Dolomitization in Hawaiian Soils — Sherman et al. 
43 
the year. Each year these soils become very 
dry, often to the extent that they show wide 
and deep cracks. During these periods of 
drought the salts are brought toward the sur¬ 
face by capillary rise of water. In the process 
of drying, much of the calcium salt is precipi¬ 
tated at lower levels, while the magnesium 
salt, owing to its higher solubility, remains 
in solution. At such periods the soluble mag¬ 
nesium may exceed the soluble calcium and 
thus create conditions favorable to the for¬ 
mation of dolomite by the action of magne¬ 
sium sulfate on calcium carbonate. When the 
soils become wet the salts are leached to a 
lower level in the soil, from which the more 
soluble salts (magnesium) can rise again. 
This would be a possible explanation of the 
gypsum accumulation in the subsoil of these 
soils and of the formation of dolomite di¬ 
rectly above the gypsum layer. 
The formation of dolomite and gypsum 
as end products of this process should be 
expected under arid or semi-arid conditions, 
since these two compounds represent the 
most insoluble compounds of calcium and 
magnesium which could be formed in the 
presence of calcium carbonate. According to 
Leather (1913), the presence of magnesite 
together with calcite in soil is extremely 
doubtful and if magnesite did exist it would 
only be present in traces. It has been found 
that when the dolomitic carbonates are 
treated with acid they give up calcium and 
magnesium carbonate in approximately a 1:1 
ratio. This fact and the effervescence which 
is characteristic of dolomite establish the 
identity of the type of carbonate formed. The 
formation of gypsum will result from the 
action of magnesium sulfate on the calcium 
carbonate and its precipitation from solution 
during wetting and drying of the soil. This 
process would lead to the formation of gyp¬ 
sum in the crystalline forms found in these 
soils. 
Dolomitization in soils can be expected to 
occur under soil conditions similar to those 
described in this report. It is likely that 
dolomitization has occurred to some extent in 
many calcareous soils. 
CONCLUSIONS 
As a result of this study of the composi¬ 
tion of the soil carbonates in a representative 
soil profile from Lualualei Valley, Makaha 
Valley, and Waianae Valley, the following 
conclusions may be drawn: 
1. Dolomitization of the carbonates has been 
established by the chemical composition 
of the soil carbonates, by the effervescence 
in cold dilute hydrochloric acid charac¬ 
teristic of dolomite, and by the presence 
of one of the two relatively insoluble end 
products, gypsum, which will be produced 
when this process occurs in the presence 
of sulfates and calcium carbonate. 
2. In the soil horizons where dolomitization 
has occurred, 50 to 94 per cent of the car¬ 
bonates are in the form of dolomite. 
3. In order that the process of dolomitiza¬ 
tion may occur, the presence of soluble 
magnesium salts is required. The soils of 
this area were found to be rich in mag¬ 
nesium, since this cation accounted for 
20 to 45 per cent of the exchangeable 
cations in the soil. The drying of the soil 
during dry weather concentrates and in¬ 
creases the soluble magnesium until it ex¬ 
ceeds the calcium in solution due to the 
precipitation of calcium as gypsum, and 
thus leads to conditions favorable to the 
formation of dolomite. Periodic rainy and 
dry seasons are essential to the dolomiti¬ 
zation process. 
4. The soil below the zone where the dolo¬ 
mitic carbonates exist contained numerous 
pellets filled with crystalline gypsum and 
crystals of gypsum. In this zone of gyp¬ 
sum accumulation the calcium-to-magne- 
sium ratio is very high. 
