Canadion MineralogistVol. 30, pp. 75-81 (1992)

ABSTRACT

The palagonite from the FAMOUS area of theMid-Atlantic Ridge at 36'N, DSDP Site 335, is mainlycomposed of smectite-like particles and their spheroidalprecursors. The crystallinity of the smectitelike particlesis good along the a and D axes, but poor along the c axis.Although they show structural similarities to stevensiteand nontronite, the smectitelike minerals are composi-tionally different from the two minerals. Their spheroidalprecursors have very poor crystallinity, and are rich in Aland Fe. Neither oxide phases of Fe, Mn, and Ti, norzeolites were found in the palagonite. The palagonite isconsidered a composite material that contains di- andtrioctahedral smectite of variable composition andcrystallinity, and their precursors.

Keywords: palagonite, smectite, mineral structure, altera-tion, X-ray diffraction, transmission electron micros-copy, Mid-Atlantic Ridge.

SoMMAIRE

La palagonite du site DSDP 335 de la rdgionFAMOUS, le long de la ride mddio-atlantique (36"N),contient surtout des particules ressemblant ir la smectite,et ses pr6curseurs sph6roides. Le degr6 de cristallinit6 desparticules i aspect smectitique est 6leve le long des axesa et b, mais faible le long de c. Tout en ayant descaractdristiques structurales semblables i la stevensite etla nontronite, la composition des particules i aspectsmectitique diff&re de celle de ces deux min6raux. Lespr6curseurs sphdroides ont un degr€ de cristallinit6 tr0sfaible, et sont enrichis en Al et Fe. Nous n'avons trouvdni oxyde de Fe, Mn et Ti, ni de zdolite dans la palagonite.La palagonite serait un matdriau composite contenant

lPresent address: Oil Sands and Hydrocarbon RecoveryDepartment, Alberta Research Council, P.O. Box 8330,Station F, Edmonton, Alberta T6H 5X2.

THE STRUCTURAL CHARACTERISTICSOF PALAGONITE FROM DSDP SITE 335

ZHIHONG ZHOUI AND WILLIAM S. FYFEDepartment of Geology, University of Western Ontario, London, Ontario N6A 587

KAZUE TAZAKIDepartment of Geologt, Shimane University, Matsue, Shimane 690, Japan

SJIERK J. VAN DER GAASTNetherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

smectites di- et tri-octa6drique de composition et degr6de cristallinit6 variables, ainsi que leurs pr6curseurs.

(Traduit par la Rddaction)

Mots-clds: palagonite, smectite, structure cristalline,alt6ration, diffraction X, microscopie 6lectronique partransmission, ride m6dio-atlantique.

INTRODUCTION

The palagonitization of sideromelane has beenthe subject of many studies since 1846, when vonWaltershausen introduced the term palagonite todescribe altered basaltic glass of hyaloclastites fromPalagonia in the Hyblean Mounts, Sicily. Despitean impressive number of publications on thesubject (e.9. , Byers et ol. I 987, Crovisier et al. 1987 ,Furnes 1978, 1980, Honnorez 1978, 1981,Jakobsson (1978), Jakobsson & Moore 1986,Jercinovic et al. 1990, Staudigel & Hart 1983), thenature of palagonite is still not well defined.Honnorez (1981) suggested that palagonite is amixture, in variable proportions, of altered,hydrated, and oxidized glass with authigenicminerals such as clays and zeolites, and proposedto abandon the term. However, studies by X-raydiffraction of carefully separated palagonite showonly a poorly resolved layer silicate (Andrews 1978,Bonatti 1965, Hay & Iijima 1968a, b, Singer 1974,Stokes l97l). Eggleton & Keller (1982) noted thesimilarity of average palagonite to smectite whenrecalculated to a cation charge of +22, andconcluded lhat palagonite is composed of adioctahedral smectite with significant Mg in thesheet of octahedra. In a previous paper (Zhou &Fyfe 1989), we discussed the chemical compositionand mechanism of formation of the palagonitefrom the Mid-Atlantic Ridge. The present com-

75

76 THE CANADIAN MINERALOGIST

munication focuses on the structural characteristicsof the palagonite.

