American Mineralogist, Volume 74, pages 1054-1058, 1989

Motukoreaite: A commonalteration product in submarinebasalts

Is,tnnl- ZaNrlnnnNo, Fnr-rcrlNo PLANA, ANroNro Ylrzeunz Instituto de InvestigacionesGeologicas "Jaime Almera" (CSIC), Marti i Franqu6ssn. 08028 Barcelona,Spain D.tvro A. Cr-,q.cuB U.S. Geological Survey, Mail Stop 999,345 Middlefield Road, Menlo Park, California 94025, U.S.A.

Ansrnlcr Motukoreaite occursin fractures and amygdulesin hyaloclastitedredged from the west- ern Mediterranean. Motukoreaite apparently forms as a low-temperature alteration prod- uct during seawater-basalticglass interaction. We report microprobe chemical analyses and the complete X-ray powder-diffraction pattern of motukoreaite to enable unambigu- ous identification in future work on submarine alteration of basaltic glasses.Comparison of our X-ray data and analysis of motukoreaite and "unknown minerals" published by severalauthors suggeststhat motukoreaite is a common basaltic alteration mineral having a widespreadoccuffence.

InrnonucrroN closeproximity to the palagonitematrix. Motukoreaite is Altered basaltic sampleshave been recoveredfrom nu- the earliest mineral to form during submarine alteration precipitation merous submarine volcanic areasaround the world. An of these .The complete sequenceis , aluminum-magnesium hydrosulfatemineral has been de- motukoreaite, and . The contacts be- tectedin some of these samples(Alexandersson, 1972; tween these three mineral phasesare sharp and lack any Rad, 1974; Bernoulliet al., 1978).A sulfatemineral iden- gradational zone. Some "floating" motukoreaite crystals tified as pickeringite has been reported by Robinson and suggesta time lag between their crystallization and the phillipsite petro- Flower (1977).In only a few caseshave similar minerals formation of and calcite. There is no been identified as motukoreaite(Rius and Plana, 1986; graphic evidenceof alteration of the secondaryminerals. Rodgerset al., 1977;' Brindley, 1979;Alker et al., l98l; X-ray analysis Ramanaidouand Noack, 1987).This work providesnew microprobe chemical analysesand the complete X-ray The motukoreaite usedfor X-ray analysiswas obtained powder-diffraction pattern of motukoreaite and empha- by hand-picking microcrystalline radiating fibers from a sizesthat it appearsto be a common alteration product thick rim and macrocrystalsfilling large amygdalesin the of submarine . basaltic hyaloclastite from the Emile Baudot Bank. The X-ray spectrum was collected using a sIEMENSp-soo dif- Rrsur,rs AND DrscussroN Occurrenceand morphology In the summer of 1980, severalolivine basalt hyalo- clastite sampleswere dredged from Emile Baudot Bank in the westernMediterranean Sea (Zamarreflo et al., 1985). Emile Baudot Bank is a Miocene seamount located be- tween CabreraCanyon and the steep,fault-defined Emile Baudot Escarpment(Fig. l). This bank has been associ- ated with the Miocene extensionaldisplacement that pro- duced both the dislocation of the Balearic margin into separateblocks and the present physiography and sub- bottom configurationof this margin (Mauffret, 1979). Motukoreaite occurs as radiating, fibrous, crystalline rims (60 pm thick) coating veinlets or as larger platelike hexagonalcrystals (150 x 15 pm) filling amygdales(Fig. 2). The macrocrystalline variety is also typical of open- Fig. l. Bathymetric chart of Emile Baudot Bank. The star cavity growth, and the crystal size increasestoward the symbol marks the location of the dredgesample containing mo- center of the void. Motukoreaite is a very soft and col- tukoreaite.The bank is characterizedby steepslopes with a com- orlessmineral with high negative relief. It is uniaxial (+) plex surficial micromorphology and is devoid of sedimentary and hasbirefringence of about 0.012.It is paleyellow, in deposits. 0003-o04x/89/0910-1 054$02.00 1054 ZATT,TENNNfrIOET AL.: MOTUKOREAITEALTERATION IN SUBMARINEBASALTS l 055

