Geoscience Reference
In-Depth Information
Smectites appear first in the zones where bases are not lacking, that
is in the depressions in dry environments. It is, therefore, logical to find
in Vertisols all kinds of accompanying salt formations (Podwojewski
1995):
Dolomite [CaMg(CO
3
)
2
], magnesite (MgCO
3
), sometimes
carbonates of sodium.
Aragonite and also calcite in the form of nodules sometimes
called
kandars
.
Baryte (BaSO
4
) and also gypsum (CaSO
4
, 2H
2
O) that could be of
marine origin and linked to sea spray, at least in New Caledonia,
up to 20 km from the coast.
Transparent, bipyramidal quartz, which could be from
neoformation (size about 5
µ
m).
The dry environments rich in Mg corresponding to Vertisols are also
favourable for the formation of palygorskite, also called attapulgite, a
clay mineral of formula (Mg,Al,Fe
3+
)
5
(Si
8
O
20
)(OH)
2
(OH)
4
,4H
2
O. It is seen
precipitated in the soil pores (Long
et al.
1997) or even in admixture
with smectites in marshland in alkaline medium (pH > 8). Some authors
believe in the transformation smectite Æ palygorskite
in situ
.
To sum up, the Vertisols have undergone several transformations:
decarbonation of the surface layer, transformation of primary minerals to
clay minerals and local enrichment in various salts. Also, they sometimes
present at depth some ferruginous and clay coatings (Nordt
et al.
2004).
But they retain the characteristics of young soils: feeble horizon contrast,
presence of primary minerals (plagioclase, micas) although they are
fragile (Rossignol 1984-1985; Srivastava
et al.
2002; Pal
et al.
2009).
The age of Vertisols is approached in different ways.
Age of Vertisols
Theoretically, some carbonate minerals that they contain can be
dated. This applies to dolomite, aragonite and calcite that contain
14
C (Podwojewski 1995). But the age obtained is unreliable and
low compared to that given by other methods, because the
concretions are formed or transformed in a horizon already in
place.
When Vertisols seem be out of step with the climate, it can
be asked when the last period favourable for their formation
occurred. For example, in the Vanuatu archipelago it is presumed
that gypsum was formed in a period of much greater aridity,
