EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 11, 557-574 (1986)
SOME OBSERVATIONS ON THE CHEMICAL WEATHERING OF
THE DARTMOOR GRANITE
ANDREW G. WILLIAMS, LES TERNAN AND MARTIN KENT
Department of Geographical Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, PL4 8 A A . Devon, U.K.
Received 29 Jury 1985
Revised 19 March 1986
ABSTRACT
The majority of geomorphological papers about Dartmoor have been essentially speculative, particularly when discussing
weathering processes and the evolution of the Dartmoor landscape. In contrast, this article presents a synthesis of several
experimental investigations aimed at studying the chemical weathering of Dartmoor granite through the systematic
analysis of soil and water samples. This involved the computation of a geochemical budget to determine the amount of
erosion in the catchment, as well as more detailed mineralogical investigations within a soil profile.
The annual output of solutes due to weathering was 116 kg ha-' a - ' of which the majority was silica (93 kg ha-' a-I).
From an examination of the soil mineralogy, it was calculated that these solutes were derived from the dissolution of
approximately 200 k g h a - l a - ' plagioclase, 90 k g h a - l a - ' biotite, and 40 k g h a - l a - ' orthoclase. As well as the
weathering of granite, there was also the production of kaolinite (150 k g h a - l a - ' ) and gibbsite (002 k g h a - l a - ' ) .
Analysis of the soil water chemistry confirmed that kaolinite was the stable mineral phase in the regolith, although in areas
where interflow was the dominant mode of water movement, the solute composition was in equilibrium with both
kaolinite and gibbsite. Examination of the clay mineralogy confirmed these results.
The microtexture of quartz grains was examined by the scanning electron microscope as another means of investigating
the hydrochemical environment in the soil. Silica was found precipitated on all the grains examined but the maximum
amount occurred in the Bs horizon. This evidence showed that, firstly, the dissolution of aluminosilicate minerals is greater
than that calculated by thechemical budget and, secondly, that models of granite weathering must take localized weathering
in the soil profile into account.
The final part of the paper highlights the limitations of calculating denudation rates for an entire catchment and stresses
the need to consider weathering as a highly localized phenomenon, particularly where there are high volumes of interflow at
hill crest sites. Observations on granite decomposition in the future should be quantitative in approach and be related to the
local site conditions.
KEY WORDS
Chemical weathering Geochemical budgets Clay mineralogy Microtexture quartz grains Dartmoor Granite
INTRODUCTION
Previous approaches to the question of weathering of the Dartmoor granite have centred on either speculations
about the nature of past weathering processes based on landform analysis, or on investigations of the various
properties of the weathered granite and periglacial 'head' deposits (Linton, 1955; Palmer and Neilson, 1962;
Brunsden, 1964; Waters, 1964; Eden and Green, 1971; Green and Eden, 1973; Doornkamp, 1974; Sheppard,
1977; Baynes and Dearman, 1978a, b). More recently, however, the present authors have approached the study
of granite weathering through systematic analyses of drainage waters in a small basin on Dartmoor (Ternan
and Williams, 1979; Williams, 1983; Williams et al., 1983, 1984).
The chemical weathering of granite involves the breakdown of the rock or regolith through reactions
between rain and soil water charged with carbon dioxide and organic acids, and the minerals which comprise
0197-9337/86/050557-18$09.OO
0 1986 by John Wiley & Sons, Ltd.
558
A. G. WILLIAMS, L. TERNAN A N D M. KENT
the rock or regolith. The products of these reactions may include clay minerals, insoluble iron oxides and
hydroxides, as well as the loss of silica and cations in the drainage waters. The geochemical budget, based on
solute outputs in the river minus solute inputs from the atmosphere, provides an initial point for chemical
weathering investigations at the basin scale. However, as many of the products of weathering may never reach
the river, the geochemical budget can only give an estimation of the total rate of weathering within the basin.
Little information is provided on which minerals are being attacked, altered or formed, or on the main locations
of weathering within the basin. This information may only be gained by detailed mineralogical analyses of
regolith samples. Study of regolith mineralogy, in conjunction with amounts of chemical erosion derived from
the geochemical budget, allow a consideration of possible weathering reactions. This facilitates a prediction of
the magnitude of weathering of the various minerals which ultimately represent the source of the ions recorded
in the river. This method has been exemplified by Cleaves et al. (1970) and Waylen (1979). A further approach
involves the use of data on the chemical composition of soil and river waters to produce stability field diagrams
in order to determine the stable mineral phase under the present clay weathering environment. For this
approach to be successful, however, data on the chemistry of water draining the horizons for which the regolith
mineralogy has been determined are desirable. Furthermore, seasonal changes in the stable mineral phase for
soil interflow waters along various pathways through the granite regolith on Dartmoor have been
demonstrated by Williams et al. (1984).
The aim of this paper is to present observations from these different approaches to the study of chemical
weathering of granite in the Narrator basin on southwest Dartmoor. This may provide a clearer understanding
of granite weathering both past and present.
THE NARRATOR BASIN
The Narrator drainage basin is located 17 km northeast of Plymouth on the southwest margin of Dartmoor.
southwest England (Figure 1).The basin has an area of 4.75 km2 and a relief of 232 m. The prevailing climate is
cool and wet, since the region is exposed to southwesterly maritime influences. Depressional activity from the
Atlantic is experienced for more than half the year and accounted for a significant proportion of the 1904 mm of
rain received in the study year of 14 February 1977-13 February 1978.
The Narrator basin is entirely located on the porphyritic granite of the Dartmoor pluton. The modal mineral
composition of the granite is shown in Table I.
