Geoscience Reference
In-Depth Information
Remaining unchanged
Primary minerals
Limited transformation
Export through dissolution
Neoformation
Fig. 2.5
Transformation of primary minerals in soils.
Some minerals stay unchanged; these are
inherited
minerals. For
example, quartz is well preserved in soils of the temperate zone.
Other minerals undergo
limited transformation processes
. There are
different possibilities. The simplest corresponds to preserving the general
physical structure and occurrence of limited chemical transformation.
For example, a biotite mica, with mineralogical structure of stacked
layers, will very easily give a clay mineral of the vermiculite type. Only
K
+
is lost through solution and hydrated cations such as Ca
++
and Mg
++
are gained.
In other cases, the transformation is considerable. For example,
palygorskite, which is a fibrous clay mineral with length perpendicular
to the plane of the cross-section (Fig. 2.6) can give a phyllitic (layered)
clay mineral, more particularly a smectite, provided the structure is
considerably reorganized. Nature seems capable of such a feat, but the
mechanism is not yet clear. Partial solubilization of the palygorskite
will furnish ions that will then combine or fill the original gaps in the
structure and rebuild layers or form a clay mineral starting from scratch
but using the existing structure as template. This will be a case of
epitaxy
(Krekeler
et al.
2005).
Palygorskite
Smectite
Interlayer space (water, ions)
Gap
Silica tetrahedra
Fragment of octahedral sheet; kind of girder elongated at a
right angle to the plane of the diagram
Fig. 2.6
Transformation of a fi brous mineral to a phyllitic mineral (Krekeler
et al.
2005).
But soils do contain minerals very different from those composing
the parent material. For example, in an eruptive vein of oligoclasite,
