Geoscience Reference
In-Depth Information
platy structure). In this case, the phase of complete breakdown
(ion soup) is not seen because it involves just a few atoms at
each moment, but it very probably does exist. In addition, stud-
ies lead to the notion that clay minerals could be formed by
triggering their crystallization on metal ions adsorbed on the
surface of quartz grains present in the immediate environment.
Actually, the quartz-metal ion association forms towards the
exterior a structure that resembles that of the clay mineral and
could serve as a template or pattern for its crystallization (Charlet
and Manceau 1994). This is
epitaxy
.
The neoformed minerals are also called
secondary
or
authigenic
.
3.3 MINERALOGICAL EVOLUTION OF CLAYS
Amongst the weathering phenomena, hydrolysis is so important that
it merits separate treatment. It affects all soils but is expressed most
prominently in the intertropical zone. The definitions of clay mineral
names were given in Chapter 2 (§ 2.2.2).
3.3.1 Definition and Examples
The boundary between dissolution and hydrolysis is rather subtle.
Dissolution can be summarized as bringing into solution all the
constituents of the minerals that are weathered. The clay minerals are
involved. Of course, they are particularly stable minerals but they are
liable to dissolve when the conditions are right. The solubility products
of the reactions have been calculated. Unfortunately, many of them are
not reliable because they are difficult to establish and therefore differ
from one author to another. The solubility of the clay minerals has
been experimentally studied. For example, kaolinite introduced in a
porous bag in a soil of Amazonia showed etch marks at the end of six
months (Cornu 2005). Resistance to dissolution increases with degree of
crystallinity.
Hydrolysis is the attack of silicate minerals by water resulting in
change of mineral species and total reorganization of the structure.
Mineral + water Æ other mineral and/or hydroxides + ions in solution
But the passage of one mineral to another often presumes an
intermediate stage of solubilization. Also, there are losses by export
in water. In short, hydrolysis is often a cocktail of solubilization and
neoformation. Generally we specify 'neutral hydrolysis' considering the
pH of the corresponding environment. But this gives rise to confusion
