, Equatorial and tropioal weathering of reoent basal ts from Cameroon I

allophanes, halloysite, metahallQysite, kaolinite, gibbsite.

by

Oaston SIEFFERMANN and Qeorges MILLOT

Offioe d. la ieoherohe Soien~if1que .~ Teohnique Ou~re-Mer

Institut de O'ologie de l'Universit' de Strasbourg

, ilSTRACT

The fvoL ution ·of allophanes_.~.~.y~unB'so11s in .Camero-on, developedI

'on ~asalts under different I tropioal "oondi tions Wtrf studitcl.

, Under • mean annual rainfall of 10 m. , without 8 dry season, 0.110-

, 'I phanes ~hange into gi~bsite and well-orystallized kaolinite;

',Under a :mean annual rainfall of 3 to 6 m. , with a dry season, 0.110-

,phanes give rise to halloysite and metahalloysite;. .

i ' -_ tUnder a mean annual rainfall of 1,5 m.,with a long dry seadon, the

1-- - .. .. - ...

,allophane l stage is very restrioted and, metahalloysite appears rapidly.

c -

In the two 'olimatio zones wit~ heavy 'r.inhLl the upper horizons

of ~h~. soils with.allophane contain, many diatoms j the silioa of. -.- .

wich plays a role lin th~~~.~formation of olay minerals.

The distrig~t'i~;-~-f- rainfall controls' the rate:- ot o.e6~tura.tion

of soils and the seasonal oonoentration of solutions, whioh are',Oipal-i&o!or~-g~~er~i~g-~~-~i~;;at~ion~----'--.. '-. ----- --, ----.--,,- .. -

__• __._~.. .. _,~_._ .. ... _. ... __ ~_. .__. __~__i

tA8 'two prin-

;

I

J

./;'

j'I .III,,.,

· I

I:I

J· .·!

;

"-: --

Equatorial and tropioal weathering of reoent basal ts from Cameroon I

allophanes, halloysite, metahallQysite, kaolinite, gibbsite.

by

Gaston SIEFFEHMANN and Georges MILLOT

Offioe de la Reoherohe Soientifique et Teohnique Outre-Mer

Institui de Oeologie de l'Universite de Strasbourg

The federal Republio of Cameroon oontains many areas oovered with

voloanio and pyroolastio rooks, 800ria, tUffs and ash of basal tio oompo­

8ition. The genesis of soils and the weathering of these rooks were stu­

died in three different olimatio zones (fig.1).

Zone I - Equatorial olimat_, always humid, on the western flank 01' Mont

Cameroun.

Zone 11 - Equatorial olimate, more. moderate, with a dr,y season, on the

southeastern and eastern part of Mont Cameroun.

Zone III - Tropioal olimate with a well-marked dry season on the Central.

Adamaoua Plateau 01' Cameroun.

Nomenclature; - The term !L!tol?~l!. here designates amorphous silica-alumina

products with a Si02/Al203 l1IoLfcuLar ratio between 0.9 and 1.9. The term

...e~l~!ij! designates the hydrated halloysi te or hydrohalloysi te of 10 1spaoing. The term !!e.!~al.!~!..i...1e designates ihe dehydrated halloysi te. ~ .ot 1.4 1 spaoing (CalLLere ~nci Hinin, "1"3 J Brindley ~t- fi/. , iq,,).

"

I. Zone I. - Andosols developed from reoent baaaltio rooks on the western

part of Mont Cameroun.

The ol~mate is equatorial with a mean annual preoipitation of about

. 10 meters (400") and without & dr,y'season. Xean annual temperature i8

29°C. Dra.1nage 18 gC?'od. . I

.... _..

2.

Six soil profiles were studied varying in thiokness from 30 to

140 om. They are"weakly developed soUs fomed "on voloanio deposits 11

(oxy-hydrandepts), rioh in organio matter, with A(B)C profile. The sum

of exohangeable bases, measured on the olay fraotion, varies between 3

and 10 meq/100g at the bottom and between 1 and 4 meq/100g at the top.

Base exohange capaoity is between 20 and 32 meq/100g. Base saturation

(ratio of the sum of exohangeable bases to base exohange oapaoity) is

low a 5 %. The pH varies ftoom bottom to top between 6.4 ana 5.

