Authigenic Zeolites in Zeolitic Palagonite Tuffs on Oahu

THE AMERICAN MINERALOGIST, VOL. 54, JANUARY_FEBRUARY, 1969

AUTHIGENIC ZEOLITES IN ZEOLITIC PALAGONITE TUFFS ON OAHU, HAWAII Azuue Irlrlra, GeologicalInstitute, Uniaersity of Tokyo, AND Kazuo H'ln.Lln, SectionoJ Geology,Chichibu Museum of l{ atural H istory, N agaloro,I{ ogami-machi, Soitamo, JaPan.

Aestnecr

flows. There is a regular succession of the zeolite formation: K zeolite+ca'Na zeolites-Na zeolites. This is explained by gradual change in composition pf percolating groundwater through the tufis deposits during palagonitization. A sodium contribution by wind-blown salt from seawater and rainfall on the oceanic island probably causes the latter occurrence of Na zeolites in even the uppermost zeolitic tufis in the semi-arid coastal area. INrnoluctroN Tephra deposits of the Honolulu Series on Oahu are middle to late Pleistocenein age and range in composition from alkali basalt to melilite

Iijima, 1968a; 196Sb).Thev precipitated in the following general se- quence: phillipsite, gismondine, chabazite,faujasite, gonnardite' natro' lite and analcime. The order is kept when any of these are lacking. Gis- mondine and faujasite are rare; the other zeolitesare common' The nature and origin of the zeolitic palagonite tuffs on oahu is de- scribedand discussedin Hay and Iijima (1968a;1968b)' This paper pro- vides the results of DTA, X-ray, chemical and electron microprobe analysesof the authigenic zeolites. ExprnnarNrer-

optical character was observed under the microscope. Specific gravities were determined by immersion of mineral grains into a mirture of bromoform and acetone. DTA was done on an M. R. K. auto-recording difierential thermal analyzer at a heating rate of l0'c/min. X-ray porvder diffractions were done on a Norelco difiractometer using cu(Ka)Ni radiation. r82 7EjL|IES IN IUFFS 183

Salt LakeCraters ! eari

Tantalus MananaIs. Craters RoundTop KokoCraters

Fro. 1. Locality map of southeastern part of Oahu showing tufts of the Honolulu Series from which zeolites studied were collected.

chemical compositions were obtained by normal wet methods and electron microprobe. Microprobe analyses were made on an A. R. L. electron microprobe analyzet, model EMX, at the Department of Geology and Geophysics, University of California, Berkeley. Si, Ti, Al, Fe, Mg, Ca, Na and K were measured using alkali feldspar and plagioclase as comparison standards. The Ti and Mg in zeolites are negligible or present only in little amounts. voiatilization of lighter elements in zeolites was significantly accelerated by using small beams of high intensity. Therefore, sublimation was kept to a minimum by using a relatively large beam of low intensity at 15 kv, 0.01 mA and 20-second exposure. The count- ings of Na, K, Ca, Al and Si did not change systematically on twenty 2-second exposures at the same spot. The result obtained by 2o-second exposure u,'as identical to that of 50- second exposute. The water content may differ considerably from the theoretical content of H2O, probably due to vaporization of zeolitic water during analysis. In this case, the numbers of atoms in the anhydrous cell unit are probably reasonably accurate although the percentage values are not. The Al reasonably balances the alkalies and alkaline earths. Two specimens of gismondine and gonnardite were analyzed both by wet methods and by the microprobe (Table 2, anal.3,4,5; and anal. 7, 8). The exact comparison of those results are difficult on account of the heterogenity of zeolite crystals. The content of Fe in gismondine is much higher in the wet analysis than in microprobe analysis. The content of ca is significantly lower but that of K is higher than in the probe analysis. The si:Al ratio is considerably higher in the wet methods than in the microprobe. rt is probable that these discrepancies are partly due to mixing small amounts of phillipsite and palagonite in the sample analyzed by wet methods, and partly due to insufficient numbers of probing spots. The content of ca in gonnardite is larger and that of Na is less in the wet analysis than in 184 ASUMA IITIMA AND KAZUO HAR-ADA the probe.The Si:Al ratio is significantly lower in rvet methods than in microprobe analysis. Insufficient numbers of probing spots probably result in these discrepancies. Pttrr,r,rpsrtB Phillipsite was found in almost all zeolitic palagonite tuffs on Oahu. It occurs commonly as a thin crust on pyroclastic grains and as prismatic crystals in vesiclesof palagonitized glass and tholeiite fragments. Small white spherulitic aggregatesare abundant in porous tuffs on the southwest sideof Round Top. Fourling twinned crystalswith a rhombohedral shape are common in extensively altered tuffs in rainy areas of Pali and the summit of Round Top. Plant remains are sometimesreplaced by white aggregatesof very minute grains of philtipsite. Crystals are commonly lessthan 0.1 mm long and rarely as long as 0.5 mm. Refractive indices of phillipsite are related to the types of original host