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Fig. 2. (a) Sketch of a vesicle filled by motukoreaite generalizedfrom a scanning electron micrograph. The crystallization sequenceis thin fibrous fringe (1), followed by macrocrystallineradiating bundles (2), and macrocrystallinebooklike packagesfilling the remainder of vesicle space(3). (b) Enlargedview framed in (a) showing well-developedhexagonal motukoreaite crystalstypical of open-cavity growth. I056 ZAMARREfrTOET AL.: MOTUKOREAITEALTERATION IN SUBMARINEBASALTS

TABLE1. Comoarisonof thecell dimensions of motukoreaite

Reference a: b This work 9.172(21A: 3 x 3.057A 33.51(1)A:3 x 11.170A Rodgerset al (1977) 9.336A:3 x 3.112A 44.72A:4x 11.18A Rad (1974) 9.36A:3 x 3.12A 34.0A:3x 11.33A Brindley(1979) 3.062A: 1 x 3.062A 33.51A:3 x 11.170A Alkeret al.(1981) 11.2164:1x 11.216A Ramanaidouand Noack(1987) 3.065A: 1 x 3.065A 33.47A:3x 11.157A lractometerwith CuKa, : 1.5405A radiation, graphite as an internal standard. The pattern quality was im- monochromator, and scintillation counter. The X-ray proved by very gently grinding the sample. The diffrac- powder pattern obtained was scannedfrom 4' to 70 20 togram was indexed using an hexagonalcell according to with 40 kV and 20 mA at l0 s per 0.05'steps,with the structuraldata ofRius and Plana (1986).

TABLE2. X-ray powder-diffractondata for motukoreaiteindexed according to the hexagonalspace group R3m

Ramanaidou Alker et al. and This work Rodgerset al. (1977) Brindley(1979) (1981) Rad (1974) Noack(1987) d.* (A) d"","(A) (hkt) ilh d*" (A) t (hkt) d* (A) /o* ?kt) 4* (A) t d*(A) / d"JA) /

11 .15 11.170 003 87 11.32 vvs 0004 11.26 10 003 11.22 100 11.5 100 11.10 8.48 tz 7.57 15 7.61 vw 1.012 7.61 0.5 /.5Y 6.41 5 o.J4 vw 1.0.1.4 5.58 5.585 006 59 5.58 s 0.008 5.59 4 006 5 609 45 J.CO OC 5.57 mw s.22 5.230 111 9 5.01 5.049 112 7 4.57 4.544 021 I 4.59 m 1.0.i.8 4.58 3 4.59 ms 4.24 4.244 107 2 4.24 w 1.0.1.9 4.22 1? 3.88 3.842 116 10 3.85 VW 3.72 3.723 009 100 3.72 vvs 0.0.0.12 3.72 7 009 3.786 80 3.77 70 3.73 s 3.51 3.467 12O 4 J.JC mw 2.0.2.6 3.40 3.395 122 4 3.40 S 3.044 031 , qqA [ 1.r 3.03 vw 2.1.2.2 2.98 s I 2.e4s 033 2.951 2.948 303 5 2 958 w 1 0.i.14 2785 2.792 0.0.12 8 2.784 w 0.0.0.16 S 2694 2.682 306 6 2.69 m ( -'-'t A?1 ' 101 z.ot , A4A J 128 1 2.650 mw 3.0.3.3 2.646 1 [ 2.615 222 m 2.571 2.576 223 9 2.578 s 3.0.3.5 2.576 4 103 2.57 2526 2.536 113 3 2.466 2.470 1 1.12 2 2.49 MS 2.444 2.434 134 1 2.391 2.392 226 8 2.386 mw 1.0.1.18 2.392 3 106 2.37 ms 2.345 2.362 309 I 2.34 vvw 2.2.4.1 2.34 ms 2.274 2.272 042 4 2.268 vw 2.2.4.5 2.272 0.3 2.28 2.235 2234 0.0.15 4 2.235 vw 0.0.0.20 2.25 2.159 2.158 229 6 2.158 mw 1.0.1-.20 2j60 2 109 2.16 [ 2.11 2 065 2.061 0.3.12 4 2.029 w 0.0.0.22 ] o.oo 1.972 1.970 143 3 1.983 w 1.98 1.922 1.921 2.2.12 I 1.921 ms 1.924 2.5 1.0.12 1.93 mw 1.859 1.861 0.0.18 4 1.870 vvw 1.88 1.767 1.765 330 2 1.763 vvw 1.725 1.731 241 3 1.74 1.709 1.707 2.2.15 5 1.709 m 1.710 t r.o.rs 1.72 1 597 1.596 0.0.21 1 VVVW 1.s28 060 4 1.528 m - 1.529 1 110 1.52 ms t'czz | f l.szz 2.2.18 1.515 1.51 5 603 1 1.513 m - 1.516 1 113 1.475 1.474 066 1 1.477 m - 1.476 0.5 116 I r.soz 2.2.21 2 1 367 vw roo/ l.^^^ I r.Jo\r r.c.ro Nofe.'vvs:veryverystrong,s:strong,ms:mediumstrong,m:medium,mw:mediumweak,w:weak,vw:veryweak,vvw:veryvery weak, vvvw : very very very weak. ZAMARREfrIOET AL.: MOTUKOREAITEALTERATION IN SUBMARINEBASALTS I057

