33. TRANSFORMATION OF OPALINE SILICA IN SEDIMENTSFROM BAY OF BISCAY AND ROCKALL BANK

Hideo Kagami, Ocean Research Institute, University of Tokyo

ABSTRACT

X-ray powder diffraction analyses of sponge spicules and radiolarianstaken from cores drilled at the Bay of Biscay and Rockall Bank in thenortheast Atlantic show marked changes in silica minerals. In Hole400A, opal-A exists in the Miocene-Oligocene sequence, opal-A coexistswith clinoptilolite which shows pseudomorph from biogenous shells inthe Eocene-Paleocene sequence, and opal-CT coexists with theclinoptilolite in the Albian sequence. Formation of the clinoptilolite maybe controlled by concentration of aluminum and alkali ions which issupported by silicate reactions observed in the interstitial water of thesite.

The mode of occurrence indicates that opal-CT alone occurs whencalcium carbonate in sediments is over 80 per cent. The marked depletionof alkalinity in Site 406 and the scarcity of aluminum and metallic cationsin interstitial solutions seems to favor precipitation of opal-CT.

The origin of the bedded cherts in the Miocene-Oligocene and Eocenesequences at Hole 400A may be explained by the sedimentary processessuch as turbidity currents and winnowing currents, for example, contourcurrents. A nodular type porcelanite at Site 406 may be formed by thedeposition of silica as colloidal state which have been dissolved from thenearby diatomite layer.

INTRODUCTION

Silica accumulates in observable quantities in marinesediments as planktonic and benthic debris in thestratigraphic sequence of high primary production.Diagenetic transformation of the natural hydrous silica leadsto classifications as follows; (1) amorphous silica (opal-A),(2) disordered α-cristobalite (opal-CT), and (3) quartz(Jones and Segnit, 1971). The whole reaction may beformed by a dissolution-redeposition mechanism, althougha solid-solid inversion was claimed for the latter reaction(Ernst and Calvert, 1969). Stein and Kirkpatrick (1976)have suggested a model involving nucleation and growth ofquartz by a dissolution-reprecipitation mechanism.

The source of the silica in the deep-sea cherts isconsidered to be biogenous opal and volcanic debris(Calvert, 1971). Heath and Moberly (1971) observed thatthe cherts contain very little biogenous opal, though thesurrounding sediments contain corroded siliceous tests.They inferred that the silica has come from the biogenousopal migrating within the sediments over distances of theorder of a few centimeters to meters. The solution of silicafrom biogenous shells or glass shards depends on their size.Preliminary examination of the dinoflagellate revealed richassemblages from sediments of middle Miocene to Recentin the Bay of Biscay sites. The majority of the dinoflagellatecysts have been dissolved out of cores older than middle

Miocene although other larger siliceous organisms are richand diverse.

The deep-sea silica minerals which are mainly opal-CTrange from Pliocene to Late Cretaceous in age, while thoseof quartz are generally, but not invariably older (Calvert,1974). As for the process and factors responsible for theformation of chert, two opposing theories have beenconsidered: (1) one is the diagenetic theory stating that thetransformation from opal-CT to quartz is mainly due to atime-dependent (aging) process (Heath and Moberly, 1971;von Rad and Rösch, 1974), and (2) the other is the quartzprecipitation theory suggesting the compositions of the hostsediments control the formation of cherts (Lancelot, 1973).Lancelot described a relationship between the mineralogy ofthe cherts and composition of the host sediments, observedthat porcelanites in clayey sediments were composed ofdisordered α-cristobalite, while cherts occurring incarbonates consisted predominantly of quartz, andconcluded that quartz precipitated directly in the carbonatematrix. Kanster et al. (1977) noted that quartz couldcrystallize from solution only when the silica concentrationis below the inferred equilibrium solubility of opal-CT, andthat in most deep-sea sediments with high concentrations ofsiliceous skeletal remains, the diagenetic sequence opal-A

opal-CT —> quartz would predominate.

