Mobilization of Al and Ti during weathering — Isovolumetric geochemical evidence

Chemical Geology, 30 (1980) 151--165 Elsevier Scientific Publishing Company, Amsterdam --Printed in The Netherlands

151

M O B I L I Z A T I O N O F Al A N D Ti D U R I N G W E A T H E R I N G - ISOVOLUMETRIC GEOCHEMICAL EVIDENCE

LEONARD ROBERT GARDNER Department of Geology, University of South Carolina, Columbia, SC 29208 (U.S.A.) (Received November 21, 1979; revised and accepted May 8, 1980)

ABSTRACT Gardner, L.R., 1980. Mobilization of Al and Ti during weathering -- isovolumetric geochemical evidence. Chem. Geol., 30: 151--165. Variations in the volumetric concentrations of A1203 and TiO 2 (ing cm -3) as a function of bulk density for 18 different saprolite suites strongly suggest that significant mobilization of Al and Ti during weathering is more common than is generally assumed. Although trends of decreasing AI~O3 (in g cm -3) with decreasing bulk density could result from dilation, textural and structural features in saprolites argue against significant volume expansion during weathering. Indeed it appears from theoretical calculations that most weathering reactions, including those in which Al is conserved, remove large quantities of various oxides and in the process should create significant amounts of void space. Accordingly, unless large quantities of smectite are produced, it seems unlikely that expansive forces should arise from such reactions. Theoretical calculations are presented that show the effect of dilation on reaction paths. Such calculations are shown to be useful in evaluating the impact of dilation (or compaction) in cases where textural and/or structural criteria are unavailable or ambiguous.

INTRODUCTION T h e c h e m i c a l b e h a v i o r o f A1 is o f f u n d a m e n t a l i m p o r t a n c e t o r o c k w e a t h e r ing. Because o f its l o w s o l u b i l i t y at n e u t r a l p H a n d its a p p a r e n t l y l o w c o n c e n t r a t i o n in m o s t n a t u r a l w a t e r s ( H e m , 1 9 7 0 ) , A1 is w i d e l y c o n s i d e r e d t o be essentially i m m o b i l e d u r i n g w e a t h e r i n g ( G o l d i c h , 1 9 3 8 ; B r o c k , 1 9 4 3 ; Buffer, 1 9 5 3 , 1 9 5 4 ; S h o r t , 1 9 6 1 ; H e l g e s o n et al., 1 9 6 9 ; T a r d y , 1 9 7 1 ; Birkeland, 1 9 7 4 ; Sarazin, 1 9 7 8 ) . Keller ( 1 9 7 8 ) has w a r n e d , h o w e v e r , against t h e u n reserved a s s u m p t i o n o f A1 i m m o b i l i t y d u r i n g w e a t h e r i n g . L a b o r a t o r y w e a t h e r ing e x p e r i m e n t s h a v e s h o w n t h a t large a m o u n t s o f A1 c a n be m o b i l i z e d u n d e r a p p r o p r i a t e c o n d i t i o n s . T h e c o n d i t i o n s t h a t p r o m o t e A1 m o b i l i z a t i o n are p H less t h a n 4.5 (Wollast, 1 9 6 7 ) a n d t h e p r e s e n c e o f c e r t a i n organic acids ( H u a n g a n d Keller, 1 9 7 0 ) . In n a t u r e t h e s e c o n d i t i o n s are generally b r o u g h t a b o u t b y t h e s l o w d e c o m p o s i t i o n o f a b u n d a n t soil organic m a t t e r . Such c o n d i t i o n s ,

