THE M I N E R A L O G Y OF SOME CEYLON SOILS
By F. S. C. P. KAtPAGI~*,B. D. MITCHELLand W. A. MITCHELL
The Macaulay Institute for Soil Research, Aberdeen.
[Received 25th April, 1963]
ABSTRACT
The mineralogical composition of the clay, silt and sand fractions from
soil profiles representative of four of Ceylon's major soil groups have
been determined using X-ray, differential thermal, optical and chemical
methods. The results show that the mineral assemblages in these
fractions can be correlated with the pedogenesis and the degree of maturity
of the profile.
INTRODUCTION
The literature on the mineralogical composition of soils in Ceylon
is scanty. Recently, however, Panabokke 0959) reported on the
mineralogy of the clay fractions (< 2/,) of samples from four profiles
in the Dry Zone of Ceylon. In the present investigation, four of the
more widespread of the fourteen major soil groups of Ceylon (Moorman and Panabokke, 1961) have been studied using X-ray diffraction,
differential thermal, optical and chemical techniques.
In soils the clay fraction consists mostly of secondary minerals, the
silt fraction contains essentially a mixture of primary and secondary
minerals, and the fine sand fraction reflects the nature of the parent
material on which the soil is developed. In a sedentary soil the
parent material is the underlying rock, whereas in a transported soil
it may be highly weathered material carried by wind, water, ice or
gravity from the original site of weathering.
MATERIALS AND METHODS
The soils examined are described in Table 1. The clay fraction
(< 1.4~) was separated by the standard method employed at the
Institute (Mackenzie, 1955) and the silt (1.4-20/,) was separated from
the clay-free suspensions by siphoning off the upper 10 cm after a
settling time of 4 min 48 sec. When the silt had been removed, the
sand fraction was divided into coarse sand (2-0-2 mm) and fine sand
(0-2-0.02 mm) by wet sieving. Each fraction was dried on a steam
bath and the clays and silts ground to pass a 100-mesh sieve.
The fine sand was further sub-divided into light and heavy fractions
using bromoform (s.g. 2-90). Approximately 50 ml of bromoform
was poured into a separating funnel, and about 2 g of the fine sand
introduced. The contents were shaken and allowed to stand, the
* Present address: Department of Agriculture, University of Ceylon, Peradeniya,
Ceylon.
308
MINERALOGY OF CEYLON SOILS
309
TABLE 1--Soil profile characteristics.
Soil Group
ReddishBrown Earth
(Young)
Horizon
Depth
A
0-9"
AB
9-18"
B21t
18-26"
Bz2t
26-33"
C
33-45"
C
45-52"
A
0-8"
AB
8-16"
Bzlt
16-24"
B2zt
24-42"
Location
Parent Material
Predominantly
biotite gneisses
of the Khondalite Series.
Madawachchiya
ReddishBrown Earth
(Mature)
Gneisses of the
Khondalite
Series,
Anuradhapura
B23t
B3
Red-Yellow
Latosol
Red-Yellow
Podzolic
Topography
2 % slope on
slightly elevated well drained
area of gently
undulating
plain; 200 ft.
elevation,
3% slope on
well d r a i n e d
gently undulating mantled
plain; 250-300
ft. elevation.
42-54"
I-
54-66"
A1
0-15"
B1
15-32"
B2
32-72"
R3
72-95"
B4
95-140"
B5
140-150"
Ap
0_3 't
A2
3-15"
Bit
15-23"
Bzat
23-39"
B22t
39-62"
B3t
62"+
Mannar
distcict.
Pelmadulla
Old (Early Pleistocene) Coastal Alluvium.
Slope colluvium with lateritic gravels over
residuum from
garnet - sillimanite schists
of the Khondalite Series.
Flat to slightly
undulating terrace on old
coastal shell;
elevation a little over 100 ft.
10~ slope;
lower slope of
a low ridge in
a sharply rolling to hilly
ridge
andvalley landscape; approx.
500 ft. elevation.
