Geoscience Reference
In-Depth Information
the minerals are found, modified by position in the profile and season
(summer, winter). According to the same authors, the value of
x
can be
used as follows to show the relationship of different iron compounds:
Table 12.3
Principal free forms of iron in soils with excess water, not including
primary minerals. In italics: the minerals found mostly in tropical environments
and related to dry conditions; they are shown here for comparison.
Forms
Characteristics
Goethite
: three- Named after the poet Wolfgang von Goethe;
a
-FeOOH, yellowish
dimensional oxy- brown color to dark brown colour; often contains a little
hydroxide manganese; up to 33% substitution of Al for Fe, orthorhombic,
solubility 7 x 10
-9
to 1 x 10
-12
mole of iron per litre; specific
surface area very variable (60 to 200 m² g
-1
); very common
in soils.
Lepidocrocite
:
g
-FeOOH, bright orange (saffron) colour; abundant in wet soils;
layered oxy- orthorhombic; metastable; characteristic of calcareous soils poor in
hydroxide Si and Al; specific surface area 180 m
2
g
-1
.
Ferrihydrite
: three- Discovered in 1971 in the former USSR. Fe
5
(OH)O
7
, 4H
2
O;
dimensional
reddish brown; rhombohedral; surface area 200 to 500 m² g
-1
;
solubility 2 x 10
-9
mole iron per litre; characteristic of natural
hydrated oxy-
hydroxide
environments rich in organic matter; abundant in springs.
Green rusts
:
Family of minerals shown in 1967 to occur in soils; named for its
colour, very labile (see below); surface area 200-300 m
2
g
-1
.
layered hydrated
oxy-hydroxide
Haematite
: three-
a
-Fe
2
O
3
; purplish red (blood red) colour
;
solubility 1.7
x
10
-13
mole
dimensional iron per litre; surface area very variable (10 to 120 m² g
-1
or more);
trigonal (~ hexagonal); tropical environments rich in Al.
Maghemite
(hydrated
g
-Fe
2
O
3
;
pK
sp
= 40.5; cubic; tropical environments.
magnetite)
x
= 0 corresponds to ferrous hydroxide Fe(OH)
2
,
when
x
becomes successively 1/3, 1/2 and 2/3, it is in the domain
of green rusts with the formulae: Fe
II
Fe
III
(OH)
7
, Fe
II
Fe
III
(OH)
5
and
Fe
II
Fe
III
(OH)
8
,
x
= 1 corresponds to Fe(OH)
3
, but this is not stable in soils
contrary to what was believed earlier.
One of these green rusts, precisely defined, has been identified as a
new mineral
fougerite
, so named because it was observed by scientists
of INRA-France in the state-owned Fougères forest near the town of the
same name in Brittany (Bourrié 1996; Trolard
et al
. 1996). This mineral
has a rather complex formula because the ferrous iron in it is more or
less replaced by magnesium. All these compounds are very labile and