152 however, appear to be generally confined to the A horizon of podzolic soils. Most groundwaters have pH values between 6 and 8 and A1 concentrations less than about 0.2 ppm (Hem, 1970). Thus while A1 translocation is only locally evident in the A and B horizons it is possible that the low A1 concentrations in groundwaters may be due to faulty sampling procedures which do n o t take into account the precipitation of A1 that might occur as a result of the loss of dissolved CO2 from samples before analysis. In view of the uncertainty regarding A1 mobility based on water chemistry data it is desirable to find evidence in the soil below the B horizon pertaining to this question. One kind of evidence would be a comparison of the A1 content of the soil to its content of Ti or Zr. Although these two elements are considered to be highly immobile they might also be mobilized under the con. ditions favoring A1 mobilization. Another way of addressing this question is to study residual soils that have retained the original texture of the parent rocks by means of the isovolumetric technique (Schoeller, 1942; Millot and Bonifas, 1955; Gardner et al., 1978). Such soils are frequently called saprolites (Becker, 1895; Grant, 1963, 1964; Plaster and Sherwood, 1971; Pavich, 1974) and are thought by many workers to have evolved by an essentially isovolumetric process. Isovolumetric weathering involves the removal (and/or addition) of elements from the weathering profile without dilation or compaction so that a unit volume of weathered rock can be considered to have evolved from an equivalent volume of fresh rock. In an earlier paper (Gardner et al., 1978) based on the isovolumetric technique, it was shown that the weathering of a granite near Columbia, South Carolina, involved substantial losses of both Al and Ti. This paper presents isovolumetric geochemical data for a variety of rock types and localities which suggest that significant mobilization of A1 and Ti is not u n c o m m o n during weathering. Also because it is not generally possible to prove rigorously the assumption of isovolumetric weathering, calculations are presented which permit evaluation of the possible effect of dilation on the apparent loss of A1 and Ti. SOURCES OF DATA Evaluation of element mobility by means of the isovolumetric technique requires the collection and analysis of a suite of saprolite samples derived from a c o m m o n parent-rock {Gardner et al., 1978). Ideally, the suite should cover the widest possible range of bulk density, from fresh rock to the most highly weathered material that still retains the textural features of the parentrock. In order to establish whether there is a trend of decreasing volumetric concentration of an element with decreasing bulk density, the number of samples in the suite should probably exceed five and should be more or less evenly spaced over the range in bulk density. Unfortunately, relatively few studies of the geochemistry of weathering residues have been conducted with the purpose of isovolumetric interpretation in mind. Most of the data on

153 which this paper is based were published by Clemency (1977), who collected and analyzed a large number of saprolite suites developed on a variety of igneous and metamorphic rocks in Brazil. At the present author's request, Professor C.V. Clemency (pets. commun., 1978) has graciously identified those of the published suites which in his judgement probably evolved in an isovolumetric fashion. In addition to Clemency's suites, the present author has included four suites of saprolites that he had collected and analyzed from the crystalline rocks of the South Carolina Piedmont. Also included are two andesite saprolite suites from California (Hendricks and Whittig, 1968). All of the saprolite samples lie below the B soil horizon. Each set of data from these sources includes measurements of dry weight (ll0°C) bulk density as well as analyses of the major rock-forming oxides. Multiplication of the weight percent of a particular oxide by the corresponding bulk density yields its volumetric concentration (in g cm-3). The mobility of an oxide is revealed by plotting its volumetric concentration vs. bulk density for a suite of samples derived from a common parent. Such plots can be thought of as empirical reaction-progress diagrams (Helgeson et al., 1969; Gardner et al., 1978) in which bulk density is the overall reaction-progress variable. RESULTS

Plots of the volumetric concentrations of Al203, TiO2 and SiO2 vs. bulk density for 18 saprolite suites are shown in Figs. 1--3. The suites shown on Figs. 1 (A--E), 2 (A, B, D, E) and 3 (A--C, E, F) show clear trends of decreasing Al203 with decreasing bulk density. For the other suites the scatter of the data is too large to give a clear trend, although the granite from Ubatuba, S~o Paulo, (Fig. 1F) may be an example of Al conservation at least in the initial stages of weathering. The suites shown on Figs. 1 (D, E), 2 (A, D--F) and 3 (A--C, E, F) have trends indicative of Ti mobilization. Note that the suites shown on Fig. 1 (B, C) have horizontal TiO2 trends indicative of conservation despite decreasing Al203 trends. In all cases SiO2 decreases in an essentially linear fashion and shows relatively little scatter. In most cases the A1 trends showing mobilization are also linear, although the trends for the Liberty Hill granite (Fig. 3C) and the Poqos de Caldas (Fig. 2F) phonolite are exceptional in that they appear to be curvilinear and concave-upward. Although the precision of the TiO2 analyses may not be sufficient to allow confident distinction between linear and curvilinear trends, there is a strong suggestion that most of the Ti trends showing mobilization are curvilinear or composed of two linear segments. Also the majority of the curvilinear Ti trends are concave-upward, indicating that Ti mobilization is more active during early stages of weathering. The Ti trends for the S~o Roque granite (Fig. 1D), the Caye diabase (Fig. 3F) and the Alto de Serra schist (Fig. 2B) are somewhat unusual in that they are convex-upward.

154

LA Q ' O 0 i

0 :'

GRANITE (A SOROCABA

O

\

SAO PAULO

oo

oo 0

\

%o.