The objective of this paper is to contribute to clarificationof the transformation of opaline silica by a coarse fraction

757

H. KAGAMI

study and diagenetic interpretation of the siliceousorganisms.

SEDIMENTARY PROCESSES OF THE SILICEOUSCHALK/PORCELANITE LAYERS AT HOLE 400A

The siliceous chalks observed in the Miocene/Oligocenesequence are thin, gritty (silty) interlayers within nannofos-sil chalk. They are often 1 to 3 cm thick with sharp lowerboundary and relatively gradational upper boundary. Theyare light bluish gray to pinkish gray in color and are gener-ally coarser toward the bottom. Parallel, lenticular, andcross laminae are rarely observed. The gritty layers consistof approximately 25 to 40 per cent sponge spicules andradiolarians on the basis of smear-slide observation. Theother constituents are 15 to 30 per cent detrital minerals and35 to 60 per cent CaCOβ. The layers observed in theMiocene-Oligocene sequence such as Cores 43 and 44 showvery sharp lower boundary and upward grading from coarseto finer particles with laminations. The sharp boundary issuggestive of slight channeling of the underlying sediments.Sometimes the layers are convoluted into recumbent foldsas a result of local slumping. As far as composition of thefraction coarser than 44 µm is concerned, the gritty layers inthe Miocene/Oligocene sequence are relatively rich in detri-tal quartz and calcareous benthic foraminifers (Figure 1).Siliceous organisms are dominantly sponge spicules.

On the other hand, porcelanite layers in the Eocene sequ-ence are composed of clean silt grains having parallel andcross laminations without obvious grading. Compositions ofthe coarse fraction in the Eocene sequence are poor inforaminifers, detrital quartz, and feldspar, but concentrationof siliceous organisms especially of radiolarians is observed(Figure 1). They are apparently associated almost entirelywith pelagic sedimentation, and winnowed by a current ac-tion during or after the deposition. Various depositionalstructures have been observed in the chert layers and thedepositional processes to concentrate siliceous organismshave been proposed (McBride and Thompson, 1970; ImotoandFukutomi, 1975).

To estimate frequency of the siliceous beds, the numberof the siliceous chalks and porcelanite layers within aknown length of the cores has been counted (Table 1). Twodominant positions of the siliceous enrichment through thesequences are observed; one is present around Cores 43 and44 where remarkable slumping occurs. The appearance ratiois 2 to 4 layers per meter for the cores. The other is presentaround Cores 51 and 52 with the appearance ratio of 2 layersper meter. Some of the layers in this group are visiblyrecrystallized to be porcelanite. The appearance ratio is plot-ted on the fourth column in Figure 1. These siliceous en-richment groups differ in mode of occurrence, sedimentarystructures, and compositions mentioned already, whichsuggests that they are formed by different sedimentary pro-cesses. The evidence indicates that the Miocene/Oligocenesiliceous layers may have been formed by a turbidity cur-rent, and the Eocene porcelanite by a contour current.

MINERAL ASSEMBLAGES IN THE BULK, COARSEFRACTIONS, AND SEPARATED SAMPLES

Form, texture, and optical properties of the coarsefractions were studied under the microscope, and some of

the results are illustrated in Figures 1 and 2. The use of anX-ray diffractometer was effective for detailed investigationof microcrystalline or cryptocrystalline minerals such aszeolite and opaline silica.

Bulk samples of siliceous chalk and porcelanite fromwhich the coarse fractions were washed in a sieve coarserthan 44 µm, and siliceous organisms separated byhand-picking with the aid of a binocular were used fortaking X-ray powder patterns. Heat treatment in the ovenhas been performed for identification of zeolite.

The bulk analysis of the siliceous chalk/porcelanite inHole 400A is shown in Table 2. A difference in authigenicminerals occurs between the Oligocene and Eocenesequences. Siliceous chalks in the Oligocene are composedof opal-Ar while porcelanites in the Eocene are ofclinoptilolite and opal-CT. Quartz is observed as a majorconstituent. Because the quartz comprises the major part ofthe coarse fraction (> 44 µm) as shown in Figure 1 andTable 3, it is considered the detrital mineral.

