Hello, one example might be the secondary dolomitisation.

Mg2+ + 2(CaCO3) --> CaMg(CO3)2 + Ca2+.

The effect is a formation of a new mineral/rock.

Hi!

There are the following ways of formation of secondary minerals in the weathering process.

1. Crystallization of solid minerals(the minerals, simple salts) from solution. This is the most common way. It is that salt in the dissolved state, the evaporation of water or lowering the temperature. kristallizuetsya. The dry conditions in the soil and the mother rock is accumulated halite (NaCl), salts (Na2SO4∙10H2O), gypsum (CaSO4∙2H2O), soda (Na2CO3∙10H2O), calcite (CaCO3), magnesite (MgCO3) dolomite (CA,Mg)(CO3).

2.Crystallization of the solid amorphous substances(minerals of the class of hydroxides and oxides). The formation of minerals in this way is widespread. This group of secondary minerals, widespread in all soil-climatic zones, consists of hydroxides and oxides of silicon (2SiO2∙пН2О), aluminum (AI2O3∙пН2О), iron (Fe2O3∙пН2О), and manganese (MnO2∙пН2О). Their content in rocks and soils up to 10% or more. Considerable spread is goethite (Fe2O3∙пН2О), гематитFe2O3, hydrogoethite (limonite)Fe2O33Н2О, гиббситАІ2О3∙3Н2О. The hydroxide of silicon - opal (SiO2∙nH2O), aging crystallizes, passes into chalcedony and quartz.

3. Hydrolysis, hydration and dehydration, oxidation–reduction reactions, dispersion, isomorphous substitution of primary and secondary minerals.As a result of these reactions formed the third group of secondary minerals found in soils. It consists of clay minerals

Mr. Sangsefidi:

Cation exchange is a well-defined process confined to clay minerals, where among other methods the cation-exchange capacity  (CEC) is used to get an idea of the amount of expandable clay minerals such as smectite present in the soil or other substrates. It is a replacement process of cations bound to a site on the surface of a solid chemical component such as the afore-mentioned clay minerals. The above reactions are hardly to be grouped among this process per definition.

The question whether cation exchange processes are conducive to new secondary minerals I would answer as follows: In certain limits restricted to a mineral group, yes. By cation exchange reactions a chemical change takes place but it does not affect the crystallography so as to get a new mineral sensu stricto as it is a surface reaction only.

Best regards

H.G.Dill

Hi Ebrahim,

It is worth reading a few papers on the formation of the HREE ion-adsorption deposits (or weathered crust elution-deposited rare earth ores) in southern China (Jiangxi, Hunan, Guangdong, Guangxi and Fujian provinces). These deposits were formed by in situ lateritic weathering of REE rich felsic rocks (mainly granites), which led to the formation of residual REE clays.

Kind regards,

Evangelos

Mr. Sangsefidi

Ion exchange usually yields secondary calcite during Na-activation of bentonites (smectite rich clays) at industrial scale. The same process may easily occur in nature when Na-rich pore waters flow through Ca-bentonites in saline/alkaline environments. Similarly ion exchange between smectites and zeolites may retain K in the system which during diagenesis may facilitate illitization (see the attached paper). On the other hand one should not confuse ion exchange with specific adsorption due to fundamental differences (stoichiometric vs non-stoiciometric process).

On the other hand, multi-valence cations such as REE do not easily exchange in clay minerals, but may be specifically adsorbed on the clay edges at non-permanent charge sites especially at relatively low pH below the pzc of the edges. Bearing in mind that REE are usually bound in trace heavy minerals, sometimes reported to have formed diagenetically by authigenic processes (e.g. monazite-see the attached paper), it is expected that specific adsorption of REE of clay minerals may restrain authigenesis of such REE minerals.

Regards

G. E. Christidis

Should we not understand the formation of vermiculite and high charge smectite from biotite in soils  at ambient temperature and pressure  as an example of cation exchange process  for the formation of  secondary minerals?

Hi Professor Kumar Pal,

Thank you for your response.

Could you please introduce some references in which this process is explained?

Does this process is a short-term one? I mean is it possible that Cation Exchange takes place during 20-40 years?

Best regards,

Ebrahim