Geoscience Reference
In-Depth Information
The fourth apex, which points inward, is formed by an OH. The formula
written from the interior outward is OH-SiO
3
Al
2
(OH)
3
according to
Ulery (2002). The Si/Al ratio is 1/2. In one turn of the tube, the number
of OH corresponding to the number of diamond points is usually 24 or a
bit less. The tubes are not isolated but are stuck together like asparagus
in bunches. The structure of the mineral seems to have a feeble negative
charge inside the tubes and another charge, positive this time, on the
external surface. Attractive forces result from this (Gustafsson 2001).
Imogolite is formed in relatively acid medium. It plays a role in
podzolization (Chap. 11).
In proto-imogolite, the tube is split lengthwise and is half open. The
Si/Al ratio remains 1/2.
The term allophane was defined in 1816 and has been in use since 1934
for characterizing mixed gels of silica and alumina of variable chemical
composition with Si/Al ratio that could be between 0.25 and 1.2. The
structure that cannot be determined by X-rays was long unknown.
All of it is still not very clear. Under the optical microscope and with
SEM,
spherules
(nanoballs) of allophane with diameter 2.5 to 5.5 nm are
seen.
Mineralogically, we can judge that the mineral corresponds to a thin
slice cut from a tube of imogolite (Montarges-Pelletier
et al
. 2005) as
shown in Figure 10.4. But this hollow ring curves back at the ends and
is transformed into a hollow sphere when the Si-OH bonds are broken
(Abidin
et al
. 2007).
Allophanes
20Å
Interior
OH SiOAl(OH)
32 3
exterior
25-55 Å
Thickness 7-10 Å
Fig. 10.4
Diagrammatic representation of a spherule of allophane (Montarges-Pelletier
et
al
. 2005) and corresponding composition.
The mineral halloysite, discovered in 1826, was named after the Belgian
geologist J.J. d'Omalius d'Halloy (1707-1789).
Its properties are as follows (Singer
et al
. 2004). The specific
gravity is approximately 2.1. Ideally, it has the chemical formula
Halloysite