The solid granite is overlain by a variable thickness of decomposed granite known as growan, the origin of
which has been variously attributed to periglacial processes (Te PDnga, 1957; Palmer and Neilson, 1962),
chemical weathering (Linton, 1955; Brunsden, 1964; Eden and Green, 1971) and marginal pneumatolysis
(Palmer and Neilson, 1962).This debate continues (Ollier, 1983).The upper horizons of the weathered granite
are often bedded; a feature attributable to either surface wash processes (Waters, 1971) or to the downslope
displacement of the growan by head materials during the Pleistocene (Green and Eden, 1973).On Dartmoor,
this periglacial head comprises a layer of blocky debris up to 2 m in depth in a matrix of gritty loam to sandy
loam material. Within the Narrator valley, the total depth of unconsolidated material from borehole evidence is
known to be greater than 35 m. Two principal soil types are present in the Narrator basin, as well as being more
wdespread on Dartmoor (Clayden and Manley, 1964;Harrod et al., 1976).The soils are dominated by iron pan
stagnopodzols (Orthods) on the moorland plateau, with a continuous iron pan at around 35 cm. Brown
podzolics (intergrades between Ochrepts and Orthods) are found on the hillslopes, with typical profiles
showing a fragipan at 7&90 cm depth. The major vegetation communities are sitka spruce forest (Picea
sitchensis), sited in the lower basin, acid grassland extensively invaded by bracken (Pteridium aquilinum) located
on the valley sides and blanket bog dominated by Molinia caerulea which is found over the upper part of the
basin (Kent and Wathern, 1980).
THE GEOCHEMICAL BUDGET
The first stage in determining amounts of contemporary chemical weathering is to compute a geochemical
budget. Solute inputs in bulk precipitation were determined on a weekly basis for the year 14 February 1977-13
CHEMICAL WEATHERING OF GRANITE
559
Figure 1. Location, topography and experimental sites in the Narrator catchment, Dartmoor
Table I. Modal analysis of coarse porphyritic biotite granite from Haytor Quarry, east Dartmoor
(Exley and Stone, 1964)
Quartz
Potash feldspar
Plagioclase feldspar
Biotite
Muscovite
Tourmaline
Apatite
Other
40.0
25.3
19.6
7.0
3.2
2.1
0.3
1.8
February 1978, as the product of the concentration of each element in the collector and the amount of
precipitation. The input figures (Table 11) include an estimate of the total amount of dry deposition. Eriksson
1955, 1960)demonstrated that large quantities of chloride, sodium and magnesium could be deposited as dry
fallout through the process of aerosol entrapment by vegetation. Bulk precipitation collectors are not efficient
at sampling the total dry deposition and trap only a proportion. As a result, many geochemical budgets
frequently overestimate the magnitude of weathering because they fail to satisfactorily collect the total dry
fallout contribution. The approach adopted here considers the annual net output of chloride to be balanced by
560
A. G. WILLIAMS. L. TERNAN A N D M. K E N T
Table 11. Geochemical balance for the Narrator basin (14.2.77-13.2.78 in kg h a - l a - ' )
output
Input
Chemical erosion
H
Na
K
Ca
Mg
SiOz
C1
005
1.05
- 1.00
103.1
96.4
6.7
13.5
10.4
3.1
19.5
8.2
11.3
14.0
11.7
2.3
92.7
0.1
926
151.4
151.4
00
the dry fallout contribution, assuming that there is no additional source of chloride in the regolith (White et al.,
1971).O n this basis, it was calculated that the unmeasured dry fallout contribution of chloride accounts for 24
per cent of the total output (37 kg ha- a-'). The calculation of the unmeasured dry fallout contribution of the
other ions follows that of Claridge (1973). He demonstrated that the ratios of the ionic concentrations in dry
fallout were identical to those in precipitation. The net input of other ions in dry deposition may therefore be
calculated from the ratio of these other ions to chloride. The solute output was derived from the product of the
solute concentration of a daily stream water sample and total discharge for the day. These daily outputs were
summed for the year to determine the total loss from the basin. The amount ofchemical erosion in the Narrator
catchment was calculated as the difference between the stream ouputs and the inputs in total dry and wet fallout
(Table 11).
The chemical erosion figures show a considerable output of silica and a significant loss of calcium, sodium
and potassium. There was a net input of 1.00 kg ha- a- ' of hydrogen which provides the driving force of
chemical weathering. Theoretically this increase of 1040Eqha-' should be equal to the loss of cations
(1130Eq ha-'), but there is a charge imbalance of about 8 per cent. This indicates an additional source of
hydrogen ions within the basin from biological sources. The loss of silica was particularly high
(92.6 kg h a - l a - ' ) , reflecting the active chemical weathering of the granite and the intense leaching
environment. The annual weathering output of solutes from two other temperate catchments is given in
Table I11 for comparison.
Glendye in northeast Scotland is predominantly underlain by Kincardineshire granite, whereas Hubbard
Brook is underlain by highly metamorphosed sedimentary rock and granite. The loss of silica is similar for the
two granite catchments. The net rate of output of solutes derived from weathering from Narrator is in the
order SiOz > Ca > Na > K > Mg which is similar to that for Glendye and Hubbard Brook (Table 111).
'
'
RELATIVE ION MOBILITIES
The establishment of the relative ion mobilities provides an insight into the rates at which minerals are
weathering and the fate of their products (Anderson and Hawkes, 1958; Feth et al., 1964). Relative ion
mobilities compare the rate of output of ions with the proportion available in the rock or regolith undergoing
weathering. In this instance, both the proportion of ions in decomposed granite and those in solid granite were
Table 111. Annual output of solutes due to weathering from three catchments
(kg ha-' a-I)
Narrator*
Na
K
Ca
Mg
SiOz
Solutes released
Rainfall (mm)
*This study.
t Reid et a/. (1982).
1Likens et al. (1977).
