vernadite having interlayer Mg
2+ to todorokite is a four-step process, starting with
the kinking of the phyllomanganate layers owing to the presence of Mn(III), then
primary todorokite particle formation, followed by oriented attachment of these
primary particles, and finally Ostwald ripening. They highlighted that the presence
of Mn(III) is an important prerequisite to the formation of todorokite. Because this
experiment was performed under laboratory reflux conditions, at 100
C, and in
solution, it may be wondered if the abovementioned mechanism of transformation is
relevant for soil. Indeed, an increase in temperature is often used to increase reaction
kinetics, but it may also change thermodynamic equilibria, and the water-saturated
conditions used in this study are not representative of most soils. To determine the
mechanisms of phyllomanganate to tectomanganate under conditions representative
of those occurring in soils, Grangeon et al. (2014, 2015) synthesized several
synthetic vernadite samples having layers containing ~0.12 layer Mn(III) and
~0.15 vacancy per layer octahedron, and 0.08–0.18 interlayer Mn(III) per layer
octahedron. These samples were left for aging for 10 years in conditions assumed
analogous to those occurring in soil, i.e., at room temperature and in the dark, and
were periodically reanalyzed during aging. It was observed that, with time, layer Mn
(III) spontaneously migrates to the interlayer, above the newly generated layer
vacancy. When the abundance of total (layer plus interlayer) Mn(III) in the fresh
samples was equal to about one-third per layer octahedron, the transformation in a
tunnel structure was observed. This was attributed to the sharing of O atoms
belonging to the coordination spheres of two interlayer Mn(III) atoms located on
neighboring layers. These connected Mn(III) formed the “walls” of the tunnel
structures, allowing for the topotactic transformation of phyllomanganate to
tectomanganate described by Bodeï et al. (2007). Such observations explain why
the transformation of phyllomanganate to tectomanganate requires a significant
amount of Mn(III). Finally, using TEM and atomic-resolution STEM, the morphological evolution observed by Atkins et al. (2014) could also be confirmed. This
transformation observed for synthetic samples is certainly analogous to that occurring in natural systems, as the particular shape of the crystals observed in the final
step of evolution (stack of layers, with rotations of Æ120
between adjacent layers) is
also observed in natural samples, for example rock dentrites (Xu et al. 2010).
7.2.3.3 Possible Influence of Light on the Phyllomanganate
to Tectomanganate Transformation
It has been emphasized here above that the transformation of phyllomanganates to
tectomanganates is favored at high Mn(III) abundance. This has many implications.
For example, exposing phyllomanganates to light induces the photoreduction of
layer Mn(IV) to layer Mn(III) that further migrates to the interlayer space, above the
newly formed vacancies (Marafatto et al. 2015). This implies that the formation of
tectomanganates from a phyllomanganate precursor may be favored in the first
millimeters of the soil where light can penetrate. Another implication is the fact
that soil pH certainly has a major influence on the transformation. Negra and
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
409
2+ to todorokite is a four-step process, starting with
the kinking of the phyllomanganate layers owing to the presence of Mn(III), then
primary todorokite particle formation, followed by oriented attachment of these
primary particles, and finally Ostwald ripening. They highlighted that the presence
of Mn(III) is an important prerequisite to the formation of todorokite. Because this
experiment was performed under laboratory reflux conditions, at 100
C, and in
solution, it may be wondered if the abovementioned mechanism of transformation is
relevant for soil. Indeed, an increase in temperature is often used to increase reaction
kinetics, but it may also change thermodynamic equilibria, and the water-saturated
conditions used in this study are not representative of most soils. To determine the
mechanisms of phyllomanganate to tectomanganate under conditions representative
of those occurring in soils, Grangeon et al. (2014, 2015) synthesized several
synthetic vernadite samples having layers containing ~0.12 layer Mn(III) and
~0.15 vacancy per layer octahedron, and 0.08–0.18 interlayer Mn(III) per layer
octahedron. These samples were left for aging for 10 years in conditions assumed
analogous to those occurring in soil, i.e., at room temperature and in the dark, and
were periodically reanalyzed during aging. It was observed that, with time, layer Mn
(III) spontaneously migrates to the interlayer, above the newly generated layer
vacancy. When the abundance of total (layer plus interlayer) Mn(III) in the fresh
samples was equal to about one-third per layer octahedron, the transformation in a
tunnel structure was observed. This was attributed to the sharing of O atoms
belonging to the coordination spheres of two interlayer Mn(III) atoms located on
neighboring layers. These connected Mn(III) formed the “walls” of the tunnel
structures, allowing for the topotactic transformation of phyllomanganate to
tectomanganate described by Bodeï et al. (2007). Such observations explain why
the transformation of phyllomanganate to tectomanganate requires a significant
amount of Mn(III). Finally, using TEM and atomic-resolution STEM, the morphological evolution observed by Atkins et al. (2014) could also be confirmed. This
transformation observed for synthetic samples is certainly analogous to that occurring in natural systems, as the particular shape of the crystals observed in the final
step of evolution (stack of layers, with rotations of Æ120
between adjacent layers) is
also observed in natural samples, for example rock dentrites (Xu et al. 2010).
7.2.3.3 Possible Influence of Light on the Phyllomanganate
to Tectomanganate Transformation
It has been emphasized here above that the transformation of phyllomanganates to
tectomanganates is favored at high Mn(III) abundance. This has many implications.
For example, exposing phyllomanganates to light induces the photoreduction of
layer Mn(IV) to layer Mn(III) that further migrates to the interlayer space, above the
newly formed vacancies (Marafatto et al. 2015). This implies that the formation of
tectomanganates from a phyllomanganate precursor may be favored in the first
millimeters of the soil where light can penetrate. Another implication is the fact
that soil pH certainly has a major influence on the transformation. Negra and
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
409