RHYO[ l i e CURITIBA

"o

•

"~

PARANA

I

I

I

I

ITAPEVA , SAO PAUIO

Ej 0")

~'

I

_

GRANITE SAO ROQUE

_

a

o.a.

:

-

i

D

GRANITE

SAO

..%

PAULO

"~.

~= O

o u u

"d' °'o.

°.

~

.---.:.°

o

-k

0

d'

~Z

I

I

E

" i

I

i

I

F

GRANITE ( B , SOROCABA SAG P A U t O

o

~J

a 2C ~

13 a

0

~ --

Q

clk~ \ ~-.2

\ •

"°--

o

"--

o\

GRANITE

x

UBAIUBA,

x

"

"o

SAO PAULO

I

I

25

2O

I :C

BULK

DENSITY

I

I

i

2.5

2O

15

g/cm'3 )

Fig. 1 V o l u m e t r i c c o n c e n t r a t i o n s o f A120 ~, TiO 2 and SiO 2 vs. b u l k d e n s i t y for Brazilian saprolites. O p e n s q u a r e s = A1203; o p e n c i r c l e s = SiO2; f i l l e d c i r c l e s = TiO 2. D a t a f r o m Clemency (1977).

DISCUSSION The trends described above clearly suggest that mobilization of A1 and Ti during weathering may be more common than is generally assumed. Furthermore, the mobilization of A1 suggested by these trends is not insignificant. This can be appreciated by comparing the slopes of the SiO2 and A1203 trends,

155 A C]'~O

AMPHIBOLITE CURITIBA, PARANA

-a

~

B

AMPHIBOLE SCHIST ALTO 0 0 SERRA, PABANA

8 o

. o'.~.

"O o. _•

"o

I

!

I

12

I

i

O

~O

I

BASALT JUPIA MATO +GR ASSO

"~ %

° a

'E u

D

O°

SYENITE POCO50E .MINAS

%

.4

o

m

L'I CALDAS,

GERAIS

Q~ ~

3

v

2

2 <~

o~ °o%

o~ v

'

! I F~NITE POCO5 DE CALIDAS, MINAS GERAIS

o.~" - _ -

I F

I

I

u D

'a

a

PHONOLITE POCOS DE CALDAS, MINAS GERAIS

,T% 0°~0

*%

,,

% o

%%0 I

I

I

2+5

2+0

1.5

"0

I 2.5

I 2.O

I °

1+S

BULK DENSITY (gcm"s) Fig. 2. V o l u m e t r i c c o n c e n t r a t i o n s o f Al~O3, T i O 2 a n d S i O 2 vs. b u l k d e n s i t y for B r a z i l i a n s a p r o l i t e s . Open squares = A I 2 0 ~; open circles = S i O 2 ; filled circles ffi T i O 2. D a t a f r o m Clemency (1977).

From the ratio of the slopes one can calculate the atom ratio in which Si and A1 were removed from the profile. In the case of the Cayce granite this ratio is ~ 5.6 (Gardner et al., 1 9 7 8 ) . Thus one w o u l d expect soil water draining through this profile with a typical SiO2 concentration of 30 ppm to have an A1 concentration of ~ 2.4 ppm, corresponding to an atom loss ratio of 5.6.

156

%

B

D

HYPERSTHENE-

ANDESIIE

~l~

CALIFORNIA

OLIVINE ANDE$11E CALIFORNIA

"\

*~ \ \

~j

o

°°

\ "x

\

\

\

•

"~

\

I

I

°¢n

I

I

I

I

C rl J~

?

GRANITE LIBERTY HILL SC BIOTII E SCHIST ENOREE S C

oi

a

~

"~i

.

d+

~.

0

O~

~: e~

",~¢~

.

c~

$@~

'Ev

_

_

0 ~[~ 0 0 0

~,@ .

o l_%,

O~

<~

0 oo

6'

I

1

I

I

E o

--

I

F 0 0

"r,3.

- .~=

?:

0"0. O"

a

n

I

DIABASE CAYCF, S C

GRANITE CAYCE S C

~'-go

"OD~.O " " O "E

Ox

O .OO"

O

0

I

I

I

I

I

I

25

20

I5

25

2.0

+5

o.q

BULK DENSITY :gcrn-3i

Fig. 3. Volumetric concentration of A1203, TiO 2 and SiO 2 vs. bulk density for saprolites from California (Hendricks and Whittig, 1968) and South Carolina (this paper). O p e n squares = Al2 0 3 ; o p e n circles = S i 0 2 ;filled circles = TiO 2.