Trer-e3. Severalanalyses of thechemical composition of motukoreaitein percentages

Na.O 85 0.71 1.10 1.83 3.41 J.50 Mgo 28 30 22.98 31.43 27.2 26.46 24.26 26.34 27.7 27.8 Alro3 19 17.87 21.33 18.63 16.64 17.5 17.5 36 23.13 23.2 't8.2 So. 14 15 1000 13.33 10.5 11.51 11.18 9.17 18.0 sio, J.5J 0.77 3.0 3.68 0.0 0.0 Fe,03 0.73 2.14 FeO 2.88 0.11 0.15 CaO 0.92 0.04 0.42 0.00 0.00 MnO 0.70 0.02 26.34 0.00 0.00 ZnO 0.56 K.o 0.10 0.01 0.13 0.00 0.00 CO, 9.32 DA 0.25 0.25 0.21 Tio, 0.01 0.01 0.00 BaO 0.28 HrO* 19.62 H.O 10.35 Residual 36.1 40.73

Note: (1) and (2) Microprobedata from Rad (1974). (3) Chemicalanalysis from Rodgerset al. (1977).(4) Microprobedata (averageof three analyses) from Robinsonand Flower (1977).(5) Microprobedata from Alker et al. (1981).(6) and (7) Chemicalanalyses (uncorrected and corrected,respectively, for the calculatedformula of motukoreaite)from Riusand Plana(1986). (8) Microprobe data from Ramanaidouand Noack(1987). (9) and (10) Microprobe data from this studv.