310
F. S. KALPAGE, B. MITCHELL AND W. MITCHELL
suspension being stirred from time to time to ensure that all the heavy
particles separated out at the bottom of the funnel. The two fractions
were filtered off separately and washed with benzene to remove the
bromoform.
In order to identify the minerals in the clay fraction by X-ray
diffraction, pressure aggregates were prepared by the method described by Mitchell (1953). Quantitative estimations of the minerals
both in the clays and in the silts were made using corundum as an
internal standard (Mitchell, 1960). Co Ka radiation was used at
36 kV and 10 mA, the exposure time varying from one to two hours.
The differential thermal curves of the clays, equilibrated at 56 per cent.
relative humidity for four days, were obtained in a nitrogen atmosphere (Mitchell and Mackenzie, 1959). The free iron oxides were
removed by dithionite and determined according to the method of
Mitchell and Mackenzie (1954). The fine sand fractions were
examined under the polarizing microscope, and samples of the light
fractions were ground for 15 min in a mechanical mortar, mixed with
corundum, and their X-ray powder patterns recorded.
THE CLAY FRACTION
X-ray diffraction. The mineralogical composition of the clay
fractions (< 1.4tz) as estimated from the X-ray diffraction photographs is given in Table 2. Where actual values are quoted the
samples were subjected to quantitative analyses: for other samples
only the occurrence of the minerals is indicated.
The clays of the reddish-brown earth (young) contain poorly
crystalline kandite, smectite and illite, in increasing order of abundance. The illite content decreases with a corresponding increase in
the smectite down the profile, the kandite remaining fairly constant.
Goethite and hematite are present throughout this profile, the
hematite content being somewhat higher than in the mature soil.
Some quartz is also present.
The clay fraction of the reddish-brown earth (mature) contains
mainly poorly crystalline kandite and illite in approximately equal
amounts; a trace of vermiculite is found in all but the lowest horizon
(B3) , which contains a small amount of montmorillonite. The iron
oxides present are goethite (about 5 per cent.) and hematite. A little
quartz and some amorphous material occur in each horizon.
In contrast to these two profiles, the red-yellow latosol clay contains no vermiculite or smectite and only a trace of illite. Poorly
crystalline kandite is the dominant mineral present and it grades into
kaolinite in the lowest (Bs) horizon. There is some hematite but no
goethite and only a trace of quartz.
The clay from the red-yellow podzolic soil differs from all the other
soil clays examined in containing an appreciable amount (about 10
per cent.) of gibbsite. Kaolinite is the predominant mineral in this
soil clay, and goethite, hematite and quartz are present in traces.
311
MINERALOGY OF CEYLON SOILS
TABLE 2--Mineralogical composition of the clay fraction (< 1 "4/0.
Soil
Group
Reddish
Brown
Earth
(Young)
Reddish
Brown
Earth
(Mature)
Kan- i
Horizon
I
Smec- I Vermi- Goetite
culite
thite
dire
lllite
A
15"
45
30
0
AB
+*
+
+
0
B21t
15
45
30
0
B2et
+
+
+
0
+
C
+
+
+
0
+
C
15
20
50
0
5
A
20
30
0
Tr
5
AB
+
+
0
Tr
B2tt
25
25
0
Tr
5
+
5
+
5
Hematite
3
+
Gibbsite
--0
-!
Quartz
<2
0
Tr*
0
<2
+
0
Tr
+
0
Tr
3
0
<2
Tr
0
Tr
+
0
Tr
Tr
0
Tr
Tr
3
B22t
+
+
0
Tr
+
+
0
B23t
+
+
0
Tr
+
+
0
Tr
B3
+
+
Tr
0
+
+
0
Tr
RedYellow
A1
+
Tr
0
0
0
+
0
+
Latosol
B1
+
Tr
0
0
0
+
0
+
B2
90
Tr
0
0
0
5
0
<2
B3
+
Tr
0
0
0
+
0
+
B4
--
Tr
0
0
0
+
0
-r
B5
90
Tr
0
0
0
5
0
<2
Ap
85
0
0
0
Tr
Tr
10
+
+
RedYellow
Podzolic
A2
+
0
0
0
Tr
Tr
+
Bit
85
0
0
0
Tr
Tr
10
Tr
B21t
+
0
0
0
Tr
Tr
10
Tr
+
+
Bz2t I +
0
0
0
Tr
Tr
B3t
0
0
0
Tr
Tr
I
85
* Figures are percentages;
I 10
-P indicates present, but not determined quantitatively;
Tr indicates trace.