These calculations illustrate that comparisons of predicted loss ratios based on saprolite data with water chemistry analyses could be used to ascertain whether the process of saprolite formation is active in the m o d e m environment. In reviewing these results, the reader should recall that the discussion in the preceding paragraph is based on the assumption of isovolumetric weathering.

157

In many cases, this assumption is often difficult to prove unequivocally or, for that matter, to refute. A volume expansion of only 15% could produce an apparent loss of A1 similar to some of the trends described above. However, since volume expansion of this magnitude is equivalent to only 5% linear expansion, its effect on textural features might not be apparent in the field. With regard to the interpretation of textural features, Thayer (1966) has described some of the ambiguity and controversy that has arisen over the question of whether the serpentization of dunite is a constant-volume process. He argued that the preservation of delicate chromite textures and the lack of disruption of anorthosite bands in some serpentines argued against volume expansion. Similarly, the present author thinks it can be shown that the evidence against volume expansion during saprolite genesis is stronger than the evidence for dilation. Aside from the decreasing trends of A1203 described above, the only evidence for dilation during saprolite genesis that the present author is aware of is the presence of spheroidal shells around core stones in some profiles. These exfoliation features are generally attributed to differential shear stresses resulting from the volume increase associated with the weathering of primary silicates to clays (Hamblin, 1975, p. 127; Leet et al., 1978, pp. 74, 75). The following equations, however, indicate that even when A1 is conserved, conversion of feldspars into kaolinite should result in a reduction rather than increase of solid-phase volume (Grant, 1963): 4KA1Si3Os + 2CO2 + 4H20 -~ A14Si4010(OH)8 + 8SIO2 + 2K2CO3 416

cm 3

198

4NaA1Si308 + 2CO2 + 4H20 -~ A14Si4010(OH)8 + 8SiO2 + 2K2CO3 402

cm 3

198

3

198

(2)

cm 3

2CaA12Si2Os + 4CO2 + 6H20 -~ A14Si40,0(OH)8 + 2Ca(HCO3)2 202'cm

(1)

cm 3

(3)

cm 3

Accordingly, these reactions should result in the creation of open space. However, the prediction of volume changes for reactions involving more complex minerals such as pyroxene, hornblende and smectite is less amenable to generalization because of the chemical variability of these minerals. The results of volume and mass changes for such reaction pairs based on typical compositions are shown in Table i. Some of these reactions can result in volume increases (e.g., biotite to nontronite). However, most of the reactions that result in volume increases also involve either the conservation or accumulation of mass, neither of which is likely in a saprolite environment characterized by intense leaching. Such reactions usually involve p r o d u c t minerals with A1/Si ratios lower than that ~f kaolinite and reactant minerals with comparatively high A1/Si ratios and thus require the uptake of dissolved SiO2 (and in some cases dissolved Fe and other cations} from solution. Thus b y choosing smectites with a higher A1 content or parents with a lower A1 content, some

158 TABLE I Ratios of volumes and weights (in parentheses) of products to reactants for weathering reactions in which A1 is conserved Reactants

Orthoclase Albite Anorthite Muscovite Pyroxene Hornblende Biotite

Products nontronite

montmorillonite

illite

kaolinite

----0.46 (0.27) 1.71 (1.06) 2.19(1.41)

-1.25 2.50 -0.28 1.02 1.33

0.53 (0.53) --1.15 (1.10) ----

0.48 (0.48) 0.48 (0.49) 0.98 (0.93) 1.04 (0.97) 0.11 (0.08) 0.40 (0.33) 0.52(0.44)

(0.87) (1.64) (0.15) (0.58) (0.77)

Formulas and molar volumes (cm 3) o f products and reactants:

Formula

Molar volume

(am 3) Kaolinite Illite Montmorillonite Nontronite

A12Si2Os(OH), Ko.,A12.,Si3.,Olo(OH)2 Ko.lNao.lCao.l(Mgo.3sFeo.19All.49)Alo.l,Si3.84OIo(OH)2 Ko.~Nao.lCao.l(Fel.sAlo.s)Alo.sSi3.sOlo(OH)2

99 143 209 208

Orthoclase Albite Anorthite Pyroxene Hornblende Biotite

KA1Si30s NaA1Si308 CaA12Si208 Nao.o5Cao.s~Feo.57Mgo.6,Alo.l5Sil.910, Ko.15Nao.4oCal.9sFeL40Mg2.soA12.21Si,.~oO22(OH)2 Ko.82Nao.~oCao.os(Mgo.9oF%.,TAlo.35)Al~.2oSi2.soO~o(OH)2

