124 
PACIFIC SCIENCE, VoL XX, January 1966 
halloysite. There is evidence in certain Hawaiian 
soils that zeolites do in fact weather to halloysite. 
Bates et ah (1950) long ago proved the mor- 
phology of halloysite to be tubular; Hawaiian 
halloysites (Nakamura and Sherman, 1963) are 
often more closely related morphologically to 
the spheroidal, Japanese allophanes described by 
Sudo and Takahashi (1955). Critical electron 
microscope examination of Hawaiian halloysites 
should be extremely interesting. 
One might expect the gibbsite content to in- 
crease with increasing proximity to the soil sur- 
face. However, samples 4, 3, 2, and 1, showing 
increasing halloysite content in the order given, 
were obtained at approximately equal depths be- 
low the surface. Mineralogical variability can be 
readily explained on the basis of certain weather- 
ing factors. 
The gibbsite content in any unit volume of 
the weathering profile depends on a number of 
factors. In general, the gibbsite content increases 
with increasing proximity to the soil surface, 
where weathering is most intense. However, 
even at depths quite distant from the surface, 
zones exist which are highly gibbsitic by virtue 
of their proximity to an internal drainage chan- 
nel. All other factors being equal, the permea- 
bility of the rock also controls rate of weathering. 
In general, any differential volume undergo- 
ing weathering may be treated as an open sys- 
tem. Two distinct volumes might be given 
special consideration here: ( 1 ) the volume occu- 
pied by a feldspar crystal, and ( 2 ) the voids or 
vesicles. From the moment atmospheric pressure 
and temperatures are attained, the feldspar is 
subject to decomposition by C0 2 ~saturated leach- 
ing waters. If the feldspar occurs near the surface 
of a lava flow, the C0 2 concentration is high 
and the reaction will be relatively rapid. At 
greater depths, the leaching solution is saturated 
with silica, alumina, sodium, and calcium, and is 
depleted of C0 2 . This saturated solution accumu- 
lates in voids, and precipitates in the form of an 
appropriate secondary mineral. 
The rate at which this process takes place will 
depend on the amount of water passing through 
this unit volume. In an open system, the dif- 
ference between the concentration of the out- 
going and of the incoming solution gives a 
measure of the weathering rate. In an open 
system, a void in the rock gains constituents 
early in its history; but, in the final stages of 
weathering, even this volume suffers loss of cer- 
tain material. Gibbsite as a final weathering 
product is extremely stable and will resist fur- 
ther decomposition; thus, it accumulates to form 
some of the world’s bauxite deposits. 
REFERENCES 
Bates, Thomas F. 1962. Halloysite and gibbs- 
ite formation in Hawaii. Clays and Clay Min- 
erals. Proc. Ninth Natl. Conf. on Clays and 
Clay Minerals 9:315-327. 
— F. A. Hildebrand, and A. Swinford. 
1950. Morphology and structure of endelite 
and halloysite. Am. Mineral. 35:463-484. 
Cline, M. G., et al. 1939. Soil Survey of the Ter- 
ritory of Hawaii. U. S. Dept. Agr. Soil Survey 
Ser. no. 25. 
Iler, Ralph K. 1955. The Colloid Chemistry of 
Silica and Silicate. Cornell TJniy. Press, Ithaca, 
N. Y. 324 pp. 
Nakamura, Martha, and G. D. Sherman. 
1965. Genesis of halloysite and gibbsite from 
mugerlte on the Island of Maui. Hawaii Agr. 
Exp. Sta. Tech. Bull. 62 fin press}. 
Sherman, G. D , and H. Ikawa. 1959. Occur- 
rence of gibbsite amygdules in Haiku bauxite 
area of Maui. Pacific Sci 13 (3) : 291-294. 
Sudo, Toshio, and H. Takahashi. 1955. 
Shapes of halloysite particles in Japanese clays. 
Clays and Clay Minerals. Proc. Fourth Natl. 
Conf. on Clays and Clay Minerals, Natl. Acad. 
Sci -Natl. Res. Council Publ. 456:67-79. 
