to corrosion attack thanhomogeneous, coarse-grained meteorites (IIA) or heter-ogeneous, but inclusion-poor duplex meteorites (IVB). Thefusion crust on iron meteorites, being largely magnetite,will be noble relative to the bulk meteorite and will en-hance the attack on metal immediately below the fusioncrust. Although time must be a factor in the amount ofcorrosive activity to which a given specimen is exposed,the complexities of individual residence histories do notlead to a recognizable correlation between degree ofweathering and tenestrial age.

The reason for the alarming breakdown of some me-teorites in museum collections is that akagan6ite residesin the interfaces between uncorroded metal and inactiveiron oxides. Moisture provided by ambient humidityslowly penetrates the oxides and reinitiates active corro-sion under what may appear to be a benign crust. Sud-denly, what appeared to be a substantial meteoritic masscrumbles. Protective measures include flushing Cl fromthe crust and reducing oxygen and humidity levels. Thereis no need to resort to the doubtful mineral lawrencite toexplain the corrosion of meteoritic metal. Phenomenapreviously attributed to lawrencite by some authors arereadily explained by the well-characterized mineral aka-gan6ite.

The Antarctic meteorites preserve the initial steps inthe degradation of meteorites. Because the temperaturemost of the time is far below the freezing point and be-cause the air is very dry, the decomposition of akagan6iteto other minerals such as goethite, maghemite, and mag-netite is inhibited. The full decomposition of akagan6itemay, however, be studied on meteorites recovered frommore temperate climates.

The specificity of the Cl accumulation process in metalcorrosion must be considered in studies that involve Clelemental and isotopic abundances. Geochemical rela-

BUCHWALD AND CLARKE: CORROSION OF FC-Ni BY AKAGANEITE

tionships that might be expected under other circum-stances may not hold where corroded metal is involved.

A final conclusion is that the corrosion mechanismproposed here and developed from studies of Antarcticmeteoritic metal has much broader implications for fer-rous metal corrosion. Work in progress convinces us thatthis mechanism is fundamental to meteoritic metal cor-rosion in all environments. Preliminary studies suggestits importance for archeological ferrous metals and forstructural metal under some circumstances.

Acxllowr,nncMENTS

We are indebted to the Meteorite Working Group and to Dr. K. Yanai

for providing specimens. J. E. Post, U B. Marvin, B. Mason, J. L Good-

ing, and E Maahn provided stimulating discussion, and E Olsen a most

helpful review. E. Jarosewich, J. Nelen, and Inger Sondergeard provided

electron-microprobe support; J. E. Post, D. Ross and S. Karup-Moller'X-ray diflraction data; and J Collins, R Johnson, T. Rose, T. Thomas,

F. Walkup, E Johannsen, and M Ketler, technical support. The work

was initiated while V. F. Buchwald was a visiting scientist in the Depart-

ment of Mineral Sciences, National Museum of Natural History, Smith-

sonian Institution, Washington, September 1987 through January 1988'

We are both indebted to the Suzanne Liebers Erickson Fund, SmithsonianInstitution, for partial financial support.

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BUCHWALD AND CLARKE: CORROSION OF Fe-Ni BY AKAGANEITE