6.7
3.1
11.3
2.3
926
116.0
1904
Glendyet
9.0
2.9
17.0
5.0
824
116.4
1375
Hubbard Brook1
5.6
1.o
11.5
2.5
37.7
58.3
1250
56 1
CHEMICAL WEATHERING OF G R A N I T E
Table IV. Relative ion mobility sequences for the Narrator catchment compared with other
granite regions
Location
Narrator, Dartmoor
New Hampshire
New Mexico
Sierra Nevada
Hubbard Brook
Glendye, Aberdeen
Sequence
Ca > Na > Mg > SiOz > K
Mg > Ca > Na > K > SiOz
Ca > Mg > Na > K > SiOz
Ca > Na > Mg > SiOz > K
Ca > Na > Mg > K > S O z
Ca > Mg > Na > SiOz > K
Source
This study (unweathered granite)
Anderson and Hawkes (1958)
Miller (1961)
Feth et al. (1964)
Likens et al. (1977)
Reid et al. (1982)
used (Table VI). This was because the regolith is the most active zone of weathering, with the action of
aggressive rainwater being concentrated in this horizon. Additionally, the decomposed granite has a greater
surface area per unit weight than solid granite and is therefore more prone to further attack. The relative ion
mobility sequence for the decomposed granite was Ca > Na > Mg > SiO, = K. For solid rock the sequence
was Ca > Na > Mg > SiO, > K. This sequence may be compared with those derived from other studies,
using the calculation for unweathered granite in order to maintain comparability (Table IV).
With the exception of the study by Anderson and Hawkes (1958),calcium is the most mobile ion with either
silica or potassium the least. In general, the cations are readily released from silicate minerals and removed by
leaching. Silica is generally immobile, although it is relatively more mobile than potassium in the high leaching
environments of Glendye and Narrator. Under high rates of dissolution, silica is completely lost from the
system (Berner, 1971) but where leaching is less excessive, silica is available for secondary mineral formation.
The low mobility of potassium is either due to its removal in the production of kaolinite or its adsorption onto
clay colloids. The relative hydration energies for sodium and potassium are such that potassium is more readily
removed from solution.
Based on these relative ion mobilities and the weathering output, the amount of material undergoing
weathering can be calculated. Calcium was chosen as the limiting element as it is the most mobile. Every year,
approximately 26,000 kg ha- of decomposed granite (CaO content 0.06 per cent) must weather to produce
11-3kg ha- of calcium (Table 11). These figures assume that the granite undergoes congruent weathering and
that all the minerals are attacked equally. Quartz, for example, which comprises about 40 per cent of the
granite, has a very low solubility, the value of which is pH dependent. Quartz is regarded as relatively inert
(Paton, 1978).
STABILITY FIELD DIAGRAMS
Stability diagrams offer a visual approach to the consideration of chemical weathering of silicate minerals.
Information is portrayed in an easily assimilated form, showing those minerals which are stable in a given
chemical environment and those which are not. The diagrams have been widely used in the field of
geochemistry since they were popularized by Garrels and Christ (1965). For example, Bricker and Garrels
(1967) used them to explore the genesis of spring waters in the Sierra Nevada, and Verstraten (1977, 1980)
considered their application to soil water and spring water in the Ardennes. However, there are several
limitations. They generally assume aluminium to be inert and in partial equilibrium; there is no information on
kinetics and only two variables can be plotted on any one diagram. For granite, in which plagioclase and
orthoclase are the two dominant minerals being weathered, there are two principal stability diagrams involving
the phases in the Na20-Al,O,-Si0,-H,O
and K,O-Al2O-SiO2-H20 system respectively. The stability
diagrams were plotted as a function of the activity of the H,SiO, and of the ratio of each of the alkali ions to
that of the H ion. The solute composition of the major hydrological pathways through a Dartmoor hillslope
regolith and their positions on stability diagrams are shown in Figure 2, and have already been discussed in
detail by Williams et al. (1984). The results, with some exceptions, fall within the stability field of kaolinite,
indicating that kaolinite is the stable mineral phase in the regolith. Shallow interflow above the iron pan
(pathway 1)in the iron pan stagnopodzol has a solute composition in partial equilibrium with either gibbsite or
562
A. G. WILLIAMS. L. TERNAN A N D M. KENT
10 0
FLOW PATHWAYS
80
K-FELDSPAR
60
Y
a
&
40
a
GlBBSlTE
;R
KAOLlNlTE
20
PATHWAY 2
Fragipan
I
1
PATHWAY 3
Saturation up
/-.,
A
0
I
.
,*. . --_
ron pan
I..,
20
I
70
60
40
1
P H SI
~
30
20
04
Figure 2. Stability diagram for water samples from soil water pathways within the grassland, springs and the river
kaolinite depending on seasonal variations in flushing and leaching mechanisms. Interflow above a fragipan
horizon (pathway 2) in the brown podzolic was in equilibrium with kaolinite alone, although the composition
of interflow resulting from saturation upwards from this horizon (pathway 3) occasionally fell within the
gibbsite field. Saturation upwards occurred during major storm events when large amounts of water were
moving as interflow and a low silica concentration was maintained. Interflow within the fragipan (pathway 4)
was also in equilibrium with kaolinite, but was markedly different from that of the other pathways, partly on
account of much lower interflow discharges from this horizon. Kaolinite was again the stable phase for spring
waters. These spring waters have a similar pH-pNa and pH-pK values to those of interflow but the pH4 SiO,
values are higher. Leaching is less powerful at depth and silica levels therefore rise. The stream waters form a
further group with a similar pH4 SO4 level but with higher pH-pNa and pH-pK values. Thus sodium and
potassium levels are similar, whereas the pH is higher. The rise in pH may be attributable to a loss of C 0 2to the
atmosphere as the influent waters equilibrate with the C 0 2 partial pressure in the atmosphere. The elongated
shape is due to the seasonal rise in temperature during the summer and its effect on the pH.
The results from the different interflow pathways taken by precipitation to the stream clearly illustrate the
importance of both spatial and temporal variations in the hillslope drainage in controlling weathering
processes and secondary mineral formation within the regolith. Examination of soil and regolith properties in
the Narrator basin serves to underline this conclusion.
563
CHEMICAL WEATHERING OF GRANITE
SOIL AND REGOLITH COMPOSITION
Results presented here relate primarily to soil and regolith samples from site G4 (Figure l), a typical improved
grassland hillslope site with a brown podzolic profile at the base of the slope. This was one of the major sites
used for regular monitoring of interflow discharges and water chemistry (Williams et al., 1984).