Sauplrs lNn ExpruusNtaL MEtHops

The samples studied are from the FAMOUSarea, at 36oN of the Mid-Atlantic Ridge, and wererecovered during Leg 37 of the Deep Sea DrillingProject (DSDP) at Site 335. The sampled portionof acoustic basement at Site 335 underlies a 454'mthick sequence of foram-bearing nannofossil oozeand consists of a very uniform sequence of pillowbasalts with numerous glass rinds and intercalationsof nannofossil chalk (Aumento et ol. 1977). A bestestimate of the basement age for this site isapproximately 13 Ma (Miles & Howe 1977). Thesamples were collected from the glass rinds ofpillow basalts (see Zhou & Fyfe 1989). The glass isprimarily dark brown to black sideromelane, witha yellowish orange palagonite rind on surfaces andalong fractures.

In order to characterize the nature of thepalagonite, glass and associated palagonite weremechanically separated and then hand-pickedunder a binocular microscope. Both the glass andpalagonite separates were ultrasonically cleaned inan acetone bath and ground into powder. Suspen-sions of the powders were made, and depositedonto glass slides for X-ray diffraction (XRD) ordeposited onto 3 mm holey carbon grids foranalysis by transmission electron microscopy(TEM). Thin sections with a thickness of about 600A also were prepared for TEM analysis. Theultramicrotomy followed procedures described byLee et al. (1975).

XRD analyses were first carried out with aRigaku X-ray diffractometer system (CuKa radia-tion). A low-angle X-ray powder-diffraction study(CoKo radiation) was further performed onselected samples according to the method describedby van der Gaast et al. (1986). The glass samplesfor low-angle XRD were milled in driedcyclohexane because dry-milling causes destructionof structures and milling in water results in clayformation (Kiihnel & Van der Gaast 1989). Thepalagonite samples were Ca-exchanged in water.Structural characteristics of palagonite were ob-served using a JEOL JEM 100C TEM, with anaccelerating voltage of 100 kV. High-resolutionTEM studies were carried out with a Philips 400STEM system equipped with EDX detector,operated at 100 kV. Electron-diffraction patternswere recorded wherever possible, but the crystallinestructure was generally destroyed by the high-ener-gy electron beam. From the electron-diffractionpatterns, d values were calculated through calibra-tion with a pulverized gold standard.

RESULTS

An XRD pattern of the parent glass from Site335 shows a hump from 4.95 to 2.43 A, with peakposition at 3.26 A (fie. 1A), The hump is typicalof glass and other amorphous materials, althoughits peak position shifts depending on the composi-tion of the glass. ̂The glass also shows a broadhump at about 12 A with low-angle XRD (Fig. lB).A change in relative humidity shows no shift orincrease in intensity at low angles. The glass showsa smooth surface under TEM, and its electron-dif-fraction pattern shows no diffraction rings or spots.This further confirms its amorphous nature.Therefore, clay minerals or clay mineral precursorsare not present in the parent glass.

An XRD study of palagonile from Site 335shows two strong peaks at 4.53 A 4nd 2.60 A anda moderate-intensity peak at 1.52 A (Fig. 2A). An"amorphous" hump similar to that of the glass canstill be seen. The basal reflection of palagonite isweak and variable from 12 to l5 A, and becomesmore diffuse after treatment with ethylene glycol.With the low-angle XRD method, one can see apeak that shifts with changing humidity (Fie. 2B).The peak is located at 16.2 A at 10090 RH, 15.8

35 31 27 23 19 15

20 Cu Kcr

1 8 1 5 ' t 2 9 6 3 0

2g 00Ko

Frc. l. X-ray-diffraction (XRD) patterns of the glass. A.