The bulk analysis of chalk/diatomite from Site 406indicates the same transformation from opal-A in the middleto early Miocene to opal-CT without clinoptilolite in the lateEocene (Table 2). Diatomites in Samples 38,CC and 39,CCare composed of opal-A and the surface appearance isnearly fresh as indicated in column 6 of Figure 2, whileopal-CT occurs in Samples 37,CC and 40,CC at theperipheral part of this concentrated diatomite. This is anodular-type porcelanite. Quartz is identified in associationwith opal-CT. It is found in the coarse fraction analysis(Figure 2) and is considered to be detrital in part. Samples37,CC and 40,CC contain over 80 per cent CaCOs. Fromthe stratigraphic sequence, opal-CT may have been derivedfrom the dissolution of the diatomite.

The coarse fractions sieved at 44 µm were examined(Table 3). The calcium carbonate content of the bulk sampletaken from Hole 400A is less than 70 per cent. The mineralassemblage of the low-carbonate sediments is characterizedby the successive transformation of silica from opal-A in theMiocene/Oligocene sequence, through clinoptilolite plusopal-A in the Eocene-Paleocene, to clinoptilolite plusopal-CT in the Albian. The absence of opal-CT in theEocene differs from the bulk analysis listed in Table 2. Thisis probably due to the size of opal-CT formed. Scanningelectron microscopy reveals the deep-sea porcelanites arecomposed of silica spheres of 10-15 µm diameter (Weaverand Wise, 1972). It is also evident from Table 3 that thecoarse fractions have the same transformation trend with thedilatory reaction time as that of bulk samples. The differ-ence in reaction time between bulk and coarse fraction sam-ples may be due to size of the siliceous organic fractions, ifthe other condition does not change.

To understand transformation of clinoptilolite, siliceousorganisms of sizes larger than 44 µm were picked up withthe aid of the binocular and examined by X-ray diffracto-meter (Table 4). They are composed mainly of spongespicules in the Miocene/Oligocene and Eocene sequences,and of radiolarians for the Eocene and Albian. Sponge spic-ules in the Miocene/Oligocene are identified to be opal-A,and those in the Eocene are opal-A plus clinoptilolite.Amount and/or crystallinity of the clinoptilolite increasedownward. Radiolarians identified in the Eocene are clinop-

758

OPALINE SILICA

39 40 41 43 44 45 46 47 49 50 51 52 53 54 55 57 58

EARLY MIOCENE

TOTAL FORAMINIFERS

PLANKTONICo-

RADIOLARIANS

QUARTZANDFELDSPAR

TURBIDITY CURRENTSAND SLUMPING

WINNOWING CURRENT ACTION(CONTOUR CURRENTS)

Figure 1. Coarse fraction analysis (>44 µm), porcelanite ratio including siliceous chalks, and sedimentary processes to con-centrate siliceous biogeneous shells.

tilolite only. From their peak intensity, crystallinity in-creases downward. The zeolitized radiolarian spherules areoften reported (von Rad and Rösch, 1974). Compared withsponge spicules in the same core, radiolarians are apt to beeasily transformed into clinoptilolite. This is becausesponge spicules have thicker and more homogeneous tubewalls which persist to dissolve for a longer time (Inoue,1973), and because radiolarians have finer and more fragilestructures. Radiolarians taken from the Albian are com-

posed of opal-CT plus clinoptilolite. The coarse fractionsfrom Samples 65,CC and 66,CC are composed entirely ofclinoptilolite with a good crystallinity, which is indicated bytheir stability after heating 8 hours at 450°C. They seem tobe tabular or platy crystals under the binocular, and mighthave been fractured from pseudomorphs of the originalradiolarian spherules. From the occurrence and siliceousspecies mentioned above, clinoptolite seems to precipitateprior to formation of opal-CT in this environment.