104 101 101 67 277 147

-- = not considered likely to occur. o f the r e a c t i o n pairs t h a t s h o w v o l u m e increases w o u l d s h o w v o l u m e decreases. F u r t h e r m o r e , the results s h o w n in Table I assume fully h y d r a t e d and e x p a n d e d smectites and thus represent t h e m o s t favorable case for v o l u m e e x p a n s i o n . H o w e v e r , full e x p a n s i o n o f smectites d e p e n d s p a r t l y o n the n a t u r e o f the i n t e r l a y e r cations a n d m a y n o t o c c u r u n d e r field c o n d i t i o n s . Also in t h i c k saprolites the o v e r b u r d e n pressure m a y be sufficient t o r e t a r d full h y d r a t i o n a n d e x p a n s i o n . Thus f r o m these c o n s i d e r a t i o n s it appears t h a t expansive reactions are p r o b a b l y limited t o h o r n b l e n d e - or biotite-rich r o c k s u n d e r c o n d i t i o n s o f restricted leaching. Evidence f o r the c r e a t i o n o f oper~ space includes s o l u t i o n pits o n w e a t h e r e d feldspars (Berner and H o l d r e n , 1 9 7 7 ; Keller, 1 9 7 7 ; N i x o n , 1 9 7 9 ; Rodgers a n d Holland, 1 9 7 9 ) a n d the loose, o p e n t e x t u r e o f kaolinite b o o k s and halloysite (?) t u b e s f o u n d b y Keller in t h e Sparta (Georgia} a n d Butler Hill (Missouri) granite saprolites. Scanning e l e c t r o n m i c r o g r a p h s o f t h e Sparta granite saprolite (Keller, 1 9 7 7 ) , in particular, s h o w delicate t u f t s o f needle-

159 like clays that clearly must have grown into open spaces. These tufts and other delicate tube~shaped clays show no evidence of bending or breakage as would be expected if significant dilation had occurred as the result of the growth of these minerals. It is also interesting to note the contrast between the loose open texture of the Sparta granite saprolite with the dense texture of petrocalcic horizons in caliches. Unlike saprolites the parent materials of caliches are forced to accumulate large quantities of CaCO3 as a result of intense evaporation. There is clear evidence that the accumulation of CaCO3 produces dilation of the detrital framework of sand grains in the parent material (Gardner, 1972). In the formation of a petrocalcic horizon, dilation of the sand-grain framework can reach 100% y e t the bulk density increases from 1.6 to 2.5 g cm -3. Thus it appears that expansive forces are generated in a situation where large quantities of material must be a c c o m m o d a t e d in a limited a m o u n t of space. Also since open space is scarce, dense textures are produced. It is therefore unlikely that expansive forces would be generated in a situation where material is being removed and space created. Furthermore, once started the process is unlikely to produce progressively greater amounts of void space, particularly in saprolites where upward expansion is usually opposed by 1.5 or more meters of overburden. Finally, with regard to expansion the reader is directed to inspect an instructive photograph of spheroidal weathering and saprolite in Gulluly et al. (1968, p. 48). This photograph shows a weathered granodiorite cut by a set of parallel joints. The joint blocks contain core stones in various stages of development. In some blocks the rock has been completely altered to saprolite whereas in others half or more of the block is still essentially fresh rock. Thus variations in bulk density from block to block must be substantial, y e t the joint surfaces remain parallel. This would not be the case if the formation of saprolite was accompanied b y significant expansion. Some aspects of the data shown in Figs. 1--3 also present difficulties for the hypothesis of expansion. In particular the Curitiba rhyolite and the Itapeva granite show trends of A1 loss and Ti conservation. If one argues that Al loss is only apparent and is simply due to dilation, then the Ti trend can be corrected for the effect of expansion by dividing each Ti volumetric concentration as well as its corresponding bulk density b y the ratio of the A1 concentration of the sample to the A1 concentration of the fresh rock. As this ratio is always less than 1.0, the effect is to shift each saprolite data point towards higher bulk density and higher Ti concentration. The corrected Ti trend will thus show an accumulation of Ti during weathering. Although this conserves Al it requires the mobilization of Ti from the upper part of the profile (or extraneous sources) and its deposition in the zone of sample collection. This t y p e of correction procedure can be extended to the preparation of theoretical reaction-progress diagrams to show the effect of dilation (or compaction) on saprolite chemistry. As will be shown, such diagrams can be .helpful in evaluating the possible effect of dilation on reaction paths a n d are