Tr
312
F . S . KALPAGE, B. MITCHELL AND W. MITCHELL
These X-ray results show that the clay fractions from the four
profiles studied differ from each other in mineralogical composition.
Both reddish brown earth soils have developed on weathered biotite
gneiss under similar conditions of climate, vegetation, and drainage.
In the mature soil pedogenic processes have proceeded for a longer
period of time and produced a deeper, more weathered profile. The
time factor may account for the predominance of kandite and illite
in the mature profile and the presence of smectite from the weathering
of biotite and felspar in the younger soil.
The red-yellow latosol and the red-yellow podzolic soil are developed on sedimentary materials which are relatively more highly
n ~ ]---I
TEMPERATURE, *C
I
FIG. 1--Differential thermal curves for clays from the lowest horizons of A--reddish-brown earth (young), B--reddish brown earth (mature), C--red-yellow
latosol, D--red-yellow podzolic soil.
weathered. This is in agreement with the absence of smectite, the
mere trace ofillite, and the dominance of the kandite minerals. The
red-yellow podzolic soil contains an appreciable amount of gibbsite
indicating that it is the most highly weathered of the soil profiles
studied. This soil is from a region where temperatures are high
(yearly average 81~
and rainfall is heavy and evenly distributed
throughout the year (mean annual rainfall 129.6 in,). Under such
conditions, weathering of the minerals in the garnet-sillimanite parent
rocks proceeds rapidly.
Differential thermal analysis. The differential thermal curves for
clay fractions from three horizons (surface, intermediate and basal)
of each of the four profiles were obtained and those from the basal
layers are shown in Fig. 1. In general the results confirm those
313
MINERALOGY OF CEYLON SOILS
obtained by X-ray diffraction. Kandite and illite predominate in
the mature reddish-brown earth with goethite as an accessory mineral.
The red-yellow latosol contains mainly kandite with no goethite,
while kaolinite and gibbsite are the principal constituents of the redyellow podzolic soil.
There is, however, one discrepancy. X-ray diffraction showed a
smectite (18A line on glycerol saturation) to be present in the reddishbrown earth (young). The differential thermal curves do have larger
hygroscopic moisture peaks than the curves for the other soils
studied, but they do not show an endothermic peak at about 700~
which is the normal dehydroxylation temperature of montmorillonite.
It would appear, therefore, that the mineral present is either nontronite, with an endothermic peak in the 400-500~ region, or an
'abnormal' montmorillonite (Mackenzie, 1957), with an endothermic
peak at about 550~
In either instance the peak would be masked
by that for the kandite and illite.
TABLE 3--Free iron oxide removed from the clay fraction by sodium dithionite.
Soil Group
Reddish Brown
Earth (Mature)
Red-yellow
Podzolic
Horizon
Fe203 removed
A
6"55
B21t
6"22
B3
5"72
Ap
5'52
Bit
5"76
B3t
5"19
(%)
Free iron oxide. The amounts of free iron oxide removed by
dithionite treatment from the clay fractions of six of the soil samples
are shown in Table 3. X-ray diffraction data in Table 2 show that
clays from the mature reddish-brown earth contain traces of hematite
and less than 5 per cent. of goethite, whereas those from the redyellow podzolic soil have only traces of both goethite and hematite.
This probably accounts for most of the free iron oxides removed, as it
is doubtful if less than 2 per cent. of hematite or goethite would be
detected; there may also be some non-crystalline iron oxides present.
The X-ray photographs showed distinct black bands indicative of the
presence of amorphous material. These bands did not disappear
completely after the clays had been treated with dithionite suggesting
that the amorphous material present consists not only of iron oxides
but also of other materials, possibly alumina and silica.