Tesr,B 1. Rnlnacrrve INlrcns ot Pnrtr-rpsrtr rN Per-acoNrrB Tulls oN O*ru

Type of original rock

Koko Craters alkali basalt |.46f_1.497 1.467-t.499 Salt Lake Craters melilite nephelinite 1.486-1.500 1.488-1 .502 and tholeiite Pali nepheline basalt | 48Vr.497 1.488-1 .504 Punchbowl nephelinite 1.482-1 .503 1. 482-1 .505 Round Top melilite nephelinite 1.48G1.504 1.488-1 . 508

rocks. They increasefrom alkali basalt tuffs of Koko Craters to melilite nephelinite tuffs of Round Top, as given in Table 1. Zonal structure is common,and refractiveindices and birefringenceare alwayslower in the outer shell. Phillipsite replacing plant stems in the Lower Salt Lake Tuff was analyzed by wet methods (Table 2, anal. 1), and the structural formula recalculatedfrom the result is (Caz.ooMgo.roNao.arKo.oa)suo(Fet+o.ooAlu.or Siro.oo)ro.ozOar'15.23Hr0.The unit cell dimensionsare a:9.96, b:14'30, and c:14.30, all +0.01 A. Chemical composition of the phillipsite of Oahu is clearly controlled by the compositions of original host rocks (Fig. 2). Phillipsite deposited in melilite nephelinite tephra of Round Top has higher Ca:Na and lower Si:Al ratios than that of the Salt Lake Tufi which contains abundant altered tholeiite fragments.The Si:Al ratio rangesfrom 1.06 to 1..5.5in phillipsite of Round Top and from 1.56 to 1.93 in phillipsite of the SaIt Lake Tuff. Slightly lower refractive indices of phillipsite in alkali basalt tephra from Koko'Craters suggestthat the phillipsite is either more sodic or more silicic, or both than that of the Salt Lake Tuff' ZEOLITES IN TUFFS 185

Tanr,r 2. CnRutcar,AN.lr,vsns ol AurrrrcnNrc Znot.tro,sow Oanu, Hewlrr vrt.70

1a 3b t1

SiOz 43.95 464 39 6r 41.1 so1 142.19140.344.O 50.8 TiOz 003 Alzoa 22.42 23.8 24 69 296 223 1283612s.6 12s.7 25.7 1237 18.2 Fe:Or 038 o20 228 o76 0 ls | 0.40 I 0.83 1.10 048 Mso 046 0.00 tr. 0.3010.001 - CaO 822 927 122 s.4 | 3.9812.4 1.2 43 NarO 183 19 1 76 2l 5r l12.Ult34 14.3 4.0 K:O 2r7 64 133 1.1 2.1 | 0.001 000 000 HzO+ 15.80 20 91 HrO- 428

Total 86 86 85.4s 1003s I 82.s3| 86.30I 90.88

Numbers of ions in anhydrous cell unit

Si 10.00 9.98 18.40 17.40 r7 27 23 70 5.56 5.88 1.97 25.45 Ti 0.00 0.00 000 AI 6.01 6.03 13 52 14 78 14.83 12.43 4.41 424 4.05 1.03 10.73 Fe3+ 0.06 003 080 0.25 0.24 006 0.04 0.08 0.11 000 018 Mg 0.15 0.00 0.00 o.20 0.00 Ca 2.O0 r.70 4.64 J J) 5.93 227 0.57 036 o.17 004 Na 081 o.79 1.60 t.73 0.87 4.69 3.28 3.65 3.70 0.91 3.88 K 0.63 I /J 080 0.59 0.95 1.28 0.00 000 0.00 0.00 1.08 HrO 15 23 32.32 504 o ?1 64 64 72 20 20 7t si/At 166 166 136 1.16 I .90 1.26 1.45 2.37 S.G. 2.25 228 2.25

