, equatorial and tropioal weathering of reoent basalts...
TRANSCRIPT
, 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-
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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.
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3, n02, p.156-166.
8rindltJ G.W., Persio De 50uza Santos. (196~ - New varieties of kaolin group
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j
Chukrov-,
f.V. J BerklLn 5.1. , Ermilova LP. J Molev. V.A., RudnitskaJa ~.5. (1963)
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/.
9.
Colmet·Daage F. , CucaLol'l F. etal. (1961) - CaraoterisUques .le qualques sols
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\'
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'I
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I
'10.
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JIB.
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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[
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