122 
Figure 7 is a photomicrograph corresponding 
to sample 3 in the X-ray, thermal, and chemical 
data. All data point to a mixture of halloysite 
and gibbsite. A thin section, however, shows that 
the gibbsite exists on the outer periphery of the 
L i 1 x 1 > 1 . I i 1 
0 200 400 600 800 1000 
TEMPERATURE °C 
FIG. 5. Differential thermal diagrams of veins 
(sample 1) and amygdules (samples 2-4) from Haiku, 
Maui. 
PACIFIC SCIENCE, Vol. XX, January 1966 
amygdules and not as an intimate mixture with 
halloysite. 
Figure 8 represents a thin section of a thor- 
oughly gibbsitized amygdule. Although these are 
not evident in Figure 8, most gibbsite amygdules 
are characterized by surface fractures. Figure 3 
clearly shows this feature; in Figure 2 the amyg- 
dules composed predominantly of halloysite fail 
to show this. 
DISCUSSION 
It is often difficult to distinguish halloysite 
from gibbsite in hand specimen. Therefore, 
X-ray, differential thermal, or chemical analyses 
are often necessary to confirm the identification. 
Differential thermal analysis is probably the best 
method for making quantitative estimates of 
these two constituents. 
Once the minerals have been identified, op- 
Fig. 6. Vesicle nearly filled with halloysite. Dark 
areas are residual grains of iron ore; clear areas in 
surrounding matrix are halloysite clays pseudomorph 
after feldspar. Plain light, X320. 