Particle size composition
The clay fraction (< 0.002 mm), as determined by the Andreasen pipette method, was very low throughout
the profile (Table V).
Although these data for the clay fraction are in broad agreement with those of Green and Eden (1973) for
weathered granite at a range of Dartmoor sites (4.5 per cent average), they are much lower than these authors
record for the periglacial head materials (6-21 per cent), in which the upper part of the soil profile is developed.
The silt fraction declines markedly at the horizons of major interflow discharge (85 cm, 92 cm). Although low
clay contents have been used to argue that the sandy type of weathered profile on Dartmoor has developed
under mesohumid subtropical conditions (Eden and Green, 1971),the association of low silt and clay fractions
with high interflow discharging horizons supports Brunsden’s (1964) conclusion that eluviation of the finer
products of weathering is occurring. Furthermore, Baynes and Dearman (1978a) suggest that the open fabric
and increased porosity of some weathered granite samples examined by scanning electron microscopy may be
the result. of eluviation processes. Gibbsite deposition in joints was also recorded. However, where active
interflow zones occur, the use ofparticle size data alone as an index of weathering must be treated with caution.
All horizons were predominantly sandy in texture, with only the A horizon having less than 70 per cent sand,
thus demonstrating the greater degree of weathering in the uppermost horizons.
Total chemical analyses
Analyses of the total chemical composition of samples of solid granite and decomposed granite from site G4
(280 cm) are presented in Table VI.
The proportion of the bases was lower in the decomposed granite, whilst silica and the sesquioxides
increased. The great loss of sodium and calcium for the solid granite can be attributed to the dissolution of
plagioclase feldspar. Biotite has also been attacked, the FeO content decreasing from 1.25 to 0.55 per cent.
Although biotite is less resistant to weathering than plagioclase, a proportion still, however, remains, as
evidenced by the decrease in MgO content from 0.47 to 0.35 per cent. Orthoclase is more resistant to
weathering than plagioclase, with the K 2 0 content decreasing from 6.9 to 6.2 per cent.
Mineralogy
The amount of chemical erosion recorded by the geochemical budget can be attributed to the alteration of
minerals present in the soil. Petrographic examination of resinated thin sections of weathered granite from 33
and 62 cm depths at site G4 was used to identify the primary minerals present. The presence of secondary
minerals was determined by X-ray diffraction. As expected from the particle size data, the regolith consists
mainly of sand sized minerals with a greater proportion of groundmass at the shallower horizon. Groundmass
consists of fine particles, too fine for study using the petrographic microscope. At 62 cm depth, the composition
Table V. Particle size composition of soil and weathered granite regolith at site G4
Depth
(cm)
&3 2
33-50
85
92
150
Horizon
A
BS
Iron stained
Main interflow zone
B/C, growan
Per cent*
sand
Per cent*
silt
Per cent*
clay
Per cent
> 2.00 mm
69.6
75.1
27.0
23.3
9.4
9.3
16.5
3.4
1 .o
2.1
1.o
2.3
15.00
32.3
88.5
89.7
81.2
*Expressed as a percentage of the < 2.00 m m fraction.
-
23.0
19.8
564
A. G . WILLIAMS, L. TERNAN A N D M. KENT
Table VI. Chemical analysisof (i) solid granite and
(ii) decomposed granite at 280 cm depth, Narrator
basin
72.24
14.91
0.82
1.25
0.12
0.47
0.55
2.64
6.90
0.27
0.22
1.05
76.40
17.48
1.29
0.55
0.10
035
006
0.14
6.15
021
0.11
4.00
was 40 per cent quar:z, 40 per cent feldspars, 10 per cent mica, the remaining 10 per cent being groundmass. At
33 cm the feldspar content had been reduced to 15 per cent and the groundmass increased to 35 per cent, again
signifying greater weathering near the surface. The potassium feldspars from both horizons had undergone
significant alteration to forni sericite along the cleavage planes. Quartz clasts occupied about 40 per cent of the
samples from each horizon. Occasionally, quartz-tourmaline aggregates were observed. In addition, zircon
haloes within biotite were present as the result of radioactivity within the granite.
The results of X-ray analyses of samples from both the iron pan stagnopodzol (Gl)and the brown podzolic
profile (G4) (Figure 1)are presented in Table VII. In the stagnopodzol profile the clay fraction is dominated by
kaolinite with illite subordinate. The proportion of kaolinite to illite increases with depth. This is reflected in a
decrease in the K 2 0 content of the clay fraction from 2.7 per cent at 10 cm to 1.0per cent at 120 cm. Chlorite is
only present in the surface horizons. In the clay fraction from the brown podzolic profile kaolinite with illite is
dominant. The K 2 0 content of the clay fraction shows no change with depth. In contrast to the stagnopodzol,
chlorite is more important. There is evidence to suggest that the Al-hydroxy interlayering reaches a maximum
in the Bs horizon. This interlayering is the result of weathering and is probably due to the degradation of
chlorite (Loveland and Bullock, 1975).
Gibbsite is of particular interest, since it does not commonly occur in temperate soils. It may be produced
either by further weathering of kaolinite (Loughnan, 1962) or by direct weathering of feldspar to gibbsite as
reported by Green and Eden (1971). Both origins indicate intense flushing of released soluble constituents. The
occurrence of gibbsite in the weathered granite on Dartmoor was confirmed by Baynes and Dearman (1978a),
who attributed its presence to continued flushing or prolonged weathering. Downslope lateral flow of water
was considered by Rice (1973) to be a possible explanation of high levels of kaolinite and gibbsite at particular
horizons within a weathered granite profile on the Carnmenellis granite, Corriwall. Although gibbsite is the
stable phase for interflow above the iron pan (pathway 1) under extreme flushing events (Figure 2), little is
present according to the X-ray analysis. Similarly, although kaolinite is the stable phase under all conditions
for interflow above the fragipan (pathway 2), the X-ray analysis indicates the strong presence of gibbsite at this
horizon. The explanation in both instances may relate to changes in the weathering environment. Higher
interflow discharges in the past under a wetter climate may have given rise to the gibbsite at site G4, in contrast
to G1, where the development of the iron pan has led to more intense flushing conditions under the present
hydropedological regime.
