Dehydration in Laterite Formation — Sherman et aL 
443 
TABLE 4 
The Relationship between Loss on Ignition and Cation Exchange Capacity 
UNDER Barren Laterite Crust and Adjacent Soil with a Vegetative Cover 
barren soil (crust) 
SOIL WITH VEGETATION (NO CRUST) 
depth 
CATION 
CATION 
PH 
organic 
LOSS ON 
exchange 
PH 
organic 
LOSS ON 
exchange 
matter 
ignition 
CAPACITY 
matter 
IGNITION 
CAPACITY 
Inches 
Per cent 
Per cent 
m.e.llOOg. 
Per cent 
Per cent 
m.e.llOOg. 
0-5 
3.8 
1.1 
2.0 
3.7 
4.6 
8.2 
27.9 
41.0 
5-10. 
4.0 
10.2 
15.6 
4.6 
26.8 
37.0 
10-15 
4.2 
25.7 
34.9 
4.4 
28.5 
38.4 
hydration is considered to be the cause of its 
reduction. 
Differential Thermal Analysis: The thermal 
curves obtained from the differential thermal 
analysis are given in Figure 2 for the soil 
profile from the barren area and Figure 3 for 
the soil with a vegetative cover. There is a 
marked difference in these two sets of curves. 
The curve for the surface horizon of the soil 
with a vegetative cover showed a strong exo- 
thermic peak at 180°C., whereas the surface 
soil from the barren area showed no thermal 
reaction at this temperature. In fact, the curve 
of the latter soil is very similar to that ob- 
tained from a sample of hematite. The thermal 
curves of the subsoil horizons of both pro- 
files exhibit thermal peaks at 180°C. to 190°C., 
but none of the soils exhibited a peak at 
710°C. which is characteristic of montmoril- 
lonite. The thermal curves of this type are 
probably due to the existence of amorphous 
hydrated oxides of iron, aluminum, and ti- 
tanium. Some of the soils have a small peak 
at 350°C. which is probably due to goethite. 
As the differential ' thermal analysis was 
made on the untreated soils, the question 
arises as to whether these curves are influenced 
by the difference in organic matter content 
of the soil. The surface soil of the profile with 
a vegetative cover has a higher content of 
organic matter than that of surface soil of 
the barren area. It will be noted that the 
thermal curves of the 10-1 5 -inch layer of the 
profile with a vegetative cover are exactly the 
same as that of the surface horizon. As this 
horizon has a low organic matter content, the 
minerals of the soil must be responsible for 
its thermal characteristics. The loss of water 
below 300°C. also substantiates the fact that 
dehydration of minerals is responsible for 
thermal characteristics of the soil, as the loss 
of water is the greatest in those soils showing 
thermal peaks at 180°C. 
The ferrous iron content was determined 
in the surface soils from the two profiles. The 
soil from the profile with a vegetative cover 
contained from two to five times as much 
ferrous iron as the soil from the barren area. 
As the surface soil from the barren areas is 
magnetic and has a low ferrous iron content, 
the presence of maghemite is suggested in 
these soils. Furthermore, this would suggest 
the existence of the lepidocrocite-maghemite 
system in these soils as maghemite is the 
product of dehydration of lepidocrocite. Fur- 
ther evidence is needed to establish the valid- 
ity of this suggestion. 
DISCUSSION 
The results of this study have established 
that the indurate laterite crust horizon is de- 
veloped by the dehydration of the hydrated 
iron and titanium minerals in the soil. The 
thermal curves from the differential thermal 
analysis and the loss of water under 300°C. 
of the two profiles have shown that hardening 