s Wet chemical analysis, analyst: Ken ichiro Aoki. Wet chemical 6 analysis, analyst: K. Nagashima and K, Nakao others are electron microprobe analyses, analyst: A. Iijima. 1' Phillipsite replacing plant remaias in lithic palagonite tufi of the Lower Salt Lake Tuff; from the clifi along PuuJoaRoad near the junction to Moanalua Road. 2. Phillipsite in cement of lithic palagonite tufi of the Salt Lake Tuff. 3' Gismondine spherulesin cement of porous palagonite tufi of the Tantalus cratergroup; from a quarry oD the southwest side of Round Top. 4. The outer shell of a gismondine spherule of the same sample as No. 3, 5. The core of the same gismondine spherule as No. 4. 6. Mosaic chabazite in cement of densepalagonite tufi on the south side of Koko Lrater. 7' Gonnardite in veinlet in palagonite tufi of Punchbowl; from the clifi of Stevenson Schoolyard on the east side of Punchbowl. 8. Gonnardite in the same veinlet as No 7. 9. Natrolite growing on t}le same gonnardite as No. g, 10' Aaalcime in vesicles of palagonitized glass in porous analcimic tufi on the south side of Koko Crater 11. Faujasite in cement of lithic palagonite tufi of the Salt Lake Tufi.

Flectron microprobeanalyses were made for threezoned crystals in the prismatic direction, as shown in Figure 3. The si:Al ratio roughry increasesoutward in the direction of growth. The distribution of cations tends to have a reverserelation to the si:Al ratio in the lower par.t,and a parallel relation in the upper part. 186 ASUMA IITIMA AND KAZUO HARADA

Co . in melilrtenePhelinite tufi r in melilitenephelinite tuff containing tholelitef ragments o in alkali basalt tuff + In acidic tutf in saline lakes and soils (HAY,1964; GUDE SHEPPARD,1966) ,,r and t zs- !.99:.!:?! Numbers show the ranges oi ,/j St:Al ratio - """.,' ,{i,ro- t / / t.go o/,,""'-"'-""" .1.56-193 ....j i o,/ ,/

l///Phillipsite//,/ +343/ T +/ / / +l ' A,' 314-344,'

-290 Cho!:ozite Anolcrme

Frc. 2. Ca-Na-K diagram of authigenic zeolites of the Honolulu Series. Silica-rich phillipsite (Hay, 1964) and chabazite (Gude and Sheppard, 1966) are plotted for comParlson.

(1) 0a.Na.K o (2) 08 r0 12 Ca:Na-Ko 08 10 12

a o o o oa

a oa

o a o o (3) E a a E o a L { i o a o o o 1 t{ oa E o E z a o o a o o a o u a o

r60 170 180 190 110 120 130 Si;Alo Si:Al . 5i:Al o

Frc. 3. Change of chemical composition within single crystal of phillipsite by electron microprobe analyses. The length and growing direction are shown on the left side of each figure. (1) rim in vesicle of basalt block in the Salt Lake Tuff' Refer to No' 4 in Table 2' (2) ditto. Refer to No. 5 in Table 2. (3) prismatic crystal in vesicle of palagonite at Round Top. Refer to No. 9 in Table 2. ZEOLITES IN TUFFS 187