160 especially useful in situations where textural or outcrop criteria are ambiguous or unavailable. To illustrate, let us prepare a hypothetical reaction-progress diagram for the weathering of an idealized granite based on the assumption that A1 is conserved during weathering. It is assumed that this granite has an initial bulk density of 2.62 g cm -3 and is composed of 30% quartz, 30% oligoclase (Na0.TsCa0.25All.2sSi2.TsOs) and 40% orthoclase (Na0.20K0.80A1Si3Os). For simplicity it is also assumed that quartz is inert during weathering and that weathering of orthoclase does not begin until weathering of oligoclase is complete. If kaolinite is assumed to be the only product of feldspar alteration, then the weathering reactions are: 2Na0.~sCa0.2sAll.2sSi2.~sO8 + 2.5H ÷ + 7.25H20 -~ 1.25A12Si:Os(OH)4 + 1.5Na ÷ + 0.5Ca 2+ + 3H4SiO4

(4)

and 2Na0.2 K0.sA1Si3Os + 2H + + 9H20 -* A12Si2Os(OH)4 + 0.4Na + + 1.6K + + 4H4SiO4

(5)

The best way of illustrating the calculations that lead from these equations to the construction of the reaction progress diagram is by means of Table II. In this table the volumetric concentrations of the oxides are portioned among the various reactant and product mineral phases. In the fresh rock A1203, for example, has a total volumetric concentration of 0.382 g cm -3 which is distributed among oligoclase (0.188 g cm -3) and orthoclase (0.194 g cm-3). The course of plagioclase weathering is calculated by incrementally lowering the CaO concentration to convenient levels (e.g., 0.041 to 0.035). As the weight ratios among the various oxides in a particular mineral phase are known, the corresponding volumetric concentrations of Na20, A1203 and SiO2 in the remaining oligoclase can be easily computed. The difference in A1203 between the initial oligoclase and the remaining oligoclase is assigned to kaolinite. The corresponding concentration of SiO2 and structural H20 in kaolinite are computed from the appropriate weight ratios to A1203. At each increment the bulk density is computed by summing the oxides. A similar procedure is followed for orthoclase weathering upon completion of oligoclase weathering. The results are shown as solid lines on Fig. 4. The dashed lines show the effect that progressive dilation would have on the process. The dashed line for A1203 is arbitrarily chosen but has a slope similar to that of the observed A1203 trend of the Cayce granite saprolite and is thus useful for comparison. The dashed A1203 line is the basis for computing the a m o u n t of dilation that has occurred at each stage of weathering. For any particular bulk density the dilation correction factor is given by the ratio of the A1203 concentration read from the solid line to the A1203 concentration read from the dashed line. This ratio can then be used to calculate the bulk density that the sample would have in the absence of dilation. For example, if the bulk density with dilation is 2.00 g cm -3, the correction factor is 1.19 (0.382/0.321). The

161

I

0

666666666

Q;

0

0

0

0

0

0

~

0

Q;

0

0 °~

0

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0

~Q

0 0 0 0 0 0 0 0 0 0

~ ~ ~

0

0

0

0 0 0

0

0 0 0

0

0 0 0

0

0

0

0 0 0

0 0 0

Q;

0 °~ Q;

~ 0 0 ~ O O 0 ~ O ~ O O O O Q

O

0

O

O

O

0

~

O

~

162 AL

CONSERVED

.~ ~

COIvS \

20

m 0

~

SiO.,

"t'OIz4 k ~

TM

~ .~4]'/0p¢

e

i

I~

a

c~

K20

,

)~q)

....

"

i

"*"

"

m ~

o

\"o o

"

~)o

o+

\

o

Na~O

/

o

\

+ +j

Z

/

\ ÷

0

I

o

~.

41r " ~ \

+

\

.+,, +

~ 2.0

Bulk

-

..-

/

2.5

o

",...

+

\

•

u

Density

1,5

\

,,,, I

~

\ !,0

Igcm-~)

Fig. 4. Theoretical reaction paths for weathering of an idealized granite with Al conservation. Solid lines show reaction paths for isovolumetric weathering; dashed lines show effect of dilation during weathering. Data points shown for comparison are from Cayce, South Carolina granite (Gardner et al., 1978). O p e n squares = A1203; large open circles = Si02; filled circles = K20; small open circles = Na20; plus signs = structural H20.