WEATHERING REACTIONS
General weathering reactions for the primary and secondary minerals known to be present may be combined
with the data on chemical erosion from the geochemical budget, to provide estimates of the amount of primary
mineral alteration and kaolinite production (Cleaves et al., 1970; Waylen, 1979).
+(+)
65
(+I
+
(+I
(+I
+(+I
1
++(+I
+
++(+)
++(+)
+(+I
+(+I
++
1
46
4-6
4 6
46
46
5-8
Quartz
++
++
+++
IA
Kaolin
tr
tr
tr
tr
xx(xI
xx(x)
xx(x)
Albite
tr
tr
tr
tr
tr
tr
tr
tr
K-felspar
The scale of relative intensity of clay minerals ranges from:? to tr (trace)to + + + + (almost monomineral).
The scale of relative intensity of non-clay minerals ranges from:? to xxx very strong reflection.
Solid rock
Decomposed rock
+(+I
+
+
6
20
G4
G4
G4
G4
96
+(+I
tr
120
G1
++
++
++
++
+
+
tr
60
tr
G1
10A
Illite
40
(+)
12A
G1
+
+
14A
10
(an)
Depth
7A:
1OA
tr
tr
tr
tr
xx
tr
tr
Gibbsite
Comments
muscovite or biotite
kaolinite has high degree of
crystallinity
Typical brown podzolic
Comments as above
Strong reflection of gibbsite
Iron pan stagnopodzol
14A chlorite, Al-hydroxy
interlayering, vermiculite
12A mica-chlorite mixed layer
(2nd order reflection)
10A muscovite or biotite
Table VII. X-ray analyses of powdered granite, decomposed granite and selected soil samples
G1
Location
(Figure 1)
~
rn
4
-z
P
2
$I
5
4
s>
5z
-
F
2-
sii
0
X
566
A. G. WILLIAMS. L. TERNAN AND M. KENT
Table VIII. Weathering reactions in the dissolution of granitic minerals used in the calculation of the amount of minerals
consumed and solutes released
1. Plagioclase into kaolinite
+
.38Si2.6208
+ 1.37H2CO3 3.18H20 + 0.69AlzSi2O5(OH),+ 0.62Na'
+ 1.24H4Si04+ 1.37HCO;
+ 0.38Ca2'
2. Biotite into vermiculite
~ [ K Z ( M ~ ~ F ~ ~ ) A I ~+S24H20
~~O~
+ ~12H2C03
( O H )+
~ 2[(Mg3Fe3)A13Si5020(OH)4~
]
8H20J
+ 6 K + +3(Mg2+/FeZC)+8H4Si0,+12HCO;
3. Orthoclase into kaolinite
+
2KA1Si308+ 2H2C03 9 H 2 0 -+ Al2SizO5(OH),+ 4H4Si0,
+ 2K+ + 2HCO;
4. Kaolinite into gibbsite
Al2Si2O5(OH),+ 5 H 2 0 -+ 2A1(OH)3+ 2H4Si0,
From Equation 1 in Table VIII, plagioclase feldspar, which constitutes about 15-20 per cent of the mineral
composition, produces sodium, calcium and silica in the ratio 0.62:0.38:1.24 as it weathers to kaolinite.
Initially sodium was used as the limiting factor, assuming that plagioclase was the only source of sodium. The
generation of the 6.7 kg ha- ' a - of sodium recorded in the geochemical budget (Table Il), however, produces
only 7.2 kg ha- ' a - ' of calcium. Calcium, since it is the most mobile ion (Table IV), may be preferentially
leached from the plagioclase. In addition, the amount of sodium in dry fallout was possibly overestimated.
When calcium is used as the limiting factor, the weathering of 198 k g h a - l a - ' plagioclase produced
11.3 kg ha- a - ' calcium and 10.6 kg ha- ' a - ' sodium (and 55.5 kg ha- ' a - ' of silica). Biotite comprises
about 5 per cent of the mineral composition in fresh rock. Within the soil, biotite is being transformed to
vermiculite and Al-hydroxy interlayered minerals. The weathering of biotite produces potassium, magnesium
and silica, although magnesium may also be released as chlorite weathers. Magnesium was used as the limiting
ion in the weathering of biotite to vermiculite, although this produces more potassium than required
(1 1 kg ha- ' a - excess). Part of the magnesium may therefore be derived from chlorite. Alternatively, the
excess potassium may be consumed in part by the fixation of potassium within the silicate structures, or by the
formation of secondary minerals. Most of the potassium is incorporated within the lattice of illite, which was
found to be present in several soil profiles in the catchment. In addition, the biomass could take up part of the
pnj~~esji~rn
Relative mobility (Table IV) is a function of the relative size of lattic energies (adsorption) and the
hydration energies for the particular combination of species which are present. Factors which determine the
values of the lattice and hydration energies include the size and charge of ions. For ions of the same charge,
hydration energy increases as the size of the ion decreases. Therefore in simple aqueous solutions sodium, a
relatively smaller ion, is more mobile than the relatively larger potassium ion (Table IV).
Orthoclase comprises approximately 25 per cent of the mineral composition and as reported above, was
undergoing decomposition, some showing alteration to sericite. The remaining silica production
(22.0 kg ha-' a - ') was partitioned between orthoclase and kaolinite minerals in the proportion o f 9 : 1 (mole
ratio). On this basis, 45.7 kg ha- a - ' of orthoclase would be consumed and 5 kg ha-' a - ' of kaolinite. The
model in Table IX shows therefore that the silica losses in the geochemical budget were derived from oligoclase
(60 per cent), biotite (16 per cent), orthoclase (21 per cent) and the remainder (3 per cent) from the dissolution of
kaolini t e to gibbsite.