The olay fraotion (less than 2Ji) is oharaoterized by a Si02/Al203

moleoular ratio between 1~, 1.3. Themal ailalysis and X-ray di:ffraction

reTeal predominant allophane with a small admixture of gibbsite and a

well orystallized kaolinite. The composition of the amorphous fraction,~ - ,

analysed by the method of S,tgalen (1968) is the following a 5i02, 23 %Al 203, .32 % J iron hydroxides, 21 % J H20, 18 %. The speoifio surfaoe

Taries between 120 ,-", 400 m2/g.

All these soils contain numerous diatoms in their upper part. The

fragment~ of their tests frequently constitute more than 10 %of the < 2P

fraction, and this ooours down to 50 cm in depth (pl.I).

ELectron micro.gYaphs .show fuzzy fLakes wich can be considered..

to be .;aLLophants (PL. Jr)) as .shown or df'.scribed. by many au~ho,s; •.Birrtl ilnd. Fieldes (11)52) i Aomine and. Yoshinaga (-fQS5) ; 5ucio and :'

Takahashi (1'56») Fitldts ~t. ,,1. (1'55, -1CJ") i Yoshinaget and.

Aomint (1962 a.) i Alonso td <11. (1"3) i Chukrov ~t ~L. (-1"3, ~"~,

Giusepp.tti. et,gl. (..",!),. Robt,.tr~on (196.3) i Kirk,,"an, Mitchelland. Mackenzit ('196&); Colmet - Daage ~t.:;1. (.J1CJ'7) i Sit'fFermann,

Yehl and. Mi.llot (1 IH8) . But ont must note in a cLeii.tion the I

pU5ence of well crystdlliz.ed. kaolil"lite (PL. IT, fig.8 .ncl

Pl. m, fig. 9) a n cL gLbb oS i t e. I' Neit heY' t vb lJ l a r 0 r glob ula Y' -

.sha.ped. halloysi.te, nor fibrous ,~allophane were obS~Tvec(.

II. Zone II. - Andosols developed from recent basaltic taoks on the eastern,

and southeastern part of Mont Cameroun.

The climate here is equatorial with a mean annual preoipitation

between 2.5 and 6 meters (90 - 250"). A moderate dry, season ocours

,,

;

three months of the year.! The mean annual temperature is between 21-29°C.

Drainage is good.

"Eleven profiles, ranging from weakly developed soils formed on

voloanio deposi tsll

to "eutropio bro~__tJ0i_~_s ~f ~,I'?pical lands "were studied.

They are generally A(B)C profiles ranging from 20 to 100 cm. in thickness.'-The sum-of exchangeabl-;-b;:~es,measuredon the olay fraotion, is between

2 - 20 mfq-. /100g. at the bottom and between 8 - 30 mt'q.j 100 g.' at the top.

Base exchange capacity l'.ang~.s from 20- 60 meq. /100 g. at the bottom to

40 - 15 m",/~oo g. at the tClp.Bu8 saturation varies between 4 - 20 ~6

at the bottom, and 20 - so % at the top. The pH varies between 6 at the~--- ---

bottom and 5 at the top.

The < 2,. f~a.0tion~_s charac~erized by a Si02/Al203 moleou lir

ratio, between 0.9 '-, 2 at the bottom and between 1.6- 2 at the top .

. The amorphous fraotion analysed by ~hE' .mt'~hod of SE'galen shows, as pre­

viously"a Si02/Al203 molecular ratio close to 1, but with a smaller per­

centage of iron hydroxides than in zone I. The specific surface varies

from 80'- 400 m2/ g. Thermal analysis and X-ray diffraotion reveal a

mixture of allophane, halloysite (10 l),metahalloysite (1.4 1) and

disordered kaolinite with traces of gibbsite at the bottom.

ELectron micrognphs show ~lophanes in the form of either

amoTp" ous fuzzy flakes or exceedingly fine~fibrous and thread-like

partioles (pl.IIl, fig.10) ( Yoshil\ag~ and Aomine (1962 b) i Aomine

and Wada (-tQ'2) j Aomine and. MiYduchi (19'5) i Mt'yauchi and. Aominr(01"'). Jaritz. (~%7) i Wad.a (1CJ&1)j Y05hinag~ et till. (-1'J(,8));

I ','

tubular or gLobular - .shaped. halloysite ( Plo ill. fig 11 ~nd. -12)

(Kino.snita and. Muchi (~CJS"+) i Sud.o and. Taakaha.shi (1'56)) Kur.ba­yashi .and. Tsuchiyc1 (-1Q&O) ,. SieFfermann -and. Mi llot (;fQ6S)); dnd

the outlines of hexagonal flakes.

