Crystallization of carbonates takes place through direct
precipitation from the soil solutions. Fundamentally three types of crystals
can be distinguished:
Microcrystals. Crystallization
produces, in most cases, very small, equidimensional crystals, generally
micrite, because normally the precipitation takes place quickly.
Acicular crystals. In other
cases the crystallization produces acicular crystals (from 2 µm
in diameter and up to 30 µm long) of calcite, called whiskers.
They occur inside discrete pores (channels, vughs), forming
pseudomycelia, build up of a network of more or less interwoven crystals
inside the voids.
They are found preferably in the upper parts of calcitic horizons
and represent a very typical case of the crystallization of pedogenic carbonates.
They form a first phase of precipitation of carbonates in the soil and are
supposed to be formed at the expense of oversaturated solutions.
Coarser crystals. Occasionally,
when conditions are more favorable for crystallization, carbonates precipitate
as crystals of large size (>50 microns).
Weathering
Carbonates may be formed as a result of the weathering
of Ca-bearing minerals, as it is frequently the case for feldspars.
Bio-synthesis
Plants and animals can synthetize carbonates as secondary
products of their metabolism.
Microcrystals. Crystallization
produces, in most cases, very small, equidimensional crystals, generally
micrite, because normally the precipitation takes place quickly.