759

H. KAGAMI

Core

3738394041424344454647484950515253545556575859

TABLESiliceous Chalk/Porcelanite

CoreThickness

(cm)

584191260303

6116

745173714415150315568

50975732190340269241201

10104

1Appearance Ratio

in Hole 400A

Number ofPorcelanite

Layer

003410

168901091

19144450000

Ratio(Layer/m) Remarks

001.21.31.602.1 Slumping4.6 Slumping1.300.701.62.01.9 Recrystallized1.9 Recrystallized2.11.21.90000

ORIGIN OF CLINOPTILOLITE AND OPAL-CTA very high alkalinity which could be observed in the

saline lakes is detected in the Bay of Biscay site.Interstitial water chemistry of Hole 400A (see Site

Report, this volume) suggests that Ca++ increases graduallydownwards, being minimal in the first 100 meters to con-centrations greater than that of seawater. This change maycorrespond depletion in HCO3-, Na+, and K+ for reasonsmentioned later. Removal of Na+ and K+ is generally ac-companied by loss of alkalinity, indicating these catons areprobably lost through silicate reactions of the reverseweathering type proposed by MacKenzie and Garrels(1966). Formation of Na-montmorillonite, illite, and theirmixed-layer minerals would be the most suitable silicatereactions.

Consumption of Na+ and K+ also occurs in the formationof clinoptilolite. Here, dissolution of amorphous silica ofthe siliceous organisms may demonstrate the following em-pirical relationship.

Aluminum hydroxyl complex and alkaline ions arecommon in the seawater which may be supplied in theinterstitial water. They are also supplied by clay dissolution.As noted already, clinoptilolite might be formed directlyfrom glass or a gel without a clay precursor when theextremely high concentration of these ions (Surdam andParker, 1972). It is also noted that no clay minerals are

amorphous silica aluminum hydroxyl complex

7 Si(OH)4 + 2 Al(H2O)k (OH)m

+(2Na+, 2K+, Ca2+) + 4 HCO3- ^

(na2,K2Ca) [Al2Si7018] 6H2O + nH20 + 4 CO2 (1)

clinoptilolite

detected on X-ray patterns from coarse fractions associatedwith the Eocene clinoptilolite.

Interstitial water chemistry in Hole 400A indicates thattotal alkaline earths increase downward and exceed that inthe seawater below the Oligocene as shown in Figure 3. Onthe other hand, alkalinity measurements indicate gradualdecrease downward to the level of the seawater at the depthof the Eocene. Since alkalinity balance is affected by all ofthe cations in the interstitial water (Manheim and Sayles,1974), alkaline earth enrichment may be balanced bydepletion of alkali cations such as Na+ and K+. Markedconcentration of alkaline earths in Hole 400A (Figure 3) isbalanced by depletion of alkali cations especially of Na+ inthe interstitial water, which may suggest silicate reactions.The high alkalinity is caused by bicarbonate concentrationby way of sulfate reduction process. Mariner and Surdam(1970) noted that the activity of silica was high in alkalisolutions, but the activity of alumina increased more rapidlywith increase in alkalinity. The Si/Al ratio is the controllingfactor for the zeolite formation and the concentration ofalumina would be favored in the Hole 400A environment.

The marked depletion of the alkaline earths in Site 406especially of Mg++ is obvious in Figure 3. This could beexplained by formation of magnesian calcite in the sedimentwith high carbonate contents, although Mg-silicate authi-genesis cannot be ruled out. This marked depletion of thealkaline earths explain the low alkalinity through the coreand, therefore, any reactions of the reverse weathering typemay be impossible at this site. As a consequence of themarked depletion of alkalinity in Site 406, the formation ofopal-CT would be favored over clinoptilolite.