corrected bulk density (1.19 × 2.00 = 2.38 g cm -3) can then be used to read off from the solid lines the concentrations that the other oxides would have in the absence of dilation. For example, at bulk density 2.38 g cm -3 the solidline concentration of SiO~ is 1.76 g cm -3. Division of these solid-line concentrations by the correction factor gives the concentrations that the other oxides would have as a result of dilation. For the SiO2 example dilation would reduce the concentration from 1.76 to 1.48 g cm -3, which value would be plotted at bulk density 2.00 g cm -3. It is in this fashion that the dashed lines for the other oxides are generated. A number of interesting conclusions can be drawn from the results shown in Fig. 4. In general, dilation produces apparent losses for oxides that are actually conserved during weathering (e.g., A1203 and K20 in oligoclase phase of weathering). For oxides that are actually being lost (e.g., Na20} or gained

163

(e.g., H~O ÷) dilation decreases the steepness of the reaction path. In the case of SiO2, dilation has little effect on the slope of the reaction path but does extend the path to lower bulk densities. Another interesting aspect of the SiO2 path is that its slope, in contrast to K20, Na20 and H20, is barely affected by the transition from oligoclase weathering to orthoclase weathering. The effect of such transitions on the slope of the SiO~ path was further explored by calculating the reaction path for the weathering of a hypothetical gabbro containing 50% augite and 50% labradorite. It was assumed that augite weathered to nontronite and that weathering of labradorite to kaolinite began after completion of augite weathering. This transition also had a negligible effect on the slope of the SiO~ path. These calculations, in conjunction with the fact that all of the SiO2 paths shown on Figs. 1--3 axe straight lines, suggest that a case in which the SiO2 path is curved or segmented probably involves reactions of unusual stoichiometry. Fig. 4 shows that in the absence of dilation complete weathering of feldspar to kaolinite should result in a terminal bulk density of 1.75 g cm -3. With the assumed dilation the terminal bulk density drops to about 1.0 g cm -3. For the purpose of comparison, data for SiO2, A1203, K20, Na20 and structural H20 from the Cayce granite saprolite are also shown on Fig. 4. Based on the K20 and Na20 data, it is clear that feldspar weathering is essentially complete at a bulk density of about 1.5 g cm "3. X-ray diffraction analysis of the low-K20 samples at bulk density 1.5 g cm -3 indicates the presence of only quartz, kaolinite and minor illite. If the observed A1203 trend for the Cayce saprolite is solely dup to dilation, then the dashed K20 path indicates that at a bulk density of 1.5 g cm -3 roughly half of the original K20 should still be present and orthoclase should have been detected by X-ray diffraction. Furthermore, the observed H20 ÷ values at bulk density 1.5 g cm -3 are about twice as great as that predicted by dilation but not quite as large as that predicted by the solid H20 line. These facts clearly indicate that the observed A1203 trend can not be due entirely to dilation. In fact, it is likely that the trend is due mainly to A1203 loss because if the weathering reactions involve a loss of A1, less kaolinite a n d structural H20 are produced and thus the terminal bulk density will be lower than that for A1 conservation. SUMMARY

Variations in the volumetric concentrations of A1203 and TiO2 (in g cm -3) as a function of bulk density for 18 different saprolite suites strongly suggest that significant mobilization of A1 and Ti during weathering is more c o m m o n than is generally assumed. Although trends of decreasing A1203 (in g cm -3) with decreasing bulk density could result from dilation, textural and structural features in saprolites argue against significant yolume expansion during weathering. Indeed it appears that most weathering reactions, including those in which A1 is conserved, remove large quantities cf various oxides and in the process create void space. It is thus not clear why expansive forces would

164

arise as a result of reactions that create void space. Theoretical calculations are presented that show the effect of dilation on weathering reaction paths. Such calculations can be useful in evaluating the impact of dilation (or compaction) in cases where textural and structural criteria are unavailable or ambiguous. ACKNOWLEDGEMENTS

I wish to thank Prof. C.V. Clemency for the use of his data from Brazil and for his helpful review of the manuscript. Chemical analyses of the South Carolina saprolites were provided by H.S. Chen. I would also like to thank Joyce Goodwin for her patience in typing several versions of the manuscript.

