Halloysite and Gibblle— ' Uehara, Ikawa, and Sherman 
121 
Fig. 3. Gibbsite or desilicated halloysite. Note 
characteristic cracked surface. Scale units in mm. 
3 
4 
1 L. 
2 6 
I ■ t .,.1 J _1 J_J 
10 14 18 22 26 30 34 38 42 
DEGREES 20 
FIG. 4. X-ray diffraction (nickel-filtered, copper KA 
radiation) diagram of veins (sample 1) and amygdules 
(samples 2-4) from Haiku, Maui. 
gibbsite line, and detection of trace amounts of 
halloysite by this method in the presence of 
gibbsite is difficult at best. Differential thermal 
analysis of this sample, however, confirms iden- 
tification of halloysite by the 550°-600° endo- 
therm characteristic of halloysite which appears 
in all four samples. 
Examination of thin sections merely con- 
firmed the X-ray and thermal data, but clarified 
the genetic relation of gibbsite to halloysite. 
Figure 6 shows a vesicle nearly filled with hal- 
loysite. The matrix feldspar has also been altered 
to this mineral and there is no evidence for 
gibbsite. 
TABLE 1 
Chemical Composition (in %) of Veins (Sample 1) and Amygdules (Samples 2-4) 
from Haiku, Maui, Showing Range in Silica-alumina Ratio 
sample no. 
(as in Figs. 4, 5) 
Si0 2 
ai 2 o 3 
Fe 2 Os 
TiQ 2 
MnO 
L.O.I. 
SiOo/ANOs 
1 
39-42 
40.58 
5.55 
1.40 
0.13 
23.85 
1.649 
2 
36.83 
44.32 
2.11 
0.44 
0.03 
16.39 
1.410 
3 
15.29 
57.33 
4.30 
0.48 
0.08 
23.85 
0.453 
4 
6.55 
64.14 
1.41 
0.44 
0.02 
28.07 
0.173 