At the bottom of the profiles allophane is the dominant phase, where­

as at the top halloysite and metahalloysite predominate)whioh indicates

a replaoement of allophane by aalloysite.

"'J

.'

"T.

Ill. Zone Ill. - Soils developed from reoent basaltio rocks on the Central

Adamaoua Plateau of Cameroon.

The olimate is the tropical mountain climate of Adamaoua. Mean

annual preoipitation is between 1.5 and 1.6 meters~with a well-markedI

dry season lasting five months. Mean annual temperature is 23°C and..drainilge is good. These sofls yange from weakly developed soils,. ..formed ~~ voloanio deposition, to humic, weakly-desaturated ferrallitio..soils.

Six soil profiles were atLLdied ranging in thickness from

'10 cm. - 2 m .. The sum at exohangeable bases varies from ~ -15 me,,!100,g.

at the bottom, and from 15 - !~ meq/100g at the top. Base exohange capa­

oi ty is between 15 30 meq/100g at the bottom and between 30 - 50meq/100g at the top. Base saturation is between 50 -- 80 %, and the

pH varies from bottom to top between 7.3 - 6.

The olay fraction is oharacterized by.an 5i02/Ala0,) moleoular ratio

of 2 - .3 for the youngest soils, . Teachi ng 2 for the most advan­

oed ones, and shoft only a small amount of allophane. The speoifio surf­

aoe of the olay fraotion varies from 74 - 150 m2 /g.

Thermal analysis and X-ray d1ffraotion show that the mixture oon-

tains little or no allophane and that it is composed of halloyaite (abun­

dant at the bottom) and of metahalloysite (abundant at the top) (Sitffermann 'and. Mi LLot I 1Q68).

ELectron Tl'licrosraphs .show amorphous masses oorrespondine to iron

oxides, tubular forms of haltoysite or m~tahalloysite and abundant glo~ular­

sh~ped. halloy~Lte. Thue globuLes ~i.theY' h;ve a. dist inct

helical or concentric structure' and are ovoi.d. or pol~ hedralin form. ~n many oases. the w1nding oonsists of reotilinear fragments wi!h I

curved. ends. These elements sometimes ·seem to separate from the glob\)lt~/:

whose or,ys1a1l1ne nature oan be disoerned by eleotron miorodiffraotion.

If disaggrega1ed by ultrasonio energy, they yield the X-ray diffraotion

peaks ot halloysite. :'i

In some profiles the only mineral ooourring at the bottom is halloy­

site, while in all the profiles metahalloysite predominates at the top.

This indioates a replaoement of halloysite by metah~llo¥site. In soils

from older Quatemary results are similar, but gibbsi~e.L' is even

more highly developed at the top of these profiles.

IV. Interpretations.

1} Allophanes. - It is difficul~ to purify allophanes. In particular the

presenoe of diatoms, cO".5titutin8 up to 20% of the olay fraction (Pl.I,

fig. -t , 2,.3 and If) makes the interpretation of the 5i021Al203 moleoular

ratio (0.9 - 1.5) difficult. Moreover the silica of the diatoms in contaot

with ~llophane having a high alumina content, may be inoorporated into"nl/Ly formed. cla~ mineral. (Pl. I', fiS' .3 ~nclll ).

Under an equatorial olimate the genesis of allophaneJ by weathering

of basaltic material, is very important. We think that h~idity and

contin.us leaching favor the preservation of allophane •

.2} ICaolinite and gibbsite in andosols. - In Zone I, wich is equatorlal and.

permilnentl.Y humid, allophanes are always present in the form of amorphous:

fu Z z1 fl a kes, never as imogoli tes (Pi .11). Halloysi te or metahalloysi te !------ ~ - - ~

in the form of tUbes or Rlobules ne'ter ooour. But the olay fraotion of

'these andosols may contain up to 10 %'of very beautiful plates of well-, :I." .. ,. ._-~,...

or,ystallized kaolinite (Pl.III, fig.9) and up to 20 % of gibbsite, .-- ---- I

espeoially at' the bottom of the profiles.