The removal of free iron oxides from the silt fractions by sodium
dithionite is referred to below.
314
F. S. KALPAGE, B. MITCHELL AND W. MITCHELL
THE SILT FRACTION
The mineralogical composition of the silt (1.4-20/,) fractions is
summarized in Table 4. The presence of clay and amorphous
material made accurate quantitative analysis difficult and therefore
only the relative abundance of the minerals present is indicated.
All the profiles studied contain appreciable amounts of quartz.
Felspar is abundant in the two profiles derived from gneissic parent
material, but occurs only in traces in the two profiles derived from
older and more highly weathered sediments. Mica occurs in the silt
fraction of the young reddish-brown earth profile, but is less abundant
in the mature profile, except in the basal horizon where pedogenic
weathering is presumably less advanced. Clay minerals are present
in the silts in considerable amounts. In a few instances all the clay
had not been removed before the silts were separated. Generally,
however, the occurrence of appreciable amounts of clay minerals in
the silt fractions would indicate the existence of either fine coatings o f
clay on the silt particles or silt-sized aggregates of clay; in either
instance the clay particles are held together by some bonding agent
such as amorphous material.
An attempt was made to determine the nature and amount of amorphous material present by treating the silts with sodium dithionite,
re-dispersing, siphoning off the clay, and determining the composition
of the silt-sized particles that remained. About 30 per cent. by weight
of the silt fraction was dispersed as clay-sized particles after dithionite
treatment. As Tables 2 and 3 indicate, sodium dithionite removed
virtually all the crystalline iron oxides originally present. Dithionite
treatment reduced the amount of amorphous material present, but
did not completely remove it.
THZ FINE SAND FRACTION
The percentages of light and heavy fractions in the fine sands,
given in Table 5, indicate that smaller amounts of light material
occur in the young reddish-brown earth profile than in the corresponding mature profile, due to the relative accumulation of the inert quartz
as weathering proceeds. The percentages of light and heavy fractions
in the two soils on sedimentary rocks remain fairly constant throughout each profile, indicating a uniformly weathered parent material.
Tables 6 and 7 show the proportions of the minerals in the light and
heavy fractions, respectively.
Some features in the mineralogical composition of the fine sand
fractions are worth noting. The assemblages of heavy minerals
(Table 7) in the two sets of profiles are different. In the reddishbrown earths derived from gneisses, iron oxide and hornblende are
the most abundant, with small amounts of micas and zircon; there is
more hornblende than iron oxides in the young profile, while the
order is reversed for the mature soil. On the other hand, iron oxides
and siUimanite predominate in both the red-yellow soils along with
some spinel, zircon, and rutile. The absence of garnet in the fine
MINERALOGY
6
+
+
+
§
§
§
+
§
~ +~
+
0
+
§
OF
§
CEYLON
§
§
+
0
0
§
§
+
§
+
§
§
+
~
315
SOILS
+
§
§
0
0
0
0
§
+
§
§
§
§
+
+
0
0
§
0
0
0
0
§
§
§
0
0
0
o
~
i
~0
0
0
0
0
0
o
0
0
0
0
0
0
0
0
0
0
0
+
+
+
§
§
§
o.
-F
o
+
§
§
0
0
+
+
+
+
+
+
Jr
+
+
I
<
N
~+~
i
o
-Oo~o
No~
~
0
~
T
0
~o
N
316
F. S. KALPAGE~ B. MITCHELL AND W. MITCHELL
TABLE5--Amounts of light and heavy minerals in the fine sand fraction
(0"2-0-02 mm).
Soil Group
(%w/w)
Heavy
minerals
(%W/W)
A
77
23
B21t
86
14
C
65
35
A
91
9
B21t
94
6
B3
97
3
A1
94
6
B2
95
5
B5
94
6
Ap
82
18
Bit
83
17
B3t
88
12
Horizon
Light
minerals
Reddish-Brown Earth
(Young)
Reddish-Brown Earth
(Mature)
Red-Yellow
Latosol
Red-Yellow
Podzolic
sand from the podzolic soil derived from colluvium overlying sillimanite-garnet schists is rather unexpected. Generally, the more
resistant minerals, such as iron oxides, zircon, spinel and rutile, are
concentrated in the more highly weathered transported soils, while
the less resistant hornblende and micas are found in the fine sands
from the soils on gneiss,
TABLE6--Mineralogical composition of the light fraction of the fine sand.