The cell dimensions of motukoreaite are a : b : and Flower (1977) may be influenced by smectite that 9.172(2):3 x 3.057L,, -- 33.51(l): 3 x ll.l70 A, lines vesicles. and these valueswere used to index the X-ray spectrum. The genesisof motukoreaite as a basaltic submarine Theseparameters are different from the values presented alteration product is poorly understood.Recently, the al- by several authors who assumedother multiples in the teration ofocean-floor basaltsby sea-waterhas been stud- brucite-likemain layer(3.057 A) and the orderedsulfate- ied in detail in the Deep SeaDrilling Project. The results and cation-bearinginterlayer (l1.170 A;. tabte I shows reported indicate that complementary studiesare needed the different cell dimensions used by various authors. to fully document this process,particularly for the con- Table 2 shows our X-ray data indexed according to ditions under which it operates:water-rock ratio, pore- spacegroup R3m, and these data are compared with the water composition, and temperature. In an extensivere- X-ray data on this mineral from other works (Rodgerset view on low-temperature alteration of oceanic basalts, a1.,7977;Brindley, 1979;Ramanaidou and Noack, 1987; Honnorez(1981) reported that'basaltic glass alters differ- and the "unknown mineral" in Rad, 1974).Thepresence ently and at different rates than crystalline basalts. He of severallines (8.48, 7.57, and,6.4t Al in the X-ray concluded that the initial stageofbasaltic glassalteration spectrum that are not consistentwith the ideal structure is characterizedby hydration and oxidation of the bulk suggeststhe partial transformation of motukoreaite, glass.Chamley and Millot (1972) suggestedthat Al is probably due to partial dehydration during the grinding releasedduring volcanic glass hydrolysis, a process ex- processin a low-humidity atmosphere.Similar basal lines perimentally confirmed by Furnes (I97 5). Therefore, the (7.61 and 6.54 A) were reported by Brindleq 0979). Al needed for motukoreaite formation may be derived Analogous grinding effectshave also been describedin a from the hydrolysis of the basaltic glass.Sea-water pro- 1l-A structureby Rius and Allmann (1984). vides SO. and Mg, although Mg may also be derived from the alteration of the glass.Therefore, the chemical com- Chemical considerations position of basaltic glassmay exert a strong effect on the Microprobe analysis of our samples (Table 3) shows composition and distribution of the secondaryminerals, SO, content considerably higher than that reported by and mineral phasesrich in the less-mobileelements, such Rodgerset al. (197'l) and Rius and Plana (1986). The as Al can precipitate. small amount of PrO, probably substitutes at the SOo- Similar occurrencesof motukoreaite have been report- CO, site. Presumably this sample has lower CO3 to bal- ed in the literature, although in certain casesthey have ance the high SOo in that site. Dehydration (Brindley, not been recognizedas motukoreaite.Rad (197Q de- 1979;Rius and Plana,1986) and contaminationby other scribeda very common "mineral x," infilling mainly ves- phases(Brindley, 1979) can explain the variations in the icles, voids, and veinlets in basalt and forming a rim l- minor-element values shown in Table 3. Differencesbe- 5 mm thick of radiating fibers that surrounds the pala- tween our chemical data and those reported by Rodgers gonitic clasts in volcanic rocks from the Great Meteor et al. (1977) are due to the impurities of quartz, calcite, and JosephineSeamounts (eastern North Atlantic). This and goethitepresent in their sample.Data from Robinson "mineral x" has the same chemical composition as mo- 1058 ZAMARREfrTOET AL.: MOTUKOREAITEALTERATION IN SUBMARINEBASALTS tukoreaite(Rad, 1974,p. 40). Bernoulli and his cowork- Alker, A., Colob, P., Postl, W., and Waltinger, H. (1981) Hydrotalkit, ers (1978) described an "unidentified material" with Nordstrandit und Motukoreait vom StradnerKogel, siidlich Gleichen- berg, Steiermark.Mitteilungen der Abteilung fiir Mineralogie des Lan- abundant Mg, Al, Si, and