Looally desaturation permits the direot regular or,ystallization of

kaolinite, partioularly due to the silica of diatoms.

More important is the direot genesis of gibbsi te immediately upon

the first stages of weathering. This ooours in a desaturated, always

humid environment without any dr,y season and vi thout transition through

halloysite.

3) ,Halloysite and metahalloysite. - Balloysite oan form in two habits:

I

!

b.

- in the form of tUbes only I in:profiles, the exclusive oocurrence

of tubes often coincides with the presence of amorphous products rich in

thread-like fine fibrous particles (Pl.III, fig. 10 and. -11).

- wi th globu.laT' shape, the fo_rm~d crystalline ~_a~llr~ o!~hich isdefi. ned. They oocur in soils in wich allophanes· predominate as amorphous

Hakes (Pl.III, fig. 12) . SubseCjuentl,y these globules give 'rise totube5 ( Plo ill, f i 2 12 ) .

The genesis of halloysite takes place only in Zone 11 and III)where

an ann~l dry season ooours. In Zone III,where the dry season is very

. intense, allophanes are oharaoterized by a high Si02/Al203 molecular

ratio (2 to 3) and the neoformation of hallolsite is sufficiently rapid

to keep the allophane content down to a few percent.

At the top of the profiles of Zone 11, and particularly of Zone Ill,

metahalloysite replaoes halloysite and in older soils metahalloysite

becomes predominant. In very anoientsoils gibbsite ooours at the top of

profiles, by degradation of metahalloysite (Sieffermann, Besnus and.

MiLLet., ~CU(8).

4) Evolution aocording to climates and time. - Two weathering sequenoes can

be discerned. :

a) Parent rook ~allophane+kaolinite sensu stricto + gibbsite (ZoneI)

b) Parent rook ~ allophane ~ halloysi te ~ metahalloysi te ~ gibbsi te

(Zones 11 and Ill)

The first sequenoe is typioal of an equatorial environment whioh is

always humid. Well-orystallized kaolinite and gibbsite form early in

andosol a., from basal ti 0 mate r1 als •

The second sequenoe and the gibbsi te formation und.er di ff~rent con- I

ditlon.s were repOTted partly or entlrely by several authors

( Sudo (~'53); FieLdes ~t .t. (1'55. 1cH6) j Sudo and. Takaha­

shi (1QS') ; Jackson (1q5~); Aomine and. Wada (1q62) ; GdS­

tuche, Fri.piat and.. .o,e Kimpe ("Q62); Keller (1q63); Wada( 1CJ 61) i Pe ch 0 and. Lubi n (19 6B) 1 Sit Ffer mann and. Mill 0 t(1CJ68) j TTichet and'Svol'onOS C"Q6a}). Here i.t i.5 ~described.., '

ibei.L.nnins with biilsaLts in intertropical regions : I.

iI

. I

7.

in soil profiles (vertioal variation)

in the transition from a humid equatorial olimate to in ter­mLttent tropical olimate J-

in the oourse of time.

5) Oeoch••ical evolution of eilicates,

In intertropical regions basal tio material of fine ~rail\ si ze- - _._-- "----

i8 rapidly hydrolyzed, giving rise to allophanes_

Gibbsite and well-orystallized kaolinite may oocur in the initial

stages of weathering in permanantly humid regions/whioh laok annual dry

seasons.

In the less leaohed zones, which have a dry season, halloysite

ocours. The seasonal oonoentration of solutions makes orystallization .I

easier, but the weaker desaturation gives rise to disordered and hydra-' II

ted orystalline lattioes. -, f

~II

In thlll upper horizons ,whioh are prrffrtntiaUy tlpo5f~ to dehydration:

metahalloysite replaoes halloysite.

In the oourse of time, a s~oond generation gibbsite oan occur,

but this requires bot)1 desaturat~,on and removal of silica by seasonal,

deesi oation.

8.