Normal XRD, B. Low-angle XRD.

0n!

o< o<o<

E E Er 9 :q

qor<

q

THE STRUCTURAL CHARACTERISTICS OF PALAGONITE 77

?0 Cu Ko

l0f/oHH -

5A6RH - - - - - -

f / o F H . . . . . . . . . .

.s / ,?" l!

MoS2(lnt SEndad)

/r,

1 8 1 5 1 2 9 6 3?0 CoKo

Frc. 2. X-ray-diffraction patterns of palagonite. A.Normal XRD. B. Low-angle XRD.

A at 50Vo RH, and L2.g L at 090 RH. This kindof peak shift with changing RH is normal forsmectite minerals (Van der Gaast el al. 1986).However, the scale of peak shift for the palagoniteis smaller than most of the smectite mineralsreported by van der Gaast et al. (1986). In additionto the shifting peak, there is a low-angle hump at20 A that can best be seen at 090 RH (Fig.2B);this feature points to the presence of curved orfolded layers (see also Van der Gaast et al. 1986).

The TEM study indicates that most of thepalagonite is composed of fibrous to lathlike orstrongly folded layered particles similar to smectiteminerals (Fig. 3A). Lattice image^s of the palagoniteparticles show a d value of l0 A @ig. 3B), whichcorresponds to the low-angle X-ray diffractionmaximum at 12.9 A at ltlo RH. This difference ind value reflects the change in the degree ofdehydration of the material in the two techniques.In low-angle XRD at OVo RH, Ca-exchangedsmectite still keeps one layer of absorbed water(Ktihnel & Van der Gaast 1989), whereas at vacuum

Frc. 3. Transmission electron micrograph (A) and latticeimage (B) of the smectite-like particles in palagonite.

conditions applied in TEM, all interlayer water islost. Electron-diffraction patterns of the palagoniteshow diffryction rings or diffuse spots at 4.55,2.61,and 1.52 A (Fig. 4A), which agrees with the XRDresults very well. This indicates that degree ofdehydration has little or no effect on these d values,and thus these diffraction maxima must belong tothe [001] zone. In fact, the electron-diffractionpattern is consistent with [CIl] diffraction pattern

78 THE CANADIAN MINERALOGIST

,:,:l:

.3320

.06

a

a aFrc. 4. Electron-diffraction pattems of the smectitelike

particles in palagonite. A. Diffuse diffraction-spots.B, Indexed diffraction pattern A.

of smectite minerals and can be indexed as (02) and(ll), (13) and (20), and (06) and (33) (Ftg. 4B).

We also noted that the particles of smectite-likepalagonite usually have well-organized crystallinityalong the a and D axes. Along the c axis, the numberof stacked layers is usually less than 7. Thecrystallinity along c axis of the smectite-likeparticles may be poor in general, as their (000 peakson XRD are weak. A high degree of crystallinity

. .11 .13O .o2a

Frc. 5. Spheroidal particles in palagonite (A) and theirreplacement by randomly oriented short line-struc-tures (arrows) (B).

of palagonite along a and D axes also was observedby Singer (1974).

Associated with the smectite-like particles areclusters of small, spheroidal particles, a fewhundred angstrdms in diameter (Fig. 5A). They arevery poorly crystalline and similar to the "cell-likestructures" described by Banfield & Eggleton(1990). These spheroidal particles may represent theprecursors of smectite-like clays. Figure 5B showsthe progressive replacement of these spheroidalparticles by small, randomly oriented line-struc-tures, which lead to formation of smectite-likeclays.