In order to explain crystallization in Eocene porcelanitesand the amorphous state in Miocene/Oligocene siliceouschalks in Hole 400A, it is worthwhile to consider thesedimentary processes which worked on these layers.Because the Eocene porcelanites are thought to bewinnowed from the original deposition which had relativelyabundant clay (50-70%) by some process of bottomcurrents, for example, contour currents, they might showhigher permeability values than the surrounding clayeyformation. On the other hand, Miocene/Oligocene siliceouschalk layers are deposited as a basal layer by a process ofturbidity currents and are, therefore, likely covered by siltylayers soon after their deposition. This is remarkablydifferent from the Eocene porcelanites which may be keptuncovered for a while by the action of winnowing currents.Since the Eocene siliceous organisms deposited asparticle-by-particle sedimentation and are later winnowed,the time required to attain 1 cm thick might be about 1000years at a minimum (Site Report, this volume). The sedi-mentation rate of the basal part of the Miocene/Oligoceneturbidite would be almost a moment and covered by theupper sequence of the turbidite instantaneously in the geo-logical time. For these reasons, natural hydration mighthave played a great role in the silicification of the Eoceneporcelanite during the initial stages. The high permeabilityof the porcelanite layer could contribute to hydration duringthe subsequent stages. Addition of clay particles to the sili-ceous organisms-seawater system causes the decrease ofthe permeability in the system and no opal-CT crystalliza-tion is detected consequently (Kastner et al., 1977).

760

OPALINE SILICA

CaCO3 BOMB (%)

20 40 60 80

O TOTAL

Δ DIATOM

• RADIOLARIANΔ TOTAL

• BENTHIC

+ PLANKTONI

Figure 2. Coarse fraction analysis (>44 µm), calcium carbonate bomb data, and observation of siliceous organ-ism under the binocular. Samples are from core catchers at Site 406 of the Rockall Bank.

761

H. KAGAMI

Site 406

TABLE 2Bulk Analysis of Siliceous Chalk/Porcelanite Layers

Age

Hole 400A

MiddleOligocene

MiddleOligocene

MiddleEocene

EarlyEocene

Sample(Interval in cm)

44-1-31

44-1-140

49-4-9

53-2-16

Lithology

Siliceouscalcareous chalk

Siliceouscalcareous chalk

Porcelanite

Porcelanite

CaCθ3

(%)

71

71

17

22

X-Ray Data

Major Component Authigenic Minerals

Calcite> Quartz Opal-A

Calcite (Opal-A)a

Calcite, Quartz

Quartz>Calcite ClinoptiloliteOpal-CT

Clinoptilolite(Opal-CT)a

Middle EarlyMiocene

Middle EarlyMiocene

LateEocene

23-2-11625-1-99

37,CC38-5-6139-2-7539-5-7440,CC

Nanno chalkCalcareous

diatomiteCalcareous chalk

DiatomiteDiatomite

Siliceous chalkLimestone

87

388745496384

Calcite

CalciteCalcite> Quartz

CalciteCalciteCalcite

Calcite> Quartz

Opal-AOpal-CT

(Opal-A)a

(Opal-A)a

Opal-AOpal-CT

aDenotes rare occurrence.

TABLE 3X-Ray Powder Diffraction Data of Coarse Fractions (>44 µm)