REFERENCES

ALonso a., Besoain E., Cat.alin~ F. (1963) - Observations de mioroscopie eleotro­

nique en argilss voloaniques de Chili et allophanes de la Nouvelle­

Z~lande. - Int.C1ay Conf. Stookholm (Sweden). I, p.167-173.

Ao"'int 5., MiyauchL N. (1965). - Imogoli te of imogo-layers in Kyushu. - Soil

Sci. Pl. Nutr., Vol.11, n05, p.28-35.

Aomi ne 5. , Wacla K. (1962) - Differential weathering of voloanio ash and pumice,

resulting in formation of hydrated halloysite. - Amer. Miner. 41,

p.1024 - 1048.

Aomine S., Yoshinaga N. (1955) - Clay minerals of some well-drained voloanio

ash soils of Japan. - Soil Soi., 19, 5, p.349-358.

BirrtL KoS., Fieldes M. (1952) - Allophane ~n voloanio ash soils. - J. Soil Soi.·

3, n02, p.156-166.

8rindltJ G.W., Persio De 50uza Santos. (196~ - New varieties of kaolin group

minerals and the problem of finding a suitable nomenolature. - Int.

Clay Conf., Jerusalem 1966, 1, p.3-11.

Cailler. S. , H~nin S. (1963) - Mineralogie des argUes. - Masson et Cie, Paris,

355 p.

j

Chukrov-,

f.V. J BerklLn 5.1. , Ermilova LP. J Molev. V.A., RudnitskaJa ~.5. (1963)

- Allophanes from some deposits of the U.S.S.R. - Int. Clay Conf•.

2, p.19-28.

Chukrov t.V. ~t ./.. (1964) - Sur le probleme des allophanes. - Izvest. Akad.

Jtauk. c.c.c.a., Ser. ,Geol. 5°4,' ~.3-19.

/.

9.

Colmet·Daage F. , CucaLol'l F. etal. (1961) - CaraoterisUques .le qualques sols

dtEquateur derives de oendres voloaniques. - Cahiers O.R.S.T.O.M.,

Ser. pedol., V, 1, 2 . and. 3.

Fieldes M. (1955) - Allophane and related mineral oolloids •.- N. Z. J. Soi.

Techn., B 31, p.336-350.

Fitldu M., WiLLiamson K.1. (1955) - Clay mineralogy of New Zealand aoils. ­

B.Z.J.Soi.Teohn., Nov. 1955, p.314-335.

Fieldts M. (1966) - The nature of allophane in soils. 1. Signifioanoe of struc­

tural randomness in pedogenesis. - B.Z.J.Soi. Teohn. 9, p.599-601.

FieLdes M., Furkut R.3. (1966) - The natu.re of allophane in soils. 2. Diffez­

enoes in oomposition. - N.Z.J.Soi.Teohn. 9, p.608-622.

Gastucht H.C., Fripiit .:J.~., DeK lmpt C. (~962) - La genese des minh'aux de

la famille du kaolin. - Colloque C.N.R.S., PaTis, N! 105 I P 51-81.

Giustppetti G., p,'gorini B., Veniale F. (1963) - Weathering materials of rooks

and sedimentary deposits from Valsesia, Italy. - Int. Clay Conf.

1963, Stockholm (Sweden), 1, p.139-148.

\'

Jackson M.L. (1959) - Frequency distributibn of olay minerals in major great

soil groups as related to the factors of the soil formation. - Clays

and Clay miner. 6 th N~t. Conf,;l1CJ51, p 133 • 1Q3.

:la ri t z G. (1961) - Ein Vorkommen van Imogoli t in Bimsboden Westdeutschlands. ­

Zei tschr. pflanzenem8.hr. DUng., Bodenkunde., 111, 1, p. 65 -77.

'I

KeLL't W.D. (1963) -lI¥drothermal kaol1niz;'Uon (endelliUzation) of voloanio

glass7 rook. - Cla78 and olay miner. (10th Nat.Conf. 1961), p.333-

. 343. "

I

'10.

Kirkman J.H., Mitchtll B.~., Mackenzie R.e. (1966) - Distribution in some

Soottish soils of an inorganio gel system related to "allophane".

- Trans.roy.Soc.Edinburgh, 66, p.393-418.

