Bicarbonate solutions circulate through the soil and when the solution becomes saturated (mainly due to losses in water or CO2), precipitation and consequent crystallisation occurs.
The formative conditions influence both the type of crystals formed and the kind of accumulations that result.
Various parameters influence the size and shape of the carbonate crystals formed. According to Folk (1974), the main ones are:
concentration of the solutions,
Mg and Na content,
speed of precipitation,
the presence of organic matter.
Only some of these have been considered in pedological studies.
The speed of their formation seems to have a decisive influence on the size of the crystals (Barthurst, 1971), in that the slower the formation, the greater the size of the crystals. Bal (1975) observes that large crystals are formed slowly in tubular voids, in soils with uniform pH values, whilst small crystals form quickly in soils with contrasting pH values as a result of the oversaturation of bicarbonate solutions when they get to the lower horizons with high pH, proceeding from the upper horizons with more acidic conditions.
Another important factor is the presence of clay particles. These act as nuclei for crystallisation and induce the formation of micrite.
Finally, the degree of oversaturation affects the speed of crystallisation and hence the size and also the shape of the crystals, which tend to be needle-shaped. Moreover, the presence of certain ions in the solution may encourage a needle-shaped tendency.
However, as is natural, the conditions of their formation not only affect the shape and size of crystals but are also responsible for the resulting mineralogy. When reviewing the conditions of the formation of carbonate minerals, Lippmann (1973) indicates that calcite forms when the temperature, pressure, salt concentration and Mg content are low. On the other hand aragonite forms at high temperatures and pressure, with concentrated solutions and a high content in Mg.
We must also consider that once crystalised, transformations in the carbonates may occur which affect the size and shape of the grains. By means of recrystallisation a filling-in process occurs which leads to the formation of sparite from micrite.
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