REFERENCES Becker, G.F., 1895. A reconnaissance of the goldfields of the southern Appalachians. U.S. Geol. Surv., Annu. Rep., 16: 289--290. Berner, R.A. and Holdren, G.R., 1977. Mechanism of feldspar weathering: some observational evidence. Geology, 5: 369--372. Birkeland, P.W., 1974, Pedology, Weathering, and Geomorphological Research. Oxford University Press, London, 285 pp. Brock, R.W., 1943. Weathering of igneous rocks near Hong Kong. Bull. Geol. Soc. Am., 54: 717--738. Butler, J.R., 1953. The geochemistry and mineralogy of rock weathering, I. Lizard area, Cornwall. Geochim. Cosmochim. Acta, 4: 157--178. Butler, J.R., 1954. The geochemistry and mineralogy of rock weathering, II. The Nordmarka area, Oslo. Geochim. Cosmochim. Acta, 6: 268--281. Clemency, C.V., 1977. A quantitative geochemical, mineralogical and physical study of some selected rock weathering profiles from Brazil. State Univ. N.Y., Buffalo, N.Y., Dep. Geol. Soc. Contrib. No. 27, 125 pp. Gardner, L.R., 1972. Age and origin of the Mormon Mesa caliche, Clark County, Nevada. Bull. Geol. Soc. Am., 83: 143--156. Gardner, L.R., Kheoruenromne, I. and Chen, H.S., 1978. Isovolumetric geochemical investigation of a buried granite saprolite near Columbia, S.C., U.S.A. Geochim. Cosmochim. Acta, 42: 417--424. Gilluly, J., Waters, A.C. and Woodford, A.O., 1968. Principles of Geology. W.H. Freeman, San Francisco, Calif., 3rd ed., 687 pp. Goldich, S.S., 1938. A study of rock weathering. J. Geol., 46: 17--58. Grant, W.H., 1963. Weathering of Stone Mountain granite. Clays Clay Miner., 11: 65--73. Grant, W.H., 1964. Chemical weathering of biotite--plagioclase gneiss. Clays Clay Miner., 12: 455--463. Hamblin, W.K., 1975. The Earth's Dynamic Systems. Burgess, Minneapolis, Minn., 578 pp. Helgeson, H.C., Garrels, R.M. and MacKenzie, F.T., 1969. Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions, II. Applications. Geochim. Cosmochim. Acta, 33: 455--481. Hem, J.D., 1970. Aluminum -- abundance in natural waters and in the atmosphere. In: K.J. Wedepohl (Editor), Handbook of Geochemistry, Vol. II-2, Springer, Berlin, Sect. 13-I. Hendrieks, D.M. and Whittig, L.D., 1968. Andesite weathering, II. Geochemical changes from andesite to saprolite. J. Soil Sci., 1 9 : 1 4 7 - - 1 5 3 (cited in Birkeland, 1974). Huang, W.H. and Keller, W.D., 1970. Dissolution of rock-forming silicate minerals in dilute organic acids. Am. Mineral., 55: 2076--2094.

165

Keller, W.D., 1977. Scan electron micrographs of kaolins collected from diverse environments of origin. IV. Georgia kaolin and kaolinizing source rocks. Clay Clay Miner., 25: 311--345. Keller, W.D., 1978. Diaspore recrystallizedat low temperature. A m . Mineral., 63: 326-329. Leet, L.D., Judson, S. and Kaufman, M.E., 1978. Physical Geology, Prentice-Hall, Englewood Cliffs,N.J., 5th ed., 490 pp. Millot, G. and Bonifas, M., 1955. Transformations isovolum~triques dans les ph~nom~nes de lateritisationet de bauxitisation. Bull. Serv. Carte G~ol. Alsace Lorraine, 8: 3--20. Nixon, R.A., 1979. Differences in incongruent weathering of plagioclase and microcline cation leaching versus precipitates.Geology, 7 : 221--224. Pavich, M.J., 1974. A study of saprolite buried beneath the Atlantic Coastal Plain in South Carolina. Ph.D. Dissertation,Johns Hopkins University, Baltimore, Md. (unpublished). Plaster, R.W. and Sherwood, W.C., 1971. Bedrock weathering and residual soil formation in central Virginia. Bull. Geol. Soc. Am., 82: 2813--2826. Rodgers, G.P. and Holland, H.D., 1979. Weathering products within microcracks in feldspars. Geology, 7: 278--280. Sarazin, G., 1978. Multivariable correction method for calculation of magmatic rock weathering budget. Geochem. J., 12: 107--113. Schoeller, H., 1942. La diorite d'Anglais (Lot). P.-V. Soc. Linn. Bordeaux, pp. 1--6. Short, N.M., 1961. Geochemical variations in four residual soils.J. Geol., 69: 534--571. Tardy, Y., 1971. Characterization of the principal weathering types by the geochemistry of waters from some European and African crystallinemassifs. Chem. Geol., 7 : 253-271. Thayer, T.P., 1966. Serpentinization considered as a constant-volume metasomatic process. A m . Mineral., 51 : 685--710. Wollast, R., 1967. Kinetics of the alteration of K-feldspar in buffered solutions at low temperature. Geochim. Cosmochim. Acta, 31: 635---648. -

-