From Table IX B, therefore, approximately 330 kg ha- a - ' of silicate minerals must have weathered in order
to account for the chemical erosional losses in the geochemical budget and would have formed about
150 kg ha- ' a - ' of kaolinite. Of this kaolinite, 5 kg ha- a - ' was consumed to produce gibbsite. The main
minerals being weathered are plagioclase, accounting for 74 per cent of the total mineral breakdown,
orthoclase, accounting for 16 per cent and biotite for 10 per cent. These form kaolinite, vermiculite and Alhydroxy interlayered minerals. Approximately 8 moles of plagioclase are weathered for each mole of biotite,
'
'
567
CHEMICAL WEATHERING OF GRANITE
Table IX. Principal weathering reactions, minerals altered, clay minerals formed and solutes released. (Numbers of
equations refer to Table VIII)
Na
Reaction
K
Solutes released
Ca
Mg
SiOz
Mineral
consumed
Kaolinite
produced
'
A. Units in k mol. ha- a-
Plagioclase to kaolinite
(Eq. 1)
0.46
0.28
093
0.74
027
0-10
033
0.16
0.51
Biotite into vermiculite
(Eq. 2)
0-19
(Eq. 3)
0.16
0.10
Orthoclase into kaolinite
0.08
Kaolinite into gibbsite
004
(Eq. 4)
B. Units in kg ha-' a-l
Plagioclase to kaolinite
(Eq. 1)
Biotite into vermiculite
11.3
10.6
(Eq. 2)
7.4
(Eq. 3)
6.4
2.3
- 0.02
55.5
198.3
15.1
88.3
19.7
45.7
131.6
Orthoclase into kaolinite
21.2
Kaolinite into gibbsite
(Eq. 4)
2.3
- 5.3
and since they are found in the proportions 4 : 1 in granite, plagioclase is weathering at a much greater rate.
Orthoclase is weathering less rapidly than plagioclase but more rapidly than biotite, 0.6 moles of biotite being
attacked for each one of orthoclase.
For comparisons with other areas, the weathering of plagioclase represented 74 per cent of the total mineral
breakdown; a figure similar to that calculated by Reid et al. (1982) for Glendye (75 per cent), but less than that
for granite in the Sierra Nevada (90 per cent) obtained by Garrels and Mackenzie (1967). Orthoclase at 16 per
cent represents more of the total breakdown than found by Garrels and Mackenzie (7 per cent), and is
quantitatively more important than previously determined by other studies.
Quartz grain textures
The scanning electron microscope (SEM) has been extensively used to study the environmental history of
sand grains. The underlying principle is that detrital quartz grains will bear surface textures which may be
related to the weathering, transportational and depositional processes that have operated on them subsequent
to their release from parent rock. Major problems, however, include the recognition of the textures of the
original grain surfaces and the effects of contemporary weathering and pedogenic processes. As part of this
broader view of weathering processes on Dartmoor, twenty quartz grains from the five horizons at site G4
listed in Table V, were examined using scanning electron microscopy. In addition, a number of grains from the
subpeat horizon of the stagnopodzol at site G 1 were similarly studied. Samples were prepared following the
procedure described by Krinsley and Doornkamp (1973) and examined using a JEOL JSM 35 scanning
electron microscope.
As this report represents only the preliminary stages in the investigation of mineral microtextures in relation
to contemporary hydrocnemical processes, no attempt has been made at a statistical treatment as
recommended by Bull (1978). Observed features were simply classified into three broad groups, (i) mechanically
produced, (ii) chemical produced and (iii) features of unknown origin, and the frequency of these features and
their relative importance at different horizons recorded.
Mechanically produced features These include fresh grain surfaces, conchoidal fractures, arcuate and
parallel steps (Plates 1 and 2). Some or all of these features occurred in all horizons investigated down to 150cm
depth at site G4, although they were most commonly observed in the A horizon ( S l O c m depth), with 50 per
cent of the grains showing a moderate to strong occurrence. Mechanical features were very rare in the 33-50 cm
Bs horizon, as a result of masking by silica precipitation. Mechanical features have been recorded from
568
A. G . WILLIAMS, L. T E R N A N AND M. KENT
Dartmoor by Doornkamp (1974) who attributes them to mechanical weathering but says nothing of the
environmental conditions responsible. It is evident from many studies that several of these ‘mechanical’
features may be produced in different environments, including glacial (Krinsley and Donahue, 1968),aeolian
and subaqueous (Margolis and Krinsley, 1971).However, a similar range of mechanical features were recorded
from untransported decompositional grus from pluton source rocks in the southern United States by Scholle
and Hoyt (1973),and were considered to be ‘original’ textures. Conchoidal fractures were said by Higgs (1979)
to be characteristic of material freshly liberated from a crystalline source, as well as being typically found in
glacial materials. Moss and Green (1975)investigated the natural fragmentation of quartz on emergence from
plutonic source rocks. Conchoidal fractures and parallel and arcuate steps were considered to be a reflection of
microfractures in the quartz grains, rather than cleavage surfaces. These microfractures and the intervening
sheets, caused by the process referred to as deformation sheeting, were considered to be the product of the
normal stressing which affects plutonic quartz between crystallization and emergence at the earth’s surface.
Weathering along grain boundaries would, according to Baynes and Dearman (1978a),allow the release of such
stresses. It seems very probable therefore that the occurrence of these mechanical features on quartz grains
from Dartmoor relates to stress relief consequent upon chemical weathering processes operating along grain
boundaries. Thus their form reflects existing microfractures within the plutonic quartz. Disaggregation
resulting from chemical weakening and stress relief could account for the occurrence of mechanical features
throughout the profile examined. Furthermore, it provides an explanation of the occurrence of chemically
produced surfaces adjacent to fresh mechanical fracture surfaces.