GrslroNtrNo Gismondine was collected only from the lower half of S0-foot quarry face in melilite nephelinite tuffs on the southwest side of Round Top. It occurs as a cement of the pyroclasts and in vesiclesof palagonitized glass. Crystals are spherulitic aggregatesless than 0.5 mm in diameter. The core of the spheruleis length-slow,having a:1.500-1.520 and T:1.503- 1.522.The outer two-thirds or three-fourthsis length-fast,having a: 1.52V1.521and 7: I.522-I.525. The two parts are separatedby an optically isotropic zone of about 0.005 mm thick. Optically is the core phillipsite and the outer part is gismondine. However, electron microprobe reveals that there is no regular differencein chemical compositions between them but slightly higher content of potassium in the core (Table 2, .anal.4, 5). Any peaksof phillipsitewere not discriminatedfrom X-ray powder diffraction pattern of the spherules. The DTA curve of gismondine is distinct. It has two endothermic peaks at 125oCand 2I5oC, and an exothermic one at 790'C (Fig. 4). The X-ray powder diffraction pattern of gismondine is identical to previous data (Walker, 1962; Fischer, 1963). The indexing of reflections on the basis of space group P21/c is a new contribution, as shown in Table 3. The unit cell dimensionsare a:1O.02, b:10.63, c:9.83, all +0.02 A; andB:92o42'+15'. The result of wet chemical analysis of hand-picked gismondine spherules is given in Table 2, anal. 3, and is recast in terms of structural formula: (Cas.56Nas.26Ko.ro)o.ar(Fe+so.roAlr.egSiz.ao)a.ogOa' 4.04H2O.

CuesazrrB Chabazite is the next in abundance to phillipsite on Oahu. It is the principal cement of the upper dense palagonite tuffs of Koko Crater. Chabazite occurs as a cement and in veinlets of tuffs and in vesiclesof palagonitized glass and basalt fragments. Crystals are present in clear transparent rhombohedra and mosaic aggregates.They are as much as 0.8 mm long, and many of them are zoned. Refractive indices are always higher in the inner part than on the outer rim. Chabazite grows on phillipsite whenever associatedwith it, but never replacesit. The results of electron microprobe analyses of five samples are illustrated in Figure 2. Chemical composition of the chabaziteis a function of the original compositions of the host rocks. Chabazite in melilite nephelinite tuffs is characterized by abundant Ca and a low Si: Al ratio of 1.48. Moderate Ca and a Si:Al ratio of 1.86 to 1.89 characterizechabazitein the Salt Lake Tuff. Chabazrtein alkali basalt tuffs of Koko Crater has a Si:Al ratio of 1.70 to 1.90 and a moderatecontent of Na. TIgLE 3. X-nav Drprn,qcrlow Poworp Dete ron Grsuoltltrqn

/(obs.) d(obs.) d(calc.) /1obs.) d(ohs.) d(calc.)

56 7.26 (t xs 110 12 2.O32 2 031 T43 I 7 tR.( r10 39 1.981 1 980 333 10 5.96 5.941 111 l.) r.914 (r.sr+ 511 10 5.79 5.77e 111 !1. e13 I t.) 59 4.93 4.916 o02 I 881 I1.884 115 4.67 4.672 021 \ 1.883 502 tt 4.46 4.461 012 l2 1.852 I 1.8s3 Dlz n1 4.25 4 267 T2r l1 RqO 234 (r g.ct 3l +- rl 4.174 2lr 324 nA 4.07 4.054 2tl 1.820 440 (r \r.82r z-cn 22r | 1.821 440 1, l''^"" 3.44 13 .43s 202 Ir. sor tlz \3.428 122 12 1.800 i 1.801 125 .)r 3.40 [s.sos 212 | 1.801 MI l3.382 22r r2 r-lll Ir.zto 215 39 3.32 .t .t.1 I 031 \ r. zzo 843 34 3.28 003 22 1.746 | 748 153 212 15 1.727 {r.zzz 522 3.19 310 \r.727 .).tr 310 12 1.709 1r.zos 225 J.IJ 131 l.1.708 433 \:.rcr 013 r.675 r.67r 352 3.03 Js.oos 811 Ir.u+ 235 I IJ 10 t.642 315 \s.oz+ )1.6M.ir.642 t9 2.968 2.970 222 qt< 12 2.779 023 Ir .o+r 261 132 10 1.615 1.615 6tl 54 2.738 32r 10 1 585 1.585 504 a)tL ir.soo 026 100 2.706 302 10 1.565 360 t1 1 3-60 2.644 2 655 040 10 1 539 361 (z.soo 140 206 2.556 \z.soo 140 1 488 036 [2.s63 041 444 32 2.488 2.488 322 172 l.) 2.417 2.413 322 10 t.+36 453 2.407 2 -405 033 064

