Halloysite and GIbbsite— U ehara, Ikawa, and Sherman 
123 
Fig. 7. Isotropic halloysite desilicating to birefrin- 
gent gibbsite along periphery of amygdule. Crossed 
nicols, X320. 
deal, methods can give insight to the relationship 
of these minerals to one another by their ar- 
rangement and distribution in a specimen. Th bi- 
section studies suggest that halloysite and gibb- 
site are not necessarily synthesized independently 
of each other, but follow a predictable pattern 
of formation. 
The source of silica and alumina for synthesis 
of halloysite is mainly in the feldspars. In the 
feldspars the aluminum ion, as well as the silicon 
ion, is tetrahedrally coordinated. Each tetra- 
hed rally coordinated aluminum substituted in 
the three-dimensional silica network imparts to 
it a negative charge, which is satisfied by either 
sodium or calcium. Halloysite, on the other hand, 
consists of a sheet of tetrahedrally coordinated 
silica joined to an octahedrally coordinated 
alumina sheet through common oxygen linkages. 
When a feldspar is altered directly to halloy- 
site, forming pseudomorphs after feldspars, 
neither hydrated silica nor alumina need move 
more than a few atomic diameters to recrystal- 
Fig. 8. Completely gibbsitized amygdule. Dark area 
represents opaque iron oxide and pore space. Crossed 
nicols, X320. 
lize in the form of halloysite. Nevertheless, all 
the sodium and calcium, and a considerable por- 
tion of the silica must be removed from the 
volume occupied by the feldspar to accommo- 
date the new mineral. Water incorporated into 
the crystal lattice of halloysite more than makes 
up for loss of the metallic cation and silica. 
Implicit in the assumption that halloysite 
precipitates in vesicles is the movement of alu- 
minum into these voids. Alumina probably 
moves early in the weathering of the rock as the 
anion A1 (OH) 4 ‘, when the sodium ion concen- 
tration is still relatively high. In an acid environ- 
ment the tetrahedrally coordinated anion is 
incorporated octahedrally into the halloysite lat- 
tice. Since the tetrahedrally coordinated alumina 
can be stabilized in acid media by silica (Her, 
1955), it is not unlikely that octahedral alumina 
sheet is the nucleus to which the silica sheet is 
joined. In systems retaining sodium and calcium, 
the secondary mineral is a zeolite rather than 