in Hole 400A

TABLE 4X-Ray Powder Diffraction Data of Siliceous Organisms

Picked up From Samples (>44 µm) in Hole 400A

Age Sample LithologyCaCθ3

X-Ray Data

Non-Authigenic AuthigenicX-Ray Data

EarlyMiocene

MiddleOligocene

EarlyOligocene

EarlyOligocene

MiddleEocene

MiddleEocene

MiddleEocene

EarlyEocene

LatePaleocene

AlbianAlbianAlbianAlbian

Aptian

Aptian

Siliceous marly40,CC nanno chalk

Siliceous marly44,CC nanno chalk

45,CC

46.CC

47,CC

49.CC

51,CC

54,CC

57.CC

63,CC64,CC65.CC66.CC

Nanno chalk

Nanno chalk

Calcareous mudstone

Siliceous mudstoneMarly calcareous

chalkMarly calcareous

chalkMarly calcareous

chalk

Marly nanno chalkCalcareous mudstoneCalcareous mudstoneCalcareous mudstone

71.CC Marly calcareouschalk

74.CC Mudstone

50

60

62

70

23

29

32

43

36

54152219

45

6

Calcite>Quartz

Quartz (Calcite)a

Quartz>Calcite

Quartz

Quartz

Quartz

Quartz (Calcite)a

Quartz>Calcite

Calcite(Quartz)a

(Calcite, Quartz)a

(Calcite, Quartz)a

Calcite

Calcite

(Opal-A)a

Opal-A

(Opal-A)a

ClinoptiloliteOpal-A

Clinoptilolite(Opal-A)a

Clinoptilolite(Opal-A)a

Clinoptilolite

ClinoptiloliteClinoptilolite

(Opal-CT)a

ClinoptiloliteClinoptiloliteClinoptilolite

RhodochrositeClinoptilolite

Pyrite

aDenotes rare occurrence.

The mode of occurrence of the minerals is shown inFigure 4, where the bulk composition of CaCCte is taken torepresent cation concentration in the sediments. It isapparent in this figure that clinoptilolite occurs whencarbonate is less than 60 per cent and opal-CT (plus quartz?)occurs when carbonate is over 80 per cent. According toLancelot (1973), quartzose nodules are restricted tocarbonate environments and opal-CT occurs in clayeysediments. Chemical analysis of opal-CT shows that theycontain an appreciable amount of alkaline and alkaline earthelements (Millot, 1970), which means that if foreign cations

Sample Sponge Spicules Radiolarians

Early MioceneEarly OligoceneMiddle EoceneMiddle EoceneEarly EoceneAlbianAlbianAlbianAlbian

40, CC45, CC49, CC51, CC54, CC63, CC64, CC65, CC66, CC

Opal-AOpal-A

Opal-A (Clinoptilolite)a

Opal-A>Clinoptilolite

ClinoptiloliteClinoptiloliteClinoptilolite

Opal-CT>ClinoptiloliteClinoptiloliteClinoptiloliteClinoptilolite

aDenotes rare occurrence.

are available in appreciable amount, opal-CT can pre-cipitate. The experimental results by Kastner et al. (1977)suggest that magnesium hydroxides are the most importantcomponents responsible for the increased rate of opal-Adiagenesis. This is analogous to aragonite, an unstable formof calcite, which precipitates in the presence of magnesiumions (Health and Moberly, 1971). In carbonates of over 80per cent CaC03, as in Figure 4, both the scarcity ofaluminum and metallic cations favor precipitation of opal-CT only.

Observation of the cores in Site 406 indicated that thediatomites in Samples 38,CC and 39,CC were very freshand composed entirely of opal-A (Table 2). Dissolution ofthe diatomites has not been extensive and no diageneticchanges seem to have occurred. Yet, observation stronglysuggests that dissolution of the diatomites is the mostprobable source of silica and the dissolved silica concentra-tion must have been above the equilibrium solubility of

762

OPALINE SILICA

Δ Mg+ + 0 Total alkaline earths (mmole/l)

Figure 3. Change in concentration of interstitial alkaline-earth ions and alkalinity measurements in Hole 400A of the Bay ofBiscay, and Site 406 of the Rockall Bank. Data are from Inorganic Geochemistory Section of this volume.

opal-CT and precipitated to form opal-CT at the surround-ing places (e.g., Samples 37,CC and 40,CC) where thecarbonate contents are over 80 per cent. Obviously, as al-ready noted by Heath and Moberly (1971), some circulationof silica-rich pore waters is required in the sediments over arange of several meters.

ACKNOWLEDGMENTS

The author gratefully acknowledges the aid and encouragementof the following people: Lucien Montadert, David G. Roberts,Robert W. Thompson, and Gerard A. Auffret. He also has bene-fited greatly from numerous discussions with Noriyuki Nasu andMasao Inoue. He is grateful to Steve E. Calvert and Minoru Utadafor their critical comments on this paper. Financial support hasbeen provided by the Government of Japan through the OceanResearch Institute, University of Tokyo, to assist Japanese IPODactivities.