22 2.373 z-51+ NJL 551 ( t tlz 640 I z.r4r 2n l-'--" nn 23M 240 t2 | +r3 1.413 640 12.34s I q1i 12.342 4tl I1.411 t2 2.332 Z. JJI 313 [1.411 236 ( t q,ot 046 12 2.281 2.285 142 t-'"'- 12 2.191 2.193 N2 12 I .393 i 1.3e3 435 2.to3 [z.ros JL5 lr.3e2 0t7 12.101 141

(Difiractometer, copper radiation, nickel filter, I : 1.54059 ZEOLITES IN TUFFS 189

10 300 5o0oczoo"c 90ooc

Frc. 4. DTA curves of gismondine and gonnardite. Heating rate l0oc/min.; reference junction 0"c; thermocouple Pt/Pt+137a Rhl reference material Alzo:; in atmosphere. (A) Gismondine from Round Top, Oahu; (B) Gonnardite from pqnchbowl, Oahu; and (C) Gonnardite from Maze, Japan.

The Si:Al ratio within a single crystal of chabazite roughly increases outward in the direction of growth. The Ca:Na*K ratio also roughly increasesoutward (Fig. 5).

GoNNenorrp.qrqo Nernolrrp The low-silica fibrous zeolites constitute the dominant minerals in cement of nephelinite tuffs of Salt Lake Craters, Ulupau Head, Diamond Head, Punchbowl and Manana lsland. They are very sparsein alkali basalt tuffs of Koko Craters. Gonnardite occurs as white fibrous aggregatesas large as 5 mm long in cements and veinlets of the pyroclasts, and in vesiclesof palagonitized 190 ASUMA IITIMA AND KAZUO HARADA

Tmrn 4. AssocrarroNsol AurilcnNrc ZsorrTBstN P.tlecoNItrzBo Tulns oN o,tnU

A. Alkali basalt tufi B. Nephelinite and melilite nephelinite tuff Ph Ph Ph-Ch Ph-Ch Ph-Ch-Go or Na Ph Gi-Ch-Fa Ph-Ch-Na-An Ph-Ch Go-Na Ph-Ch-An Ph-Ch-Go or Na Ph-An Ph-Ch-Fa Go-Na Ph-Go-Na-An Ph-Go An Ph-An

Abbreviation: Ph phillipsite, ch chabazite, Gi gismondine, Fa faujasite, Go gonnardite. Na natrolite, and An analcime. glass and tholeiite blocks. It sometimes replaces plagioclase and plant remains. It grows on phillipsite or chabazite when associatedwith them. A film or crust of natrolite less than 0.1 mm thick usually fringes the gonnardite fibres with a sharp optical contact. Small rods or needlesof natrolite are sparsely found in tuffs of the Koko Crater group, not associatedwith gonnardite. Natrolite replacesphillipsite in a few samples of the tuffs of UIuPau Head. Chemical composition of gonnardite collected from the Stevenson Schoolyard on the east side of Punchbowl was determined by wet methods.The chemicalcomposition falls into the compositionalrange of gonnardite proposedby Meixner et al- (1956) and Foster (1965)' Re- iractive ind,icesare also compatible with the previous data. Elongation is Iength-slow which is inconsistent with generalizationby Deer, Howie and Zussman(1963, vol. 4, p.374).In anotherpaper (Hay and Iijima, in press), the gonnardite is described as "thomsonite" on the basis of elongation character. However, the X-ray difiraction pattern is identical to that of gonnardite (Meixner et al-, 1956; Harada et al', 1967)' The occurrenceof thomsonite on Oahu is probably doubtful' The result of difierential thermal analysis of the gonnardite from punchbowl is slightly different from that reported by Eva and vela (1966).The gonnarditefrom Punchbowl showsthree endothermicpeaks at 70oC,385oC and 450oC,and an exothermicone at 950oC,all of which agreewell with thoseof gonnarditefrom Maze, Niigata, Japan (Harada et at.,1967)(Fig. a). The fibrous zeolites on Oahu form an isomorphous series of solid solution; that is, gonnardite Car.s+-0.soNar.zz-s.osAl(Fe'+)n.rz-o.rzSis. ao-r.soOzo' rHzO ZEOLITES ]N TUFFS

(1) Ca.Na*Ko e) 1.82 0 222.42.62.8 3.0 Ca.Na*Ko O. 1.51.6 1.7 1.8 19 2.0 Oo .o Oo o .o oO Oo o E O. o E O. E oO LO E \f o. .o O \ Oo O oO t{ o o = O. oO o O. oO t- () tJJ Oc oO q Oo oO 180 1.90 2.00 1.80 1.90 2.00 Si:At o Si:Al .