S in basaltic brecciascored at desmuseumJoanneum, 49, 1-13 (in German) Hole 373A (Tyrrhenian Basin). It occurs as rosetteswith Bernoulli, D., Garrison, R E., and McKenzie, J. (1978) Petrology,isotope a platy or fibrous habit and forms cement rims around geochemistry,and origin of dolomite and limestone associatedwith basalticclasts, particularly in the most palagonitizedones. basaltic breccia, Hole 3734, Tyrrhenian Basin. Initial Reports of the Petrographicfabric, microprobe analysis,and X-ray data DeepSea Drilling Project,42,541-558. Brindley, G.W. (1979) Motukoreaite-Additional data and comparison suggestthat this "unknown mineral" is motukoreaite. with related minerals. Mineralogical Magazine, 43, 389-390. Motukoreaite mostly occursas cement in hyaloclastiteor Chamley, H., and Millot, G. (1972) Neoformation de i volcanic breccias(Alexandersson, 1972; Rodgerset al., partir de diatom6eset de cendresdans les sedimentsmarins. Academie 1977; AIker et al., I 98I Ramanaidouand Noack, 1987). des Sciences,Comptes Rendus Hebdomadairesdes Seances,Serie D, , 134 (in French). The magnesium-aluminum SciencesNaturelles, 274(8\, ll32-l sulfate reported by Robin- Furnes,H. (l 975) Experimentalpalagonitization ofbasaltic glassesofvar- sonand Flower (1977)occurs as vesicle fillings lined with ied composition. Contributions to Mineralogy and Petrology,50, 105- smectlte. I 13. Honnorez, J. (1981) The aging ofthe oceanic crust at low temperature. CoNcr-usroNs In E Emiliani, Ed., The oceaniclithosphere, p. 525-58?. Wiley, New York. present The study allows us to conclude that motuko- Mauffret, A. (1979) Etude g6odinarniquede la marge des iles Baleares. reaite can be a common mineral, formed during subma- M6moires de la Soci6t6G6ologique de France, 132, l-90 (in French). rine basaltic glass alteration, and it can be accurately Rad, U. (1974) Great Meteor and JosephineSeamounts (eastern North identified by using the complete X-ray diffraction pattern Atlantlc): Composition and origin of bioclastic sands,carbonate and pyroclastic I l-6 I provided in this article. The alteration to form motuko- rocks. "Meteor" Forschungergebnisse,9, Ramanaidou,E., and Noack, Y. (1987) Palagonitesofthe :A reaite must have taken place at relatively low tempera- new occurrenceofhydroxysulphate. Mineralogical Magazine,51, 139- ture conditions as the mineral assemblagedoes not in- 143. clude any minerals that form at temperatures near or Rius, J., and Alknann, R. (1984) The superstructureofthe double layer fiir Kristallographie, I 68, 133-144. higherthan 150 "C. We encouragefuture studiesto look mineral wermlandite Zeitschrift Rius, J, and Plana, F. (1986) Contribution to the superstructureresolu- for motukoreaite as a low-temperature alteration product tion of the double layer mineral motukoreaite. Neues Jahrbuch {iiLr of basalt in the oceansof the world. Mineralogie, 6, 263-272. Robinson, P.T., and Flower, M.J. (1977) Low temperaturealteration of AcxNowr,socMENTS oceanicbasalts, DSDP Leg 37. Initial Reports ofthe Deep SeaDrilling Project,37,775-793 We thank J Bischoff(U.S. Geological Survey, Menlo Park) for stimu- Rodgers,KA., Chisholm, J.E., Davies, RJ., and Nelson, C.S. (1977) lating suggestionsand J. Rius (Instituto de Cienciasde Materiales,CSIC, Motukoreaite, a new hydrated carbonate sulphate, and hydroxide of Barcelona)for previous manuscript revision. We are also indebted to M. Mg and Al from Auckland, New Zealand Mineralogical Magazine,41, Marsal (U.P.C.) for sEu technical assistanceThis researchwas supported 337-340. by the United States-SpainJoint Committee for Scientific and Techno- Zamareflo, I., Plana, F, and Vazquez,A (1985) Motukoreaita, filipsita logical Cooperation y calcita: Una secuenciade alteraci6nsubmarina de basaltosen el mar- gen Acta Geol6gicaHispanica, 20, 8l-93 (in Spanish). RrrnnnNcBs crlno Sur-Balear Alexandersson,T. (1972) The sedimentaryxenoliths from Surtsey:Tur- MeNuscrrpr REcETvEDJ,cNUARv 9, 1989 bidites indicating shelf growth SurtseyProgram Repons, 6, I 0 1- 1I 6. Mexuscnrvr ACcEPTEDMev 8. 1989