Ki nosnita K., Muchi M. (1954) .. Bauxi. tio olay derived from volcanio ash. ­

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Kura ba~ashi. 5., Ts uchi ta 1": (1960) -Quantitative estimation of hydrated hal­

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N° 5, p. 19-22.

Pedro G., Lubi n J. CL. (1968) -Sur I' 'volution des gels alumino-siliciques en

milieu lessiv~, influenoe de la nature des agents d'alteration sur

l'~difioation de boehmite ou d~gibbsite au sein des produits elu­

viaux. - C. R. Acad. Se;., Penis , 2", ser. D, P 551 - SSIi.

Robert'son R. H. S. (19' 3) - ALLoph.3nic soil from Trail Bridge, Oregon, 'With notes

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SeiaLe" p. (1968) - Note sur une m~thode de determination des produi ts mine­

raux amorphes dans dertains sols &hydroxydes. - Publ. O.R.S.T.O.M.

29 p.

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phyllites dans les Vieux sols rouges du Centre Cameroun. ~ Soi. Sol

N! 2 , 1%8, P 105 ~ 111- • I,

"Sieffermann G., Millo t G. (1968) - L'halloysi te des sols jeunes sur basal tes

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Sieffermann G., YehL G., Millot G. (1968) - Allophanes et minaraux argileux

des alt~rations aes basal tes du 140nt Cameroun. - Bull. Qr. Fr. ArgUes- 20, 2 (sous presse).r'. A

11.

Sudo T. (1953) - Clay mineralogioal aspeots of the alteration of voloanio glass

in Japan. - Clay Miner.Bull., 2, p.97-106.

Sudo T.-,Takihashi. H. (1956) - Shapes of halloysi te partioles in Japanese olayse

- Clays and Clay Miner. (-~ th. Nat. Conf. -1QS5) p. 61 - 69.

Trichet J., SvorOllos D. (1968) - Etude des premiers stades d' al teration d'uh

verre wloani,ue. - C. R.Aoad.So~ .• , Paris, 266, sar.D, p.1~07-1209.

Wada K,(1967) - A .truotural soheme of soil allophane. - Amer.Miner., 52,

p.69O-708.

Yashinaga N., Aamine S. (1962) I~ Allophane .in. some Ando-so11s. - Soil.So1. Pl.

Nutr. (Tokyo), 8, 2, p.6-13.

Yoshinaga N., Aominf S. (1962) h- Imogolitel,in some Ando-so11s. - Soil Sc1. PI.

Nutr. (Tokyo), 8, 2, p.114-121.

Yo,hinaga N•• Yotsumoto H., Tbe K. (1968) - An eleotron miorosoopio study of

soil allophane with an ordered ~truoture. - Amer.Miner., 53, p.319-

JIB.

i,

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CAMEROON

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20NE I

ZONE 1I

figure 1

,,~

Localization of the three climatic zones and annual rainfalldietributlon.

Plate I: Samples from Zone I .

Fig 1 Test of diatom (d) coated with allophane (a) - x 14 000

Fig 2 Fragmebt of diatom less than 0,2J1 (d) in an allophanic mass (a)- x 100 000

. Fig 3 : Unperforated fragment of diatom (d) coated with allophane (a) - x 10 000

Fig 4 : Fragment of diatom in pTOCfSS of dissolution (d) coated with allophane (a)- x 45 000

Plate 11 : Faoies of allophahes of Zone I

Fig 5 Felt-like facies of allophane (a) - x 100 000

Fig 6 Fuzzy flakes of allophane (a) - x 100 000

Fig 1 Allophane (a) and unidentified mass In process of organization - x 100 000

,:Fig 8 Allophane (a) and kaolinite (k) - x 100 000

Plate III

Fig 9 : Allophanic mass (a) and kaolinite (k) - x 60 000 - Zone I .

Fig 10 : lmogolite (lm) - x 80 000 - Zone ][.

Fig 11 : Metaha.1loysite (MH) and thread-like fine fibrous particles (a) whichm~ be imogolite - x 80:: 000 - Z one If.

\r. !

I

,I

Fig 12 :' GLobular and tubular shaped. halJ;oysite (Hall) and amorphous mass Ca)- x 210 000 • ZOlltJ[

!

~

[

[

~ ,', ..

2

9 f----l 0,1 fA

1---1 0,1 fA 12 I-----il 0,1 fl