Chemically producedfeatures These include the effects of silica precipitation from soil water and chemical
etching by soil acids. Amorphous silica precipitation is the most commonly recorded form of precipitation
(Plate 3) and occurred in all horizons. In the 0-10 cm horizon at site G4 only 20 per cent of the grains showed
abundant evidence of amorphous silica precipitation in contrast to over 75 per cent in the Bs horizon. At deeper
levels, this form of precipitation was still evident, although not as abundant as in the Bs horizon. The range of
quartz grains examined in these horizons indicates a progressive masking of mechanical features by silica
precipitation (Plates 3 and 4). Different forms of silica globules were observed only at the deepest levels
examined (150 cm) (Plates 5,6 and 7). X-ray analysis by means of an EDAX system showed that these globules
were predominantly silica, although some potassium was also present. These globules characteristically
occurred in hollows or conchoidal fracture surfaces and have been attributed to precipitation from near
stagnant solutions that are supersaturated with silica (Le Ribault, 1975; Higgs, 1979). This agrees with the
hydrochemical observations on interflow pathway 4, where flow along the type of pathway was characterized
by low interflow discharges and the highest solute concentrations (Williams et al., 1984). Horizons of major
interflow discharges (90 cm depth) showed abundant mechanically produced features (Plate 2), although these
were often softened or partially obscured by minor silica precipitation (Plates 3 and 4). No evidence of high
energy chemical etching was observed in the brown podzolic profile at G4. Samples collected from the subpeat
horizon of the stagnopodzol at G1 did, however, reveal crystallographically orientated etch pits or orientated
Vs (Plate 8) formed by the dissolution of the quartz grain. At high pH, these features are usually attributed to
the slow solution of quartz along cleavage or fracture planes, through long contact with alkaline solutions.
Similar features have been observed on millstone grit sediments of the Pennines by Wilson (1979), who suggests
that they may have been initiated by acid solutions derived from overlying peat deposits. This suggestion was
strongly supported by laboratory simulations using peat waters of pH around 3.2. It is highly probable that the
features recorded at site G4 on Dartmoor owe their origin to very acid peat solutions and are of recent origin.
Such an origin may also provide the explanation of high energy chemical weathering features in the head
deposits at Shilstone Pit on Dartmoor reported by Doornkamp (1974),rather than incorporation of material
from deeper more chemically weathered granite from upslope.
Features of unknown origin These include the four to five sided angular pits previously described by
Doornkamp (1974)and referred to as ‘bright rimmed hollows’. These seem to be associated with clean fracture
surfaces and most probably represent voids in the original quartz grain or former mineral inclusions as
suggested by Bull (1978). These hollows sometimes appear to be slightly obscured by more recent silica
precipitation.
Conclusions from the SEM analysis point to the continued importance of chemical weathering on
CHEMICAL WEATHERING OF GRANITE
569
B
c
u
a
*
m
ct:
,-.i
u
*
z
C"
.-a
0
c
c
a:
e,
Y
m
5
570
A. G. WILLIAMS, L. TERNAN A N D M. KENT
0
c)
5a
k-
0
w
-a
0
c
6
8
13
9
v;
-a
0
c)
f
2
U
571
CHEMICAL WEATHERING OF GRANITE
Dartmoor. Silica derived from feldspar decomposition is frequently reprecipitated on quartz grain surfaces in
forms related t o the particular hydrochemical environment. Silica output in streams therefore considerably
underestimates the amount of chemical weathering occurring. Chemical weathering may also be important in
releasing intergranular stresses to produce mechanical textures on quartz grain surfaces. Acids draining from
peat horizons also appear to be producing high energy chemical weathering features.
DENUDATION RATES
Chemical Denudation: Denudation rates, generally expressed in mm 1000 a-', are calculated from the rate of
output of weathering derived solutes from the catchment (Table 11). The total loss of solutes during the year was
116 kg ha- a - This sum is less than the 330 kg ha- a - of primary minerals undergoing alteration, since at
least half of the weathering product remains as kaolinite. Rates of denudation are approximately 5 mm
1000 a - based on solute removal and assuming a specific gravity of 2.5 for granite. There are a number of
problems with the approach, both those inherent with the calculation of the solute budget and the assumption
that the removal of soluble products is not accompanied by a decrease in bulk density. Other difficulties include
the extent to which one year's data are representative of the longer term and the manner in which this chemical
denudation is distributed spatially within the catchment.
Although the rate of solutional denudation for the Narrator basin is corsiderably lower than that recorded
for limestone regions (Smith and Atkinson, 1976) it is significantly higher than that recorded for old resistant
sedimentary rocks (Table X).
Chemical and mechanical denudation of the Narrator basin are 6.5 mm 1000 a- in total. Solute yield in
temperate regions normally exceeds sediment yield (Carson and Kirkby, 1972; Gregory and Walling, 1973)
and in the Narrator basin solutional removal is over three times greater than mechanical denudation. Although
evidence of accelerated weathering and erosion in the afforested part of the basin has been presented elsewhere
(Ternan and Williams, 1979; Murgatroyd and Ternan, 1983)these data on total denudation may be considered
as a close approximation to the geologically normal rate of denudation for Dartmoor.
' '.
' '
'
CONCLUSIONS
This investigation has shown that chemical weathering of the Dartmoor granite is both an active and
continuing process. Chemical denudation amounted to 116 kg ha-' a-' with silica accounting for 80 per cent
of this total. This denudation is the result of the alteration of 335 kg ha-' a-' of primary minerals. Seventyfour per cent of the total mineral breakdown can be attributed to the weathering of oligoclase, whilst orthoclase
(16 per cent)and biotite (10per cent) account for the remainder. Although SEM evidence of solutional attack of
quartz by peat-generated acids has been presented, quartz generally has a low solubility and probably
contributes little to chemical denudation. Hydrolysis is the most significant process in the conversion of these
Table X. Rates of chemical and mechanical denudation for small catchments in the British Isles
Lithology
Location
Chemical
weathering
rate
(mm 1000a-l)
Mechanical
weathering
rate
(mm I W a - ' )
Granite
Narrator, Dartmoor
5.0
1.5
Old Red Sandstone
Silurian greywackes
Keuper Marl/Upper
Greensand
Middle JurassicLiassic clays and shales
E. Twin Brook, Mendip
1.6
Mid Wales
E. Devon
16-2.1
14G33.8
1.o
0.4
3.8-20.8
N. York. Moors
20.2
3.4
Source
Present study
Murgatroyd (1980)
Waylen (1979)
Oxley (1974)
Gregory and Walling
(1973)
Imeson (1973, 1974)
572
A. G . WILLIAMS, L. TERNAN A N D M.K E N 1
primary minerals to kaolinite, 92 per cent of the hydrogen coming from external sources and the remainder
from internal biological origins. Chemical denudation in the Narrator basin was 5.0 mm lo00 a-', and is
evidently more active than for old resistant sedimentary rocks in Mid Wales and the Mendips. Reviews by
Douglas (1969, 1978) indicate, however, that the weathering loss of silica from Narrator at 0.4 m3 km-' a-',
although falling within the range for extratropical rivers (0-2-8.6 m3 km-2 a - '), is considerably lower than
those for tropical rivers (2.3-24.7 m3 km-2 a - ' 1.