Horizon
Quartz
Felspar
B21t
F.A.
V.A.
B21t
F.A.
V.A.
Red-Yellow
Latosol
B2
V.A.
C.
C.
Red-Yellow
Podzolic
Bit
V.A.
C.
C.
Soil Group
Reddish-Brown Earth
(Young)
Reddish-Brown Earth
(Mature)
Mica
KEY: V.A.--Very abundant, 60-100%; F.A.--fairly abundant, 10-30%
C.--common, 5-10%.
MINERALOGY
OF
CEYLON
317
SOILS
"O
O
,z
=O
E
O
I
-6
o~
t:l
o6
o6
06
o6
06
6,
rd
O6
o6
o6
~v
~6
06
o6
06
I
0)
o6
>: o6
~O
.4
>:
O
O
.-&
,.Q /.
O
T
~5 ~5
~
,'
0)
C,
kD
<
'
r-,
0)
l=
>:
i
>
L.
<.
>
O
~ "-6 *&
318
F. S. KALPAGE, B. MITCHELL AND W. MITCHELL
The heavy fraction of the fine sand from the B 2 layer of the redyellow latosol was separated into magnetic and non-magnetic
minerals. X-ray diffraction confirmed the presence of zircon,
sillimanite, rutile and spinel in the non-magnetic fraction, while the
magnetic portion contained ilmenite and another component, the
diffraction pattern of which showed similarities both to that of
8-ferric oxide and rutile except that some strong rutile lines were
absent or very weak. On heating to various temperatures up to
550~ for one hour the pattern was unchanged, showing that ~-ferric
oxide was not present. After heating to 800~ for one hour, the
ilmenite lines disappeared, a number of new unidentified lines appeared, and the incomplete rutile pattern persisted. After heating
to 1000~ a pattern agreeing closely with rutile was found, along with
that of pseudobrookite, FeoTiO 5.
CONCLUSIONS
A detailed study of the mineralogy of four Ceylon soils has shown
that the mineral assemblages can be correlated with the maturity of
the soil profile, and that the relative stability of the various minerals
in the parent materials and of the secondary minerals is dependent on
differences in pedogenesis.
Examination of the clay fractions has shown that there is a gradation
in the degree of crystallinity of the kandite present which can be
correlated with the stage of development of the soil profile; in the
young reddish-brown earth this component is poorly crystalline
kandite whereas in the red-yellow podzolic soil the highly crystalline
form predominates. Gibbsite, a reliable indicator of advanced
pedogenic weathering, was observed only in the red-yellow podzolic
profile. The degree of development of the soils is also indicated by
the variation in the ratio of triphormic to diphormic clay minerals.
The most significant feature of the silt samples is the occurrence of
stable aggregates of clay-size particles, in all possibility bonded by
the amorphous material and crystalline iron oxides shown to be
present.
The heavy mineral composition of the fine sands of the reddishbrown earth profiles shows variations which can be related to maturity, the lower content of heavy minerals in the older profile being due
largely to the relative instability of the hornblende.
REFERENCES
MACKENZIE, R. C., 1955. Clay Min. Bull., 3, 4.
MACKENZIE, R. C., 1957. Bull. Groupe frar, c. Argiles, 9, 7.
MITCHELL, B. D., and MACKENZIE, R. C., 1954. Soil Sci., 77, 173.
MITCHELL, B. D., and MACKENZIE, R. C., 1959. Clay Min. Bull., 4, 31.
MITCHELL, W. A., 1953. Clay Min. Bull., 2, 76.
MITCHELL, W. A., 1960. Miner. Mag., 32, 492.
MOORMAN, F. R., and PANABOKKE, C. R., 1961. Trop. Agriculturist, 117, 3.
PANABOKKE, C. R., 1959. Soil Sci., 87, 67.