Calcisols and other Soils on Carbonate-Bearing and Sulphate-Bearing Materials - Major Soil Groups of the World - page 226

precipitate (in the dry season). In short, the silicates are replaced,
volume for volume, by lime: this is epigenesis . This mobility of silica in
calcareous environment explains other phenomena: appearance of flint
nodules in chalk, manufacture of certain grindstones (“meulières” of
Paris region)…
Without this phenomenon of replacement, such accumulations of
CaCO 3 in crystalline rock will be impossible. Indeed, the porosity of the
soils does not exceed 50 per cent. Thus, if the imported lime had only
blocked the voids of an originally acid soil, without solubilizing and
replacing the minerals present, the CaCO 3 present could not exceed 50
per cent. But it can be as high as 90 per cent or more!

Crusts are obstacles to the agricultural utilization of soils.

Tufas are concretions formed at the outlets of subterranean water sources.
They are manifestations of the reheating of the water and its degassing
(loss of CO 2 ). In caves too there is degassing because the air is ventilated
and its CO 2 content is lower than that of the soil situated on the roof of
the vault. Stalactites and stalagmites are formed.

Other macroscopic forms of accumulation

7.4.3 Microscopic Features of the Accumulations
The fine examination of calcretes has shown that they consist of a
mixture of all kinds of accumulations, most of them of a partly biological
origin (Zhou and Chafetz 2009). Plant roots, fungi, algae, bacteria,
cyanobacteria and Actinomycetes act simultaneously. We give only some
examples of forms from among the most important and spectacular
ones.

Extraordinary proliferations of mineral needles ( needle-fibre calcite ,
needle-shaped calcite ) are formed, in the dry season, in the space that
separates the root from the adjacent soil ( rhizosphere ) or even within
calcretes. These are whisker crystals composed of calcite, perhaps the
variety lublinite . The rhombohedra, instead of stacking up in three
dimensions, are preferentially associated in a single direction and thus
form needles. Figure 7.15 gives an example from a hundred that we
could procure, all spectacular.

Needle-fibre calcite