Data on chemical denudation rates are, however, incomplete as they provide no information on the
localization of active zones of weathering either spatially or within profiles. Considerable evidence has been
presented that chemical weathering within the Narrator catchment is strongly localized principally by
hydrological factors. High volumes of interflow, together with the presence of gibbsite and the occurrence of
crystallographically orientated etch pits on quartz grains point to hillcrest areas as being zones of particularly
active chemical weathering and high removal of soluble weathered products. These results agree with the
observations of Eden and Green (1971)and Doornkamp (1974)on the localization of weathering in relation to
decomposed granite and for development.
In addition to such spatial variations in weathering within the basin, vertical and seasonal variations in
weathering within individual profiles occurs. Zones of active concentrated interflow are characterized by a
sandier texture resulting from lateral eluviation of fines. The importance of fines eluviation in the development
of weathered granite profiles was discussed by Ruxton and Berry (1957,1961),Brunsden (1964)and Baynes and
Dearman (1978a)but thought by Eden and Green (1971)to be oflittle significance on Dartmoor. Microtextures
of quartz grains taken from these hydrologically active horizons also showed limited evidence of silica
precipitation, unlike horizons with low interflow discharges. Stability field diagrams of the active zones
illustrate this variability of weathering, with the stable mineral phase changing seasonally from kaolinite for
much of the year to gibbsite under extremely wet flushing events.
In contrast, hydrologically less active horizons showed silica precipitate forms associated with near stagnant
conditions and only kaolinite was the stable mineral phase from these horizons. A simple downward
weathering model is therefore not appropriate for Dartmoor regoliths undergoing active interflow.
In order to explain the decomposed granite on Dartmoor, many geomorphologists (Linton, 1955; Waters,
1957; Brunsden, 1964; Eden and Green, 1971; Doornkamp, 1974)have evoked to a varying degree faster rates of
chemical weathering under the warmer climatic conditions prevailing in the Tertiary or interglacial periods.
The evidence presented here indicates that only minor variations in the present climatic and pedohydrological
regimes are necessary to explain the properties of weathered granite on Dartmoor. Although eluviation of fines
appears to be occurring, the very low clay contents (< 4.0 per cent) and the high feldspar to quartz ratio (1.0)
point to a lower degree of chemical weathering than found in some tropical environments, for example Hong
Kong (Ruxton and Berry, 1957),and are in accord with present day weathering conditions. Furthermore, the
kaolinite-gibbsite combinations found in weathered granite regoliths are often associated with humid tropical
conditions, and are thus generally attributed to the Tertiary period. However, stability field diagrams based on
seasonal changes in the chemistry of soil interflow waters show clearly that kaolinite is the stable mineral phase
under present day weathering conditions, with gibbsite being formed during wetter periods or under very freely
drained conditions where leaching is rapid. In addition, the process of erosion and deposition since the Tertiary
will have drastically changed the regolith so that the kaolinite-gibbsite combination is unlikely to be relict. Rice
(1973) suggests that soil formation was initiated in head deposits following the periglacial and that kaolinite
formation only commenced at the beginning of the Bronze Age (2000 B.C.). High rainfall, the absence of tree
cover and good drainage led to increased leaching and initiated its formation.
Much speculation has also appeared on the relationship of the weathered granite profiles with landform
evolution on Dartmoor. Data presented here demonstrate that chemical weathering is an active and continuing
process, and hence recourse to a two-stage model for tor evolution as Linton (1955) suggested is unnecessary.
Granite bedrock and tors will emerge in locations and at periods when chemical denudation and erosional
stripping exceed the local rate of weathered profile development.
Several implications for further work on granite weathering have emerged from these observations of
chemical weathering on Dartmoor. Firstly, evidence has been presented that the mineralogical and textural
properties of weathered granite profiles are closely related to the local hillslope drainage and hydrochemistry.
CHEMICAL WEATHERING OF GRANITE
573
Consequently interpretations of weathered granite exposures would be greatly assisted by a more complete site
evaluation of those topographic factors likely to influence or have influenced drainage conditions in the
vicinity of the exposure. Secondly, these observations and most other work on Dartmoor have largely focused
on profiles developed on valley side or interfluve locations. Although eluviated fines may be lost from the
drainage basin via emerging interflow (return flow) and in stream waters, it is probable that some may have
accumulated on or within valley bottom profiles. Boreholes in Dartmoor valleys have revealed significant
lenses of fines within these profiles (Alexander, 1983), although little research on the properties of these
materials has been carried out.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the cooperation of South West Water for permission to instrument the
Narrator valley for experimental work and to Plymouth Polytechnic for financial support. We would also like
to thank Dr P O’Neill for valuable advice, D Bosworth and K Solman for technical assistance, Mrs S
Cockerton and Ms J Wyatt for cartographic assistance, Mrs M Luscott-Evans for typing the manuscript and S
Johnson for photographic work. Work on the total chemical composition of the soils and clay mineralogy was
conducted under the supervision of Professor J M Verstraten, University of Amsterdam. Financial support for
AGW for this work was provided by the British Council and is gratefully acknowledged.
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