The chemical composition of palagonite isknown to be variable (Honnorez 1981). Thecompositional changes during palagonitization atSite 335 has been discussed by Zhou & Fyfe (1989).Using a scanning transmission electron microscopeequipped with an EDX detector, we were able toaralyze palagonite particles having different mor-phology. Compositional variations are quite large,

a

THE STRUCTURAL CHARACTERISTICS OF PALAGONITE 79

TABLE I. CIIEMICAL OMFOSIIION OF SBIJCTED PARTICLESIN THE PAI.AGOMTE FROM DSDP SITE 335,

Srmti$[ke fbrdcLs1 2 3 4

' Dffihd bt @irsio elffi@ niouoopy ei& @E-dispqsi@X-E' sp€dr8i Epqed s fl% of qid€ @ ehld@s bssi& z Tdslh@s&Oj.

even within each individual sample (Table l). Themajor oxides present are SiO2, Al2O3, Fe2O3, MgO,TiO2 and K2O. The spherical particles (Table l,analyses 5 and 6) are rich in Al and Fe, whereasthe smectite-like particles (Table l, analyses l-4)are Mg- and Fe-rich silicares.

DIScUSSIoN

The structural characteristics of smectiteJikeminerals described above are similar to those ofstevensite and nontronite. Table 2 compares theXRD data of palagonite and those of stevensite,saponite, and nontronite. The match between thepalagonite and stevensite data is very good. Noronly the positions of the diffraction peaks of thepalagonite but also their relative intensities aresimilar to those of stevensite. Stevensite wasestablished as a member of smectite group by Faust& Murata (1953). This was confirmed in subsequentstudies (Brindley 1955, Faust et al. 1959), althoughthe detailed structural characteristics of the mineralremain to be defined (Brindley & Brown 1980). Thesmaller shift of the 001 peak position of palagonitewith changing RH (as compared with most smectiteminerals) also is consistent with the low cation-ex-

TABLE Z X-RAY-DIFFRACIToI.I DATA FOR STEVENSIIE SAPOMTI.N(I{TRONTTq AND PAI.AGOMTB FROM DSDP SIIE 335

Siereld&' Samolte!

A t A r

change capacity and small swelling ability ofstevensite (Brindley 1955).

However, the chemical composition of thepalagonite from Site 335 is variable and can besignificantly different from that of stevensite (Tablel). The structural formula of stevensite wassuggested by Faust & Murata (1953) to be:[Mgr.rrMn6.62Feo.er3+]SiaOre[OH]2, which differsfrom saponite in lacking essential aluminum.Stevensite found in experimental alteration ofbasaltic glass (Trichet 1969) is a nearly pureMg-silicate hydrate. The Mg-rich smectite-likeparticles in the palagonite invariably containsignificant amount of Fe or Al (or both). Thus,they are compositionally incompatible with stevens-ite. In chemical composition, the smectite-likepalagonite particles are similar to saponite, but th^e060 peak of the p-alagonite has a d of 1,52 Acompared to 1.53 A for saponite (Table 2).

The XRD data could also be accounted bynontronite, although the fit is not as good as withstevensite. However, those nontronite peaks thatare absent in the palagonite are of the 00/ type. Asdiscussed above, the crystallinity of palagonite ispoor along the c axis, and the discrepancy couldresult from this poor crystallinity.

However, nontronite alone cannot account forthe observed chemical composition of thepalagonite either. Its structural formula (Brindley& Brown 1980) is [M*".nHzO][Fd*2j[Si4-*AlxOr0l[OH]2, i.e., it should contain little or noMg. In contrast, the smectitelike particles in thepalagonite usually contain more than l0 wt.9o MgO(Table l, analyses l-4).