Fro. 5. Change of chemical composition within a single crystal of chabazite by electron microprobe analyses. The dimension and growing direction are shown on the left side of each figure.

(1) Prismatic crystal growing on phillipsite rim in vesicle of basalt block in the Salt Lake Tuff. Refer to No. 3 in Table 5. (2) Rhombohedral crystal growing on (1). Refer to No. 4 in Table 5.

and natrolite

Ca6.17-6.66Naa.ez-+.rsAl(Fe+t) r.rr a ssSiE,gr-o.o+026. rH2O.

Refractiveindices of gonnarditeincrease from 7: 1.504to 1.528with the increaseof Ca and Al. Electron microprobe analysesof single crystals of the fibrous zeolites show that the compositionschange abruptly at the contact of gonnardite with natrolite. The Si:Al ratio in the gonnardite that is not associated with a natrolite rim increasesnear the outer end, whereasthe Ca: Na*K ratio roughly decreasesoutward in the direction of growth (Fig.6). t92 ASAMA IIIIMA AND KAZUO HARADA

(2) Ca Na.K 0123

(l) .o Ca:Na*Ko .O 01234 .o (4) aO (3) Ca Na.K o .o CarNa.Ko 12345 2345 1 ao I I E E ao E E a o o d Ul a a I 120 140 160 120 140 160 140 160 140 Si:Al r Si:Al e Si:Al . Si:Al

Frc. 6. Change of chemical composition within single crystal of gonnardite and natrolite. The length and growing Cirection are shown on the left side of each figure. (1) and (2) Natrolite on gonnardite fibre from Ulupau Head. Refer to Nos. 5, 10 in Table 6. (3) and (4) Gonnardite in the Salt Lake Tufi. Refer to No. 4 in Table 6.

Faulesrrr Faujasite, a rare speciesof zeolite, was recognized in palagonitized tuffs at three localities: the clifi along Puuloa Road near junction to Moanalua Road on the east side of Salt Lake Craterl a roadcut along Puuloa Road at the junction to Peltier Avenue; and a quarry 0.4 mile southwestof Round Top. It occursas small clearoctahedra with rounded edgesin cement of the pyroclasts.The largest crystal is 0.17 mm in diameter. Faujasite also occursas white aggregatesof very minute grains which replaceplant stemsin the Lower Salt Lake Tuff. Refractiveindex of faujasite is 1.471+0.002and isotropic. X-ray diffraction powder data are very similar to those of synthetic C-faujasite (Barrer etal., 1956).The unit celldimension is o: 24.69!0.02 A. f]he result of electron microorobe analvsis is recast in terms of the structural formula:

(Caz.szNaa.ssKr.os)z.zs(Fe+3e.1sAl16.73Siru.nr) tu.tuOtr' rHzO.

AN,q,r-cruB Analcime was found in rather extensively palagonitized tuffs of all localities except Manana Island and the Tantalus Crater group. It is common as a cement of tufis of Koko Craters and Diamond Head. Analcime rarely occurs as clear trapezohedra as large as 0.15 mm ln ZEOLITES IN TUFFS 193

diameter, but is commonly rounded grains lessthan 0.05 mm in diameter. Crystals are isotropic with refractive indices n:1.489-1.494. Analcime characteristically forms a coating on palagonite or other zeolites, but it replaces phillipsite in several samples and gonnardite in at least one sample (Hay and Iijima, 1968b). It rarely replacesplagioclase. Three samplesof analcimefrom Koko Craters were analyzedby microprobe (Table 2, anal. 10). They are of the low-silica variety with the Si:AI ratio o{ 1.91,1.94 and 2.07, averaging1.97. This is consistentwith the Si:Al ratio as determined b)' the (639) d spacing method (Saha, 1961; Coombsand Whetten, 1967).The valuesrange from 1.68 to 2.30, averaging1.98 in 15 samplesfrom Koko Craters.Potassium occurs only in trace amounts.The content of CaO may be as much as 1.14percent. Analcime in nephelinite tuffs may have the lower Si: Al ratio than that in alkali basalt tuffs. The Si:Al ratio as obtained by X-ray result ranges from 1.64 to 2.1I, averaging1.81 in six samples.

