Occurrence of Gibbsite Amygdules in Haiku Bauxite Area of Maui^ 
G. Donald Sherman and Haruyoshi Ikawa^ 
Areas of bauxite deposits have been dis- 
covered on the islands of Kauai, Maui, and 
Hawaii of the Hawaiian Islands. These de- 
posits have been described by Sherman (1957) 
and Fellom (1957). In many of these deposits 
the free aluminum oxide occurs as a constitu- 
ent of irregular-shaped nodules varying in 
size from 1/100 inch to over 6 inches in their 
longest dimensions. The alumina content of 
these nodules as determined in the labora- 
tories of the Hawaii Agricultural Experiment 
Station ranges from 40 per cent to 62 per cent. 
Iron oxide was found to be the other major 
constituent of these nodules ranging from a 
little over 2 per cent to 40 per cent. Mineral- 
ogical analysis of the nodules has identified 
the minerals as gibbsite, the trihydrate of 
aluminum oxide, and goethite, an iron oxide. 
Recently, layers of pea-sized gibbsite amyg- 
dules were found in several of the deep road- 
cuts occurring on the new highway to Hana, 
3-5 miles east of Pauwela, Maui. The white 
gibbsite amygdules are exposed in the banks 
of the road-cuts as irregular layers from 12 to 
20 feet below the surface soil. These layers 
are easily recognized because the numerous 
white amygdules occur in dark gray brown 
rocks which are completely weathered. These 
weathered rocks still retain the structure of 
the original parent rock. The material is soft 
and when crushed between one’s fingers it 
^ Published with the approval of the Director of the 
Hawaii Agricultural Experiment Station as Technical 
Paper No. 400. Manuscript received September 25, 1957. 
2 Senior Soil Scientist and Junior Soil Scientist, re- 
spectively. Dept, of Agronomy and Soil Science, Ha- 
waii Agricultural Experiment Station, University of 
Hawaii, Honolulu, Hawaii. 
readily breaks down to a clayey material, and 
the hard round amygdules. The amygdules are 
shown in Figure 1. 
A close examination of the amygdules 
revealed that they have been developed by 
the precipitation of hydrated aluminum oxide 
in the cavities of the parent rock. The unfilled 
cavities can be found in adjacent parent rock 
and in the unweathered rock of the same 
formation. The cavities were formed by gas 
bubbles in the original lava. The precipitation 
of the aluminum oxide initially occurred on 
the wall of the cavity. Subsequent precipita- 
tion of additional aluminum oxide has con- 
tinued until the cavity was completely filled. 
The contents of the cavity harden to form a 
resistant amygdule. The size and shape of the 
cavity determine the physical shape of the 
amygdule. The aluminum is transported in 
percolating waters to the cavity as aluminum 
hydroxide and is precipitated as the hydroxide 
on exposure to air in the cavity. It then is con- 
verted to trihydrate of aluminum oxide on 
aging in a drier atmosphere. The evidence 
for the mode of deposition of the aluminum 
oxide in the cavities is as follows: (a) the 
occurrence of similar unfilled cavities having 
similar shapes and volume in the adjacent 
weathered material, (b) the concentric deposi- 
tion layers of the amygdule, (c) the occur- 
rence of hollow amygdules which would 
indicate the initial precipitation of the hy- 
drated oxide on the wall of the cavity (several 
of the amygdules in Fig. 1 have hollow cen- 
ters), and (d) the observation of the formation 
of aluminum and iron oxides on exposure of 
seepage waters exuding from ditch and road 
cuts during the wet season. 
291 