We suggest here that the discrepancy is due tothe composite nature of the palagonite. Bothdioctahedral nontronite and trioctahedral stevensite(or saponite) may be present. The close associationof the smectite minerals of variable composition ata submicrometer scale leads to a variable chemicalcomposition that cannot be accounted for by eithermineral alone. The chemical analysis of thepalagonite also is complicated by the sphericalparticles that are closely associated with thesmectite-like particles. In this respect, it should benoted that the area covered by electron beam duringEDX analysis was about 0.05 pm in diameter. Thissize is much greater than that of the smectitelikeparticles in the palagonite. Therefore the chemicalcomposition determined as such may be that of acomposite material. In fact, diffuse diffractionspots and diffraction rings observed for thepalagonite (Fig. a) provide direct evidence that thematerial is both polycrystalline and poorly crystal-line. It is also interesting to note that the basalreflection of the palagonite varies between 12 and15 A, and may be split (Fig. 2A). This againsuggests that the palagonite is composite in nature.

Sph@tdal5 6

47.4 49.9u.8 r9.015.3 19.50.9 3.22,1 7.1r3 0.27.6 I.t

99J 100.0

sio, 5L2 53.0 52.3 532Arp' 18.7 r7.r 5.9FqOr. 15.2 74.O 7S 9.8rfrq 1.6 r.4 l.EMgo 10.3 llJ 31.7 353CaO - 1.8Kp 2.r 2.9 0.9Torsl tm.l lm.l lm.r 100.1

No!lrodte'

A r A

Pdeni!e

I A

? A T l Z O s -14.6 v -

rZ4 lm U.2 l0O 14.6 100 13.0 r2.U w7 A l 0 7 1 6 l 0 _

5.0 m 4.96 4 4.98 10 _4J4 lm 4.57 50 4.53 lm 4.53 vs 4J3 v83J0 50 3.67 & 3.67 m 3.60 e 3.49 s3.0 - 2-g l0 3.01 30 3.Ot e -2.62 N 2J8 m 2.il 50 2.fi w 2.60 w2.X 4 2.n m 2.n l0 2.28 w1.73 & 1.73 2n L72 m rJs ,lJ2 90 1.53 90 1.52 80 1.52 s 1.52 s

'AsrM s 7-357; bAsrM $t3-86: "ASTM $34{42

80 THE CANADIAN MINERALOGIST

Thus the observed structural and compositionalcharacteristics of the palagonite from DSDP Site335 can explained by the microscale association ofdioctahedral nontronite and trioctahedral stevensite(or saponite). This finding contrasts with theobservation of Eggleton & Keller (1982) that theirpalagonite is composed of a dioctahedral smectitewith significant Mg in the octahedral sites.Dioctahedral (nontronile) and trioctahedral(saponite) clays also were suggested to occur in thepalagonite of Contrada Acqua Amara, neuuPalagonia (Crovisier et al, 1987).

The spherical particles are probably the precur-sors of the smectiteJike minerals. Eggleton & Keller(1982) noted the formation of smectite fromspherical particles during palagonitization. Ourobservation indicates that spherical particles arefirst replaced by short-range crystalline domainsthat eventually develop into smectite-like minerals.The presence of spherical particles may account forthe "amorphous" hump seen in XRD pattern ofthe palagonite (Fig. 2A).

It should be noted that oxide phases of Fe, Mnand Ti and zeolites were not found in the palagoniteitself, although they are closely associated with it(Zhou & Fyfe 1989). Therefore, it may beinappropriate to consider this palagonite as amixture of altered glass with clays, oxides andzeolite. Rather, the palagonite from DSDP Site 335can be considered as a composite material thatcontains smectile minerals (of variable compositionand crystallinity) and their precursors.

AcrNowl.gocEMENTS

We are grateful to Dr. P.T. Robinson, Depart-ment of Geology, Dalhousie University, forproviding the DSDP..samples. We thank R.Humphery of the University of Guelph for hisassistance with the STEM analysis. The technicaland editorial comments from referees and editorare appreciated. The financial support of this studycame from an operating grant from the NaturalSciences and Engineering Research Council ofCanada to W.S. Fyfe. "

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THE STRUCTURAL CHARACTERISTICS OF PALAGONITE 8 l

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Received February 6, 1991, revised manuscript ac-cepted August 6, 1991.