Gibbsite Amygdules — Sherman and Ikawa 
extremely uniform. The remarkable fact 
brought out by the analyses is that the amyg- 
dules have a high concentration of aluminum 
oxide while the matrix has a high concentra- 
tion of iron and titanium oxides. The silica 
content of both materials is very low. This 
same observation has been made in other 
types of precipitation of gibbsite from perco- 
lating waters. 
The data presented in Figure 2 were ob- 
tained from the differential thermal analysis 
of a representative sample of the amygdules 
and the matrix. The strong endothermic peak 
occurring in these curves at temperatures be- 
tween 350° C. and 375° C. is due to the min- 
erals gibbsite and goethite. The endothermic 
action of gibbsite is much stronger than 
goethite, thus it is safe to identify the alumi- 
num oxide mineral of the amygdule as gibb- 
site, the trihydrate of aluminum oxide. 
The occurrence of gibbsite amygdules in 
the Hawaiian bauxite deposits is of consider- 
able interest. Prior to their discovery, the 
gibbsite had been found only in nodular form 
and in sheets of aluminum oxide with iron 
oxide in seepage channels of percolating wa- 
ter. Pisolitic deposition of gibbsite has been 
described by Alexander et al. (1955). In the 
soils above the decomposed layer, the amyg- 
duloidal gibbsite occurs in aggregates and 
thus might easily be described as being a 
pisolitic occurrence of gibbsite. 
They observed gibbsite fills in spaces left 
293 
Fig. 2. The representative differential thermal analy- 
sis curves of a sample of amygdule and the matrix. 
Amygdules are from the Haiku bauxite area east of 
Pauwela, Maui. 
by shrinkage in the hardening of laterite. The 
recently discovered bauxite of Cape York, 
Australia, is a pisolitic type of concentration. 
Its development has been attributed to the 
flooding of the coastal areas during the cy- 
clones of the monsoon season and its subse- 
quent drying. The development of pisolites 
TABLE 1 
The Chemical Composition of Gibbsite Amygdules and Their Matrices 
FROM THE Haiku Bauxite Area East of Pauwela, Maui 
(Samples from road-cuts on Hana road) 
SAMPLES 
SAMPLE 
PER CENT 
Si02 
PER CENT 
AI2O3 
PER CENT 
Fe 203 
PER CENT 
TiO. 
PER CENT 
LOSS ON 
IGNITION 
PER CENT 
Amygdules A 
51.1 
1.54 
61.72 
5.64 
0.80 
30.96 
Matrix A 
48.9 
2.43 
28.68 
45.37 
9.43 
15.11 
Amygdules B 
49.5 
1.65 
61.60 
5.83 
0.73 
31.06 
Matrix B 
50.5 
2.65 
27.80 
46.30 
9.62 
14.92 
Amygdules C 
50.4 
1.72 
61.72 
6.08 
0.88 
30.60 
Matrix C 
49.6 
2.52 
27.24 
46.42 
9.62 
14.84 