REFERENCES

Calvert, S.E., 1971. Composition and origin of North Atlanticdeep sea cherts, Contrib. Mineral. Petrol., v. 33, p. 273-288.

, 1974. Deposition and diagenesis of silica in marinesediments,Spec. Publ. Int. Assoc. Sediment, v. 1, p. 273-299.

Ernst, W.G. and Calvert, S.E., 1969. An experimental study ofthe recrystallization of porcelanite and its bearing on the originof some bedded cherts, Am. J. Sci., v. 267-A, p. 114-133.

Heath, G.R. and Moberly, R., 1971. Chert from the westernPacific, Leg 7, Deep Sea Drilling Project. In Winterer, E.L., et

al., Initial Reports of the Deep Sea Drilling Project, v. 7, Pt. 2;Washington (U.S. Government Printing Office), p. 991-1008.

Imoto, N. and Fukutomi, M., 1975. Genesis of bedded cherts inthe Tamba Belt, southwest Japan, Assoc. Geol. Collabor. Ja-pan, Monogr., v. 19, p. 35-42.

Inoue, M., 1973. Crystallization and recrystallization of siliceoussponge spicules in some marine sediments of Japan, /. Geol.Soc. Japan, v. 79, p. 277-286.

Jones, J.B. and Segnit, E.R., 1971. The nature of opal, 1.Nomenclature and constituent phases, /. Geol. Soc. Australia,v. 18, p. 57-68.

Kastner, M., Keene, J.B., and Gieskes, J.M., 1977. Diagenesis ofsiliceous oozes-1. Chemical controls on the rate of opal-CTtransformation — an experimental study, Geochim. Cos-mochim. Acta, v. 41, p. 1041-1059.

Lancelot, Y., 1973. Cherts and silica diagenesis in sediments fromthe central Pacific. In Winterer, E.L., Ewing, J.I., et al., Ini-tial Reports of the Deep Sea Drilling Project, v. 17:Washington (U.S. Government Printing Office), p. 377-405.

MacKenzie, F.T. and Garrels, R.M., 1966. Chemical mass bal-ance between rivers and oceans, Am. J. Sci., v. 264, p. 507-525.

Manheim, F.T. and Sayles, F.L., 1974. Composition and origin ofinterstitial waters of marine sediments, based on deep sea drillcores, The Sea, v. 5, p. 527-568.

Mariner, R.H. and Surdam, R.C., 1970. Alkalinity and formationof zeolites in saline alkaline lakes, Science, v. 70, p. 977-980.

McBride, E.F. and Thomson, A., 1970. The Caballos Novaculite,Marathon region, Texas, Geol. Soc. Am., Spec. Paper, 122.

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H. KAGAMI

100

80

60

5 «.2 OS o

J Λ CO

Eoo

CD

4 0 -

2 0 -

0

Opal-CT

A 406 Bulk sample

I0 400A Bulk sample

X 400A Coarse fraction

I

Clinoptilolite

Opal-CT-Opal-A

X

XX

20 40 60 100 120 14080

Age (m.y.)

Figure 4. Mode of occurrence of silica minerals in Hole 400A of the Bay of Biscay and Site 406 of the Rockall Bank.

Millot, G., 1970. Geology of Clay: New York (Springer-Verlag).Stein, C.L. and Kirkpatrick, R.J., 1976. Experimental porcelanate

recrystallization kinetics: A nucleation and growth model, J.Sediment. Petrol, v. 46, p. 430-435.

Surdam, R.C. and Parker, R.D., 1972. Authigenic aluminosilicateminerals in the tuffaceous rocks of the Green River Formation,Wyoming, Geol. Soc. Am. Bull., v. 83, p. 689-700.

von Rad, U. and Rösch, H., 1974. Petrography and diagenesis ofdeep-sea cherts from the central Atlantic, Spec. Publ. Int. As-soc. Sediment, v. 1, p. 327-347.

Weaver, F.M. and Wise, S.W., Jr., 1972. Ultra morphology ofdeep sea cristobalitic chert, Nature, Phys. Sci., v. 237, p.56-57.

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