Accumulation of carbonates

REPLACEMENT

It is about the replacement of primary minerals by carbonates, conserving the original forms (epigenesis).

This process is very common in horizons with a high calcite content. The well expressed alkaline pH of these horizons provokes a destabilization of the quartz and other silicates (especially feldspars and micas) and their substitution by calcite, due to the opposite trend in solubility of silicium and carbonates in function of the pH.

This process requires high pH values, which occur only during short periods, and probably even only once a year. It requires a high concentration of carbonates and is favoured by the presence of coarse grained carbonates which absorb less CO2 on their surface.

Feldspar replacement

It is revealed by a number of features:

Brecciation

The minerals grains are, by the action of the carbonates, fragmented in a series of pieces which are separated one from the other, but generally with preservation of the optical orientation of the original crystals. This is frequently observed in grains of quartz, feldspar and mica.

The replacement of these grains generally gives rise to colorless sparite crystals, sometimes with clear secondary halo´s.

In some cases it is possible to reconstruct the original shape of the detritical grain.

Floating grains

The replacement of silicate minerals is also shown by the presence of felspar and quartz grains floating in a carbonate matrix and separated from the latter by a more or less large empty pore.

These pores are attributed to the (congruent) dissolution of quartz and feldspar in arid areas.

At very high pH values the solubility of Si and Al increases, whereas simultaneously an oversaturation for carbonates may be reached (their solubility decreases) causing their precipitation.

This very high pH is not required for the global horizon, but it is sufficient that it is reached periodically and very locally.

Ghost features

The presence of ghost features is very diagnostic for replacements.

Phantomorphic fabrics may originate also from recrystallizations, but in those cases the crystals are colorless, whereas carbonates due to replacement are frequently grayish or show different stains.

Gypsum replacement

In gypsum marls, the replacement of gypsum by carbonates is sometimes observed, as shown in the following images.

Increasing replacement sequence:

Clay replacement

Replacement of clays is easier than that of detritical grains of quartz, feldspar or mica. The replacements are especially visible when illuvial clay coatings (exhibiting a strong orientation) are substituted.

During the initial phase, in which replacement has barely started, only the crystallization of the carbonates is observed.

In the intermediate phase the presence of partially replaced clay coatings is common.

In the final stage a total assimilation of the clayey material can take place, preserving as only proof a pinkish color, or more frequently a yellowish one, as a result of the incorporation of iron originally bound to the clay.

The replacements are most frequently of micritic size, sometimes of sparitic size.

Organic matter replacement

Carbonate replacements may affect remains of soil organic matter.

Index | Processes | Previous | Next