498 
PACIFIC SCIENCE, Vol. XX, October 1966 
The main amorphous mineral colloid is allo- 
phane-like, excepting that the silicia-to-alumina 
ratio is too low for the value of allophane pro- 
posed by Ross and Kerr (1934). The mineral 
is amorphous to X-rays, and differential thermal 
analysis indicates only a single, low temperature 
endotherm at about 180°C. It resembles so- 
called stable allophane (Jackson, 1956) in that 
it is resistant to decomposition by dilute hydro- 
chloric acid of pH 3.5 or 2% sodium carbonate 
solution. Characteristics of the allophane-like 
fraction of Akaka soil and other amorphous 
mineral colloids are shown in Table 1. 
The Akaka soil is a member of the Hydrol 
Humic Latosol Group, and was described by 
Cline et al. (1955). Akaka soils have developed 
from volcanic ash, under a tropical rain forest 
with an average annual rainfall of more than 
120 inches. They are highly hydrated in the 
natural state; the water content sometimes ex- 
ceeds 300% of the oven-dry weight of soil. The 
Akaka soils have very low bulk densities rang- 
ing from 0.1 to 0.6. On dehydration they lose 
volume and exhibit an increase in particles of 
sand size; the dehydration is irreversible. An 
explanation of this irreversibility has been given 
by Sherman (1957) ; he reported that the pro- 
cess of dehydration caused the crystallization of 
the hydrous oxides to gibbsite, to crystalline 
iron oxide minerals, or to alpha quartz. 
Monopotassium phosphate was used in the 
experiments. It is a product (reagent grade) 
of General Chemical Division, Allied Chemical 
Corporation. The pH of the phosphate solu- 
tions was adjusted with hydrochloric acid. 
Preparation of the Reaction Products for 
Characterization 
The reaction products were prepared by the 
following procedure. A 5-15 g sample of wet 
Akaka soil was placed in a 200-ml pyrex beaker 
or pyrex glass container of suitable size, to 
which was added 100 ml of potassium phos- 
phate solution ranging from 0.2 to 0.6 molar, 
with an initial pH value of 2. The glass con- 
tainer was wrapped in a plastic bag and kept 
at room temperature (about 25°-30°C). The 
contents were filtered after standing from 1 to 
200 days, and the pH of the filtrate was mea- 
sured. The pH of the filtrate serves more or less 
as an indication of the pH of the reacting sys- 
tem, which, in turn, predicts the possible species 
of ions that enter into reaction. The residue was 
washed with 60% methanol, 80% ethanol, 
TABLE l 
Characteristics of the Allophane-Like Fraction of Akaka Soil as Compared 
with Other Amorphous Mineral Colloids* of Soils 
DIFFERENTIAL 
THERMAL 
TYPE OF 
AMORPHOUS 
MATERIAL 
INDICATION 
Endotherms 
°C 
Exotherms 
°C 
Si0 2 /Al 2 0 3 
SOLUBILITY 
IN DILUTE 
ACID AND 
ALKALI 
Allophane: 
unstable 
150 
960 
0 
OJ 
1 
K> 
o 
high 
stable 
150 
960 
2.0 
low 
Silica 
165-573 
— 
— 
high 
Alumina 
110-500 
— 
— 
high 
Iron oxides 
160 
350-450 
— 
high 
Titanium 
oxides 
195 
465 
— 
low 
Allophane-like 
fraction of 
Akaka soil 
180 
— 
0.8 
low 
Jackson (1956:528). 