Assocr,s.uoNsoF AurHrGENrc ZEoLTTES Among the authigenic zeolites in palagonite tuffs of the Honolulu Series,the following associationswere distinguished and are arranged in order of precipitation (Table 3). Phillipsite containing the most abundant potassium is always the initial zeslite. Chabazite, the next common in potassium, generally deposited after phillipsite. The next is gonnardite, which is followed by natrolite. Analcime is the latest. Gismondine probably precedeschabazite, and faujasite possibly follows it. The general se- quence of precipitation is K zeolites--+Ca.Na zeolites--+Nazeolites (in ultramafic tuff), and K zeolites--+Nazeolites (in mafic tuff). This cor- respondswell with the generalsequence of zeolite formation, K zeolites---+ analcime,in silicictuff in saline-lakedeposits (Hay, 1966).The changeof chemical compositions of these successivezeolites are well illustrated schematicallyon a Ca-Na-K diagram in Figure 7, based on Figure 2. Ca:Na and Si:AI ratios of K zeolitesare strongly affectedby the compositions of original host rocks. Low-silica fibrous zeolites are restricted in nephelinite tuffs, exclusive of very sparseoccurrence in tuffs of Koko Craters. The fibrous zeolitesof Koko Craters are limited to the area along coastal line, suggesting addition of some cations from sea waters. The leachedAl, Si and K and high proportions of the Na and Ca from altering sideromelanewere fixed as zeolites on or near the palagonitized sideromelane(Hay and Iijima, 1968b). Hence chemical compositions of the zeolitesdepend on the original compositions of host rocks. Precipita- tion of zeolites at the surface condition is controlled by the nature of solution such as pH, the aqtivity of silica, the concentrations of alkalies and alkalineearths, etc. (Hegs,t966;I{ay,1966). The regularsequence oI t94 ASUMA IITILTA AND RAZUO HAIIADA

ana, nat Na Fro. 7. Trends of the succession of authigenic zeolite formation in different types of original rocks during alteration of volcanic glass, dranw on the basis of Fig. 2' (Mn)-in meiilite nephelinite tufi, (Mn-T)-in melilite nephelinite tuff containing abundant tholeiite fragments, (Ab)-in alkali basalt tuff, and (A)-in acidic tufi in saline lakes and soils. Ca:Na and Si:AI ratios of earlier-formed K-zeolites are strongly controlled by the types of original rocks. However, the final zeolite is analcime in all the tufis. zeolite formation should be explained by successivecompositional change of percolating groundwater through porous tephra deposits as reacting with the pyroclasts. The concentration of K+ or K/(Ca*Na) ratio would be sufficient to precipitate phillipsite at the initial stage of zeolitization The concentration of Na+ should increasedownward, for an appreciable amounL of ex- cess Na+ is dissolved and migrated with percolating water through porous tuffs. As the K:Ca*Na ratio decreases,chabazite begins to crystallize. Then gonnardite forms under a low-silica environment as long as Ca:Na ratio is favourable.The concentrationof Na+ eventually be- comes to the point to deposit Na zeolites. Analcime is more abundant than natrolite. Natrolite is universally rare in sediments and altered vitric tuffs on account of a kinetic condition (Senderov and Khitarov, 1966).Almost all of the natrolite of Oahu grows on gonnarditeas a film following the fibrous structure. Analcime would be more stableand eas- ier to form than natrolite. The vertical zonation of zeolitic palagonite tuffs of Koko Crater and parts of Salt Lake Craters,Manana Island and Ulupau Head may be explained by the above hypothesis. , ZEOLITES IN TAFFS 195

