Why are some soils so red? Mafic rocks? What does this have to do with acid soil?

Photo taken in the Australian outback in the Pilbara region of Western Australia in June 2016.


We don't need to go very far to see soils with this coloration but what makes that soils here of Brazil and for example from the other side of the planet are so red?
The answer lies in the presence of a mineral in the soil, the hematite (mineral rich in iron).
Hematite is a mineral of red coloration and that when present in large amounts in the soil gives it the same strong coloration.
Okay, but what makes the soil so rich in hematite?
This question is deeper but I will try to be brief. Soils are formed by the degradation of pre-existing rocks. In this case, some soils may be formed of rocks already rich in hematite and other minerals (olivines, pyroxene, biotite, mica, etc.) that are rich in iron (mafic rocks). On the other hand even soils that are formed by rocks not so rich in iron can become very red as well.
With the passing of geological ages some minerals are degraded (mainly by the action of rain, the water allows reactions to occur) in the soil and releasing mainly potassium, sodium, calcium and magnesium. These elements are then transported to the sea and there they have been accumulating, it is not by chance that the sea is salty. However, other elements such as iron (greater quantity), aluminum and manganese are not easily leached and accumulate in the soil. As they accumulate they form amorphous minerals (iron oxides and hydroxides) in the soil. The iron as it accumulates gives the red tonality to the soil.
The more iron and aluminum accumulate in the soil, the more it becomes more acidic but that will be the subject for another time.

 See you soon folks!

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