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PACIFIC SCIENCE, VoL XX, January 1966 
Fig. L Fragments of halloysitic veins from Haiku, 
Maui. Dark coatings are surface manganiferous de- 
posits. Scale units in mm. 
Closer examination of the saprolytic material 
reveals numerous milky-white, spherical bodies 
(Figs. 2, 3). In distribution and dimension they 
appear to be vesicle fillings, and henceforth will 
be referred to as amygdules. Similarly, joints and 
crevices in the rock have become filled with this 
material (Fig. 1) and appear as veins. Differ- 
ential dispersion of the matrix by rain, and 
protrusion of amygdules and veins give these 
features exaggerated prominence. In selected 
hand specimens, however, as much as 50-75% 
of the volume may be occupied by these amyg- 
dules. 
RESULTS 
Halloysite and gibbsite were identified as the 
two important crystalline minerals in the amyg- 
dules and veins by X-ray diffraction analyses. 
Fialloysite was in the form of the fully hydrated 
(4H 2 0) form, indicated by the appearance of 
the 10 A (8.8° 20 ) spacing. 
Fig. 2. Halloysitic amygdules from Haiku, Maui. 
Note relatively smooth surface. Scale units in mm. 
Figure 4 shows the X-ray diffraction diagrams 
of the veins and amygdules ranging in mineral- 
ogy from essentially pure halloysite to pure 
gibbsite. 
Table 1 gives the chemical composition of 
these same samples. Halloysite is normally given 
the formula 2Si0 2 - Al 2 0 3 -4H 2 0 and, therefore, 
should have a mole Si 0 2 /A1 2 0 2 ratio of 2. The 
values given in Table 1 are all less than the 
theoretical value of 2. This is understandable 
since both the X-ray and differential thermal 
data (Fig. 5) show traces of gibbsite even in 
the predominantly halloysitic samples. Even 
when no gibbsite can be detected, Hawaiian faal- 
loysites, in general, never attain a Si 0 2 /Al 2 03 
ratio of 2 (Nakamura and Sherman, 1963). 
Pure gibbsite should be free of silica, but the 
sample containing the largest quantity of gibbs- 
ite still retains 6.55% Si0 2 . The 02 (hk) re- 
flection for halloysite which appears at 20° 20 
in the X-ray diagram is masked by a major 