Na zeolitesoccur in even the uppermost zeolitic tuffs in the southwest part of the Koko Crater group and Diamond Head which are situated in semi-aridcoastal lowland (Fig. 1). Wind-blown salt from seawaterand rainfall on the oceanicisland would contribute sufficient Na+ to increase the Na: K ratio to the point of producing Na zeolitesas the supply of K+ from altering sideromelanediminishes. Relatively high salinity of water due to evaporation would result in zeolite formation even near under the surface. Most of the Oahu phillipsite, chabazite and gonnardite are zoned. The Si:AI ratio increasesoutward in the direction of growth. This phenom- enon is specifically remarkable in phillipsite which grows on pyroclasts. The rate of diffusion of aluminum ions that leach from sideromelane would be more sluggishthan that of silicic ions. The difierencein the rate of diffusion causesa chemical gradient of the Si: Al ratio which decreases outward. Hence the zoned crystals may form. The distribution of cations is complicated by large capacities of cation exchange of zeolites. The change of Ca:Na*K ratio is not always compatible with that of the Si:Al ratio in terms of the type of substitution Si(Na, K)3AICa. This is significant in phillipsite and chabazite which have more open structures than gonnardite and natrolite.

Coupenrsox Wrrn Orupn Zror,rrrs Authigenic phillipsite of Oahu is characteristic in low Si: AI and Na : Ca ratios, as compared with other analysed "sedimentary" phillipsite (Hay, 1964;1966).It has rather very similar chemical compositionsto the phillipsite commonly found in the cavities of mafic Iava flows (Dunham, 1933; Harada et al., 1967). For purposesof comparison,phillipsite in altered silicic tuffs of saline-lakedeposits (Hay,1964) are plotted on the Ca-Na-K diagram of Figure 2. It is remarkable that authigenic phillipsite comprisesnearly the same content of potassium ranging from 17 to 43 percentof the total cations.The Si:Al and Na:Ca ratios are strongly affected by the original compositions of host rocks. Gismondineof Round Top on Oahu is the first occurrenceof authigenic origin. It has similar chemical composition to the low-potash gismondine which occurs in the cavities of basalt lavas in Ireland and Iceland (Walker, 1962). Chabaziteof Oahu is characterizedby much lower Si:Al and Na:Ca ratios than those of authigenic chabazite in altered rhyolitic tuff of the Barstow Formation of Southern California (Gude and Sheppard,1966), as illustrated in Figure 2. It shows the chemical similarity to the chabazite forming in cavities and veins of mafi.clava flows (Majer, 1953;Kihara and Negishi, t967). Chemical compositionsof authigenic gonnardite and natrolite of Oahu ASUMA IIJIMA AND KAZUO HARADA are identical to the gonnardite and natrolite commonly found in the cavities of mafic lavas (Foster, 1965). The Oahu analcimehas a lower Si:AI ratio than most of the analcime in sedimentaryrocks (Coombs and Whetten,1967). Chemicalcomposi- tions of the Oahu analcime are rather similar to the analcime occurring in the cavities of mafic lava flows (Saha, 1959). The low silica and rather high calcium contents may reflect the relatively low silica and high calcium contentsof the originalhost rocks.However, very Iow potashcontent in the analcime of Oahu is different from the igneous analcime which sometimescontains high KzO as much as4.48 percent (Peters et aL.,1967). Authigenic analcime in altered silicic tuffs of saline-lakedeposits always replacespotash zeolites(Hay, 1966).Most of the analcimeof Oahu was directly precipitated as a cement and in vesicles. In conclusion,the authigenic zeolitesof Oahu show chemical similarity to the zeolites commonly found in the cavities of mafic lava flows. How- ever, the sizesof crystals are much less than the latter.

AcrNowmoGlmNts Support by the National Scientific Foundation (Grant GP 3995) of the field work done by A. Iijima is appreciated. The authors are indebted to Professor R. L. Hay of the Uni- versity of California, to Professor S. Iwao of the University of Tokyo for critical reading of the manuscript and helpful suggestions Dr. B. W. Evans and Mr. A. Smith of the Univer- sity of California assisted in electron microprobe analyses. Professor K. Nagashima and K Nakao of the Tokyo University of Education assisted us in providing wet chemical analyses. Thanks are due to Professor T. Sudo of the same University for permitting the use of chemical equipments.

Rnlnr.oNcrs

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Manuscript rueiad,, f une 26, 1968; orcepteilJor publi.cation